All 115 313A Practice Questions & Answers

Cycle order: Compressor → Condenser → Expansion valve → Evaporator. The compressor pumps high-pressure hot gas to the condenser (rejects heat), then through the expansion valve (pressure drop), then to the evaporator (absorbs heat from space), then back to the compressor.

A cooling coil dehumidifies because its surface sits below the dew point of the air passing over it. Air holds water vapour; cool any surface below the temperature at which that vapour would begin to condense — the dew point — and water comes out of the air onto the surface, exactly as it does on a cold glass. On a coil it runs down the fins into the drain pan and leaves the building as condensate. Removing that moisture is latent cooling; lowering the air temperature is sensible cooling, and a coil does both at once, which is why a coil handling humid air spends part of its capacity on water rather than on temperature.

The refrigerant never touches the air — it is sealed inside the tube, so it can neither absorb the moisture nor be affected by it, and the low pressure inside that sealed circuit has no reach into the airstream. The drain pan only catches what has already condensed. Two practical consequences follow from the real mechanism: a coil running warmer than the dew point cools without dehumidifying, and a coil driven below freezing turns the condensate to ice and blocks its own airflow.

Condenser: rejects heat, converts vapour to liquid. High-temperature, high-pressure vapour from the compressor enters the condenser. Heat is rejected to the outside environment — ambient air (air-cooled) or water (water-cooled). As heat leaves, the vapour condenses into a high-pressure liquid ready for the metering device.

Two superheat measurements answer two different questions, and the gap between them is heat the suction line picked up on the way. Evaporator superheat is taken at the coil outlet against the saturation temperature at that point, and it says how the metering device is feeding the coil. Total or compressor superheat is taken at the compressor suction, and it says how hot the gas reaching the compressor actually is. Here the coil reading of 10°F sits in the normal band for a thermostatic expansion valve, so the valve is feeding correctly and the coil is not starved; the extra 18°F was added after the refrigerant left the coil. Look for missing, wet or displaced suction line insulation, or a line run through a hot ceiling space or mechanical room.

It matters because return gas cools the compressor motor and sets the temperature the compression stroke starts from — hot return gas means a hotter discharge line, and heat is what breaks oil down. Note the contrast: if BOTH readings were high, the coil itself would be underfed and the fault would be charge or a restriction, not the line. The compressor cannot be the cause of a suction-side reading, because its heat of compression is added after the suction port, on the discharge side. A bulb that had lost its charge would close the valve and drive the coil-outlet superheat up as well, which is not what is measured.

Subcooling is the saturation temperature for the pressure on the high side minus the actual temperature of the liquid. Take the pressure at the liquid line or the discharge service valve, read the saturation temperature for that refrigerant off the pressure-temperature chart, then measure the liquid line temperature with a clamp probe: at the condenser outlet when the condenser and the charge are what you are judging, or at the metering device inlet when what you want to know is the condition of the liquid actually arriving at the valve. The difference says how far below its condensing temperature the liquid has been cooled, which is what tells you whether solid liquid rather than a mixture of liquid and vapour is reaching the metering device. Probe contact and insulation decide whether the number is worth anything, because a small probe error is a large share of the whole measurement. On a blend with temperature glide, subcooling is worked out from the bubble point column of the chart.

Where the reading is taken decides what a low or a high number means, so name the point before naming the fault. At the condenser outlet, low subcooling means too little liquid is backed up in the condenser: an undercharge or a leak. High subcooling at the condenser outlet means the liquid is there but something is holding it back: an overcharge, or a restriction downstream of the condenser such as a plugged filter drier, a kinked liquid line or a blocked metering device screen. Past that restriction the picture reverses, because the liquid has lost pressure and part of it has flashed, so a reading taken at the sight glass or the metering device inlet shows little or no subcooling; that is the same event a sight glass reports as bubbles. Ordinary pressure drop along a long liquid line trims the subcooling arriving at the valve in the same way without any fault upstream. Where non-condensables are present the calculated subcooling reads high because the gauge pressure is inflated, so it is apparent rather than real.

The other combinations pair readings that have nothing to do with one another. Suction pressure belongs to the low side and says nothing about the temperature the refrigerant is condensing at, and the suction line lies downstream of the evaporator, so neither of them belongs in this calculation. Comparing the liquid line temperature against the air entering the condenser is not subcooling, and it is not condenser split either: subcooling measures the liquid against the condensing saturation temperature, while condenser split measures that same condensing saturation temperature against the entering-air dry bulb. Neither measurement is a liquid-to-air comparison.

Head pressure that has climbed while suction pressure stayed where it belongs says the condenser is not getting rid of its heat. A condenser can only reject heat into what it is given, so a fouled or blocked coil, a fan that has failed or is turning the wrong way, discharge air recirculating around the unit, a high ambient, or non-condensable gas occupying condenser volume all force the refrigerant to condense at a higher temperature than it should, and the pressure rises with that temperature. The compressor is then pumping against a wider difference, doing more work, which is exactly what the raised amp reading is reporting.

Suction pressure stays near normal on this machine because the thermostatic expansion valve throttles to hold superheat and goes on feeding the coil at the rate the load calls for. The same fault reads differently on a fixed-orifice machine: there the higher liquid pressure pushes more refrigerant through the orifice, so suction pressure rises along with the head and the pattern becomes high on both sides.

The other three faults move the pressures in other directions. An expansion valve stuck closed starves the coil, so suction pressure falls well below normal and discharge falls with it. Worn or leaking compressor valves let gas bleed back instead of being pumped, so the machine still turns but pumps poorly: suction pressure runs higher than it should because the compressor cannot pull it down, discharge pressure runs lower than normal because it cannot build head, the two pressures come closer together, and amp draw falls rather than rises. An undercharge leaves too little refrigerant in the system, so suction pressure, discharge pressure and subcooling all read low together.

Non-condensables raise head pressure and push the condensing temperature up with it. Air and other non-condensable gases collect in the top of the condenser and cannot be condensed, so they occupy surface that should be rejecting heat. With less effective surface the refrigerant has to condense at a higher temperature than normal - so both the pressure and the actual condensing temperature rise. The head pressure rises further than the temperature alone would explain, because by Dalton's law the gauge is reading the refrigerant's own saturation pressure plus the partial pressure of the trapped air sitting on top of it. That gap is the diagnostic clue: measure the actual condenser temperature and it will read BELOW the saturation temperature the P-T chart gives for the pressure showing on the gauge. To confirm, shut the system down and let the condenser settle to ambient temperature, then compare the standing head pressure against the chart value for that ambient. A standing pressure higher than the chart predicts for the measured temperature means non-condensables are present, and the charge must be recovered and the system evacuated properly.

A vacuum on the low side of a running system means almost nothing is reaching the evaporator. With the box calling for cooling, the compressor is pulling vapour out of the coil faster than the metering device can replace it, so the pressure falls until it drops below atmospheric. Two faults do that: a severe undercharge, where there is barely any refrigerant left to feed the coil, and a blockage that stops the flow - a metering device stuck closed, a plugged strainer or filter-drier, or a kinked liquid line. Either way the compressor loses the vapour flow that returns its oil and, on a suction-cooled motor, carries the winding heat away, so shut the system down and diagnose before it is damaged. Running below atmospheric pressure also draws air and moisture into the low side through any leak, fitting or seal, which puts non-condensables and acid into the system. The other readings do not fit. An overcharge raises suction pressure rather than lowering it. Leaking compressor discharge valves let high-side gas bleed back into the cylinder, which pushes suction pressure UP while head pressure falls toward it. And a pump-down draws the low side down only after the call for cooling has ended and the liquid-line solenoid has closed, not while the box is still calling.

Condenser outlet: subcooled liquid under normal operation. The condenser removes heat from the refrigerant in three stages: 1) desuperheating (vapour cools to saturation), 2) condensing (vapour → liquid at constant pressure and temperature), 3) subcooling (liquid cools further, below saturation). Subcooling is what stops vapour bubbles forming in the liquid line before the metering device. Typical subcooling is 10–15°F (about 6–8°C), but the value to work to is the one on the equipment nameplate, since some equipment specifies outside that band.

High suction + high discharge simultaneously = refrigerant overcharge or a condenser that cannot reject heat. An overcharge backs liquid up in the condenser and raises both pressures; a dirty or blocked condenser, a failed condenser fan, or high ambient temperature prevents heat rejection with the same result. Check condenser cleanliness, fan operation, and refrigerant charge. Contrast this with worn compressor valves, where suction rises while discharge FALLS and the two pressures converge toward the middle — that is compressor inefficiency, not this pattern.

Letting the nitrogen out is not the same as taking it out. When the gauges read zero the system is still full of nitrogen at atmospheric pressure, and only a vacuum pump removes it. The sequence after a nitrogen test is release, evacuate, then charge; this technician went from release straight to charging, so the refrigerant was weighed in on top of a system full of nitrogen. Nitrogen does not condense at any pressure or temperature the condenser can reach, so it is carried round to the condenser and stays there, occupying surface that should be rejecting heat and adding its own partial pressure to the refrigerant's. Head pressure climbs beyond anything the ambient and the load can justify, the high-pressure control trips, the unit restarts when the pressure falls back and trips again, and the little running it does is at reduced capacity. There is no bleeding it off: recover the charge, evacuate to a deep vacuum proven with a micron gauge on a standing test, and weigh the charge back in.

The other faults are real but each writes a different signature. A plugged filter drier is a liquid line restriction: it starves the evaporator, so suction pressure runs low with high superheat and the drier outlet runs colder than its inlet; head pressure does not climb and the high-pressure control has no reason to trip. Moisture freezing at the metering device also starves the coil, intermittently, with the same low-side picture and a moisture indicator turning colour; again nothing pushes the high side up. Compressor valve damage is the opposite of what was found: a compressor with damaged valves still turns and moves some gas but pumps poorly, so suction pressure runs higher than it should, discharge pressure lower, the two converge and amp draw falls, which is a machine that cannot reach its high-pressure cut-out rather than one tripping it repeatedly.

COP: dimensionless efficiency ratio. EER: BTU/h per watt. Both measure the same concept — useful cooling output per unit of work input, in different unit combinations. COP = Qc / W (cooling capacity in watts divided by power input in watts). COP = 3 means 3 units of heat removed per 1 unit of electrical energy consumed. EER = cooling capacity in BTU/h divided by power input in watts = COP × 3.412 (because 1 watt = 3.412 BTU/h). A modern high-efficiency air conditioner might have a COP of 3.5–5.0 and an EER of 12–17. SEER (Seasonal EER) averages over a cooling season.

Boiling absorbs latent heat, and that is far more heat per kilogram than simply warming the refrigerant. Heat that turns a liquid into a vapour is latent heat, and for a refrigerant it is many times the sensible heat picked up by warming that same mass a few degrees. The evaporator is therefore built to boil the refrigerant off, and only the last stretch of the coil adds sensible heat as superheat. That is why a modest mass flow can carry a large cooling capacity, and why flooding or starving the coil, which changes how much of the coil is actually boiling, costs capacity so quickly. It is also why the refrigerant leaving the coil must be all vapour: any liquid still in it represents latent heat the coil failed to use, and it is heading for the compressor.

Metering device (TXV or fixed orifice): creates the pressure drop separating the high and low sides of the system. The TXV (Thermostatic Expansion Valve) or fixed orifice/cap tube meters refrigerant flow, dropping pressure rapidly. This pressure drop causes partial flashing (evaporation), dramatically lowering refrigerant temperature. The resulting cold liquid/vapor mixture then enters the evaporator where it absorbs heat and fully evaporates.

Superheat: additional sensible heat added to vapor above saturation temperature at a given pressure, ensuring no liquid droplets enter the compressor. Liquid refrigerant cannot be compressed — it would damage the compressor (liquid slugging). Superheat at the evaporator outlet confirms all refrigerant has evaporated. Typical evaporator superheat: 8–12°F (4–7°C). TXV controls evaporator superheat. Low superheat = risk of liquid slugging. High superheat = evaporator not fully used, reduced efficiency.

Heat pump: standard refrigerant circuit with a reversing (4-way) valve that reverses flow direction, switching the roles of indoor and outdoor coils to provide both cooling and heating. In cooling mode: outdoor coil = condenser, indoor = evaporator. In heating mode: reversing valve switches — outdoor coil = evaporator (extracts heat from outdoor air), indoor coil = condenser (releases heat indoors). Heat pumps are more efficient than resistance heating because they move heat rather than create it. Efficiency measured as COP (Coefficient of Performance) or HSPF.

Charge each system by the measurement its metering device does not already hold constant. A thermostatic expansion valve exists to hold superheat: add refrigerant or take it away and the valve simply moves to keep superheat where its spring is set, so superheat tells you almost nothing about charge on that system. The surplus liquid has to go somewhere, and it shows up as subcooling at the condenser outlet — so a TXV system is charged to the subcooling value on the equipment nameplate. A fixed orifice has no feedback at all: whatever charge is in the system shows up directly at the coil outlet, so superheat is the measurement that moves with charge, and it is checked against the manufacturer's superheat table for the measured indoor wet bulb and outdoor dry bulb.

Swapping the two is the common mistake, and it produces an overcharged fixed-orifice machine or a TXV machine charged blind. Suction pressure on its own is not a charging method — it moves with load, airflow and ambient long before it moves with charge. Amp draw tells you the compressor is loaded, not whether it is loaded by refrigerant, by heat or by a fouled condenser. And whichever method applies, the air side comes first: a system with a dirty coil or a dead blower cannot be charged correctly at all.

The condenser must reject everything the evaporator picked up plus the energy the compressor added. The electrical energy driving the compressor does not vanish; it enters the refrigerant as heat of compression, which is why discharge gas leaves a compressor hot. Total heat of rejection therefore equals the net refrigeration effect plus the heat of compression, and that is the reason an air-cooled condenser is physically larger than the evaporator it serves for the same capacity. Anything that widens the compression ratio, such as a starved evaporator, a fouled condenser or a low charge, makes the compressor work harder, adds still more heat of compression and loads the condenser further, which is one reason those faults tend to compound. Energy is indeed conserved, but not between those two coils alone: the compressor is the third party in the balance.

Filter drier: removes moisture and debris from the refrigerant stream — must be replaced whenever the system is opened to atmosphere or moisture/acid contamination is found. Moisture in a refrigeration system reacts with refrigerant and oil to form acids (hydrolysis), causing copper plating, valve erosion, and compressor failure. The desiccant (molecular sieve or silica gel) in the drier absorbs moisture. Once exposed to atmosphere, desiccant begins absorbing ambient humidity immediately. Also traps debris that could clog the metering device. A clogged/saturated drier shows as a temperature drop and frosting across it.

Suction accumulator: liquid trap in suction line that holds back liquid refrigerant and oil that might otherwise enter and damage the compressor (liquid slugging). Positioned between the evaporator and compressor, it allows vapor to pass through while holding back liquid. A small metered orifice slowly returns trapped oil back to the compressor. Common on heat pumps (liquid can return during defrost) and systems with large refrigerant charges. Oil trapped too long = compressor oil starvation.

Low outdoor temperature = low suction pressure = low refrigerant density = reduced mass flow = reduced heat extraction capacity — the heat pump approaches its balance point. The coefficient of performance (COP) of a heat pump decreases as the temperature differential between indoor and outdoor increases. Below a certain balance point temperature (typically -5°C to -15°C depending on equipment), the heat pump cannot meet the building heat load and supplemental heat (electric strips or gas) is required. Cold climate heat pumps (inverter-driven) are engineered to operate efficiently down to -25°C or below.

Discharge temperature depends on two things: how hot the gas is when it reaches the suction port, and how far the compressor then compresses it. Both gauges read normal here, so the compression is ordinary and the heat has to be arriving with the suction gas. Compression ratio is absolute discharge pressure divided by absolute suction pressure, so it takes BOTH readings to know it; a normal head pressure on its own would settle nothing, because a normal head sitting over a low suction is a wide ratio, not a normal one. In this system the low side and the high side are each where they belong, so the compressor is doing an ordinary amount of work on the gas. Compress gas that arrives hot and it leaves hotter still, and that is the whole of the fault. Hunt for the heat between the evaporator outlet and the compressor: missing, wet or displaced suction line insulation, a suction line run through a hot ceiling space or mechanical room, or a suction-cooled compressor sitting in a hot, poorly ventilated space. Measure superheat at the coil outlet and again at the compressor suction; a normal coil reading with a much higher compressor reading puts the heat gain in the line rather than in the coil.

It matters because the return gas cools the motor of a suction-cooled compressor and sets the temperature the compression stroke starts from. Sustained high discharge temperature thins the oil and breaks it down and is hard on the valves, so treat it as a compressor-life problem rather than a nuisance reading. Take the discharge temperature on the discharge line close to the compressor, at the point the compressor manufacturer's published limit refers to, and record where it was taken, because the line a short way out runs well below the discharge port itself and a temperature without its measuring point is not a reading.

The other three answers fail against the gauges or against the machine. A fouled condenser does raise discharge temperature, but it raises condensing temperature and head pressure along with it, and head pressure here is normal. A partly closed discharge service valve is a restriction on the compressor's own outlet: throttling does not add heat to the gas passing through it, and the compressor would be discharging against the restriction, so the pressure at the compressor discharge would read high rather than normal. And reverse rotation is not a discharge-temperature fault on a reciprocating machine at all: a piston compresses the gas the same way whichever direction the crank turns, so a compressor running on a swapped phase pair pumps as before and its discharge runs no hotter for it. The very loud machine that pumps nothing and damages itself within minutes is the scroll's reverse-rotation signature, a different compressor from the one in this stem.

Where liquid and vapour sit together, pressure and temperature are locked to one another. In the two-phase (saturated) part of a coil the refrigerant can only be at the saturation temperature that matches its pressure, and that pairing is exactly what a pressure-temperature chart lists. It is why a suction gauge reading gives the technician the evaporator temperature and a discharge gauge reading gives the condensing temperature, with no thermometer on the coil at all. Air temperature and fan speed matter, but they act on the pressure: more load drives the pressure up and the saturation temperature rises with it, so the temperature follows the pressure rather than the other way round. Once the refrigerant is all vapour (superheated) or all liquid (subcooled) the link is broken, and temperature must then be measured with a thermometer as well as a gauge.

R-22 (HCFC) = ozone depleter. Because of its ozone-depleting potential, Canada phased out R-22 production and import under the Montreal Protocol. As of 2020, R-22 can only be used from recovered/recycled stock. Common replacements include R-410A (new systems) and R-407C (retrofit).

A cylinder that will hold recovered refrigerant is a pressure receptacle for the transport of dangerous goods, so it has to carry a specification Transport Canada authorizes and it has to be within its requalification date. Transport Canada's own container pages describe the permanent TC markings a specification cylinder carries under CSA B339, and the requalifier's registered mark and date stamped on the cylinder each time it is requalified; CSA B340 governs the selection and use of those containers. Cylinders sold here are frequently dual-marked, because Transport Canada also lists DOT cylinders manufactured to the American rules among the authorized container types — so 'DOT-rated' on its own is US shorthand, and what actually matters is a specification Transport Canada accepts, plus a current requalification date.

A single-use disposable cylinder is out on both counts: it was never built to be refilled, it has no overfill protection, and refilling it is prohibited. A shop-built vessel carries no specification marking at all, whatever fittings are hung on it. And a pressure rating is not a specification: a cylinder can be strong enough for the gas and still be unmarked, unauthorized or long out of date. Fill by weight against the cylinder's tare and rated capacity so a vapour space is left, and keep a separate, clearly labelled cylinder for a burnout or for mixed refrigerant.

R-410A at a 35°C (95°F) condensing temperature sits near 295 psig on the high side, so 290 psi is the closest choice. R-22 at that same 35°C sits near 182 psig, so the gap at condensing conditions is about 60 per cent. Lower down the curve the gap is wider: at a 40°F evaporator temperature R-22 is near 69 psig while R-410A is near 118 psig, about 70 per cent higher. There is no single multiplier that covers the whole curve, which is why any comparison of the two has to name the saturation temperature it is made at, and has to compare gauge with gauge — reading one refrigerant as gauge pressure and the other as absolute makes the gap look far smaller than it is. On a hot day the measured head pressure runs higher still, because the condensing temperature sits well above ambient. All of this is why R-410A systems require components, hoses, gauges and recovery equipment rated for the higher pressure.

POE is hygroscopic: it absorbs atmospheric moisture far faster than mineral oil, and it holds that water chemically rather than simply carrying it along. That is what makes it a field problem. Free water in a system can be boiled off with a deep vacuum, but moisture bound into POE largely will not come out on the vacuum pump, so an open pail left on the truck for an afternoon can carry water straight into a circuit that was correctly evacuated. Water plus POE plus heat gives hydrolysis, hydrolysis gives acid, and acid gives copper plating, varnished valves and eventually a burnout. Working practice follows from that: buy oil in the smallest sensible container, keep the cap on until the moment of use, pour rather than leave it standing open, never return poured oil to the container, and fit a fresh liquid line drier whenever the circuit has been opened. POE is used in the first place because HFC and HFO refrigerants are not miscible with mineral oil, so this handling discipline is the price of that compatibility.

All refrigerant must be recovered with certified recovery equipment into an approved recovery cylinder. In Canada that means a TC specification cylinder. Transport Canada's requirements for manufacturing, inspection, testing, marking and requalification of cylinders are set out in CSA B339, which CSA B340 invokes for TC specification cylinders authorized for use in Canada under the Transportation of Dangerous Goods Regulations. DOT is the United States Department of Transportation, so a DOT stamp is the American specification; cylinders sold in North America are commonly marked to both, and the marking to look for on this side of the border is TC. Releasing refrigerant to atmosphere is prohibited: the Federal Halocarbon Regulations, 2022 made under CEPA 1999 forbid it for systems under federal jurisdiction - Crown-owned systems, federal works and undertakings, and systems on federal or Indigenous lands - and everywhere else the use and handling of these substances are regulated by the province or territory through its own halocarbon regulation. There is no small-quantity exemption and no acceptable way to vent slowly. Never mix refrigerants in one cylinder; a mixed cylinder cannot be reclaimed and becomes waste for destruction.

Liquid refrigerant expands strongly with temperature and is very nearly incompressible, so a liquid-full cylinder has nothing left to absorb that expansion. Leave a vapour space and a warm afternoon simply walks the pressure up the saturation curve, which is what the cylinder is built for. Fill it solid and the same temperature rise has nowhere to go: pressure climbs hydrostatically, far faster and far higher than any saturation pressure, and the cylinder can rupture. That is why recovery cylinders are filled by weight, checked against the tare weight and the capacity marked on the cylinder itself, and why many are fitted with a float or a shut-off that stops the machine before the cylinder fills. Two related rules come from the same physics: never put a flame or other heat source on a cylinder to speed up a transfer, and never refill a disposable single-use cylinder, which has no overfill protection and was not built to be filled again.

A2L means lower toxicity and the lower-flammability subclass: A for toxicity, 2L for flammability. Under the ASHRAE 34 safety group system the letter is toxicity — A is lower toxicity, B is higher — and the number is flammability: 1 is no flame propagation when tested to the standard, 2 is lower flammability, and 3 is higher flammability, the group the hydrocarbons fall into. 2L is not a fourth class but a subclass sitting inside class 2, for refrigerants whose burning velocity is 10 cm/s or less. R-32 will burn, but slowly and only in a fairly narrow set of conditions.

That does not make it something to treat casually. A2L work calls for no open flames or other ignition sources, adequate ventilation, leak detection where the standard requires it, and recovery equipment and tools rated for A2L service. The remaining answers each fail on one half of the group: an A1 refrigerant would be lower toxicity with no flame propagation, higher toxicity would be a B group, and the highly flammable group is 3, not 2L.

Zeotropic blends: always charge from liquid phase (cylinder inverted). Zeotropic blend components have different vapour pressures — if charged as vapour, lighter components vaporize first, leaving the heavier ones behind. This changes the composition in both the cylinder and the system. Charging from liquid phase maintains the correct blend ratio.

Near-azeotropic still means a blend, and blends are charged as liquid. Two things get confused here, and they are separate. Glide is about how the blend behaves inside the system: R-410A is near-azeotropic, meaning its bubble point and dew point sit almost on top of each other, so it can be read off a pressure-temperature chart much like a single fluid. Composition is about what leaves the cylinder: R-32 is the more volatile of the two components, so vapour drawn off the top of a cylinder is richer in R-32 and both the cylinder and the system end up off-specification. That is fractionation, and a small glide does not prevent it. Charging liquid, through the cylinder liquid valve or with the cylinder inverted, delivers the blend at its intended ratio; meter it into the suction line so the compressor is not slugged, or weigh it into the high side with the system off. R-410A is also an HFC that runs on polyol ester oil, not mineral oil.

R-600a (isobutane) is a highly flammable A3 refrigerant — flammability is the primary hazard. Hydrocarbon refrigerants (R-600a, R-290/propane) have excellent thermodynamic properties but they burn. They are used in small-charge domestic appliances, where the charge is small enough that a full leak stays below the lower flammability limit in the room. Servicing: ventilated areas only, no open flames or sparks, non-sparking tools, and recovery equipment rated for flammable gases. These appliances must not be serviced in an enclosed space. R-600a is a low-GWP refrigerant and it operates at lower pressures than the common HFCs, so neither a GWP argument nor a high-pressure argument is what drives the precautions.

Burned-out compressor: refrigerant is acid-contaminated — use dedicated equipment and send for reclamation. A burned motor creates acid and carbon deposits from the breakdown of motor windings (copper, varnish, refrigerant oil). This acid-contaminated refrigerant cannot be recycled back into a system — it must be sent to a reclamation facility (not just recycling). Use dedicated "burnout" recovery equipment to prevent contaminating your regular recovery machine, and clearly label the recovery cylinder as contaminated.

GWP: climate impact relative to CO₂ over 100 years (CO₂ = 1). R-410A has a GWP of ~2,088 — one pound of R-410A released has 2,088 times the global warming impact of one pound of CO₂. Canada's HFC phase-down is set out in the federal Ozone-depleting Substances and Halocarbon Alternatives Regulations, made under the Canadian Environmental Protection Act, 1999, which implement the Kigali Amendment to the Montreal Protocol by progressively reducing the supply of HFCs allowed into Canada and the use of high-GWP HFCs in manufactured products. Technicians must understand why refrigerants are transitioning to low-GWP alternatives (R-454B, R-32, R-1234yf, CO₂).

Liquid refrigerant boils the instant it leaves the system, and it takes the latent heat it needs from whatever it lands on. On skin that is a freeze burn in a moment; in an eye it can cause permanent damage. That is why gloves and eye protection go on before a hose is connected or broken, not after. Treat the injury as frostbite: flush with lukewarm water — never hot — do not rub, do not apply direct heat, cover loosely and get medical attention. Rubbing a frozen area, which one of the answers above recommends, drives ice crystals through the tissue and makes the injury worse.

The other hazards are real but they are not this one. Common HFC and HCFC refrigerants are not acids in the cylinder, although acids do form inside a system that has run wet or burned out. They are not absorbed through the skin as poisons; the genuine inhalation hazards are oxygen displacement, because refrigerant vapour is heavier than air and pools in pits, basements and machinery rooms, and the toxic products formed when refrigerant meets a flame or a hot surface. Venting vapour also carries oil and debris with it, which is the second reason for eye protection.

The 700 series is reserved for inorganic refrigerants, and the last two digits are the substance's molecular mass. Ammonia has a molecular mass of 17, so it is R-717; carbon dioxide is 44, so it is R-744; water is 18, so it is R-718; air is taken as 29, so it is R-729. The series is worth knowing because a number tells you immediately what family you are dealing with. The 400 series is zeotropic blends and the 500 series azeotropic blends, and both are numbered in the order they were registered rather than by what is in them — which is why R-407A and R-407C hold the same components in different proportions, are different refrigerants, and are not interchangeable. The 600 series covers organic compounds, including R-600a, isobutane. Halocarbons take their numbers from their chemical formula.

What a refrigerant number never encodes is a pressure rating or a safety property. Toxicity and flammability come from the separate ASHRAE safety group — the letter-and-number code such as A1, A2L or A3 — and pressure comes from the refrigerant's own saturation curve, which is why the pressure-temperature chart exists.

R-410A works far above R-22, so gauges, hoses and recovery equipment rated only for R-22 can rupture on it. Compare the two gauge to gauge at the SAME saturation temperature, or the difference becomes whatever the two machines happened to be doing that day. At a 40°F evaporator temperature R-22 sits near 69 psig and R-410A near 118 psig, about 70 per cent higher. At a 110°F condensing temperature R-22 sits near 226 psig and R-410A near 364 psig, about 60 per cent higher. So the gap is widest at low-side conditions and narrows on the high side, and there is no single multiplier that covers both ends.

What follows is practical. Use a manifold set and hoses marked for R-410A service pressures and a recovery machine rated for it. The service ports differ as well — 5/16 inch flare on R-410A against 1/4 inch on R-22 — so the fittings themselves discourage cross-connection. Nothing about the higher pressure reduces the duty to recover: the charge still comes out with recovery equipment. Charge quantities are set by the equipment, not by the pressure. And the two refrigerants are not interchangeable in service equipment, in components, or in oil: R-410A runs on polyol ester oil, R-22 on mineral oil.

Fractionation is a zeotropic blend separating into its components, and it happens whenever the blend leaves as vapour: through a leak from a part of the system holding vapour, or when vapour is drawn off the top of a cylinder. The components of a zeotropic blend do not share a boiling point, so the vapour standing above the liquid is always richer in the more volatile one. What escapes through a vapour leak, and what comes out of the valve of an upright cylinder, is therefore not the blend that went in. The mixture left behind is short of that component, so both the cylinder and the system end up off specification, and the operating pressures and the capacity move away from what the pressure-temperature chart for that refrigerant says they should be.

Near-azeotropic is not an exemption. R-410A is a mixture of R-32 and R-125, R-32 is the more volatile of the two, and vapour drawn off the top of an R-410A cylinder is richer in R-32 exactly as it is with R-407C or R-404A. What near-azeotropic describes is glide, meaning how close the bubble point and the dew point sit and therefore how much like a single fluid the blend reads inside a running system. A small glide says nothing about composition at the cylinder valve. Only a true azeotrope, whose vapour and liquid hold the same composition at the boiling point, resists this.

The other three describe real things under the wrong name. A zeotropic blend changing temperature while it boils at one pressure is temperature glide: a property of the blend at its intended composition, read as the bubble point and the dew point on its chart, and it is present whether or not any fractionation has occurred. Refrigerant breaking down into acids in the presence of moisture and heat is hydrolysis, the contamination a filter drier and an acid test are there to catch. Liquid flashing to vapour ahead of the metering device is flash gas, a loss of subcooling that a sight glass shows as bubbles; the composition of the refrigerant has not changed at all.

Practice follows from the definition. Every zeotropic blend, the near-azeotropic ones included, is charged as liquid, through the cylinder liquid valve or with the cylinder inverted, metered into the suction line so the compressor is not slugged. A leaking system is never topped up, whatever the refrigerant: recover, repair the leak, pressure test, evacuate and weigh in the charge. With a zeotropic blend there is a second reason for that rule, because what remains after a vapour leak is no longer the refrigerant named on the cylinder, so the recovered remainder is not put back into the system.

On a privately owned commercial system on private land, the halocarbon rules that bind the technician are the province's or territory's own, not the federal ones. The Federal Halocarbon Regulations, 2022 (SOR/2022-110), made under the Canadian Environmental Protection Act, 1999, open with an application section: they apply to air-conditioning or refrigeration systems, solvent systems, fire-extinguishing systems and containers located in Canada that are owned by Her Majesty in right of Canada, a board or agency of the Government of Canada, a Crown corporation as defined in the Financial Administration Act, or a federal work or undertaking, or that are located on aboriginal lands or federal lands. A supermarket owned by a private grocery chain on private land is none of those, so that instrument does not reach this job.

Inside its own scope the federal regulation is strict, and it is worth knowing in its own terms because federal sites do come up. It provides that a person must not release, or allow or cause the release of, a halocarbon contained in an air-conditioning or refrigeration system, a fire-extinguishing system or a container, or in equipment used in the reuse, recycling, reclamation or storage of a halocarbon. The prohibition does not apply if the release is for the purpose of calibrating leak-detecting devices with equipment designed specifically for that purpose and the manufacturer's recommended procedures are followed; results from connecting or disconnecting hoses that are less than 1 m in length and used for charging with, or recovering, a halocarbon; results from a purge system on an air-conditioning or refrigeration system, including any associated recovery equipment, that emits less than 0.1 kg of halocarbon per kilogram of air purged; or is from a fire-extinguishing system for the purpose of fighting a fire that is not set for training purposes, or of testing the system in a military vehicle as authorized by a permit. Note the boundaries as written: a hose of exactly one metre is not inside the hose exception, and a leak-detector calibration is only inside its exception when both conditions, the purpose-built equipment and the manufacturer's procedure, are met. The regulation also provides that only a certified person may install, service or recover a halocarbon from an air-conditioning system or refrigeration system. None of that changes who the regulation applies to.

Outside that scope, Environment and Climate Change Canada describes provincial and territorial legislation on ozone-depleting substances and halocarbon alternatives as complementing the federal regulations, and lists among its requirements proper labelling and handling of equipment, training for equipment service providers, procedures for installing, removing, servicing, repairing or decommissioning products, and a prohibition on recharging leaking equipment. So the rules that actually bind this job, meaning who may handle the refrigerant, what has to happen to it instead of a release, and what has to be recorded, come from the halocarbon regulation of the province or territory where the work is done, and they differ in detail from one province to the next.

The other two instruments are federal and are real, which is what makes them tempting. The Ozone-depleting Substances and Halocarbon Alternatives Regulations control the manufacture, import, export, sale and use of the substances themselves, through prohibitions, permits and allowances; they contain no release prohibition for a refrigeration system and say nothing about who may service one. The Transportation of Dangerous Goods Regulations do bear on this trade, but on the recovery cylinder, its TC specification and its movement on the road, not on how the system is serviced.

Correct leak repair procedure: Recover → Repair → Pressure test → Evacuate → Recharge. 1) Recover all refrigerant (cannot vent). 2) Repair leak. 3) Pressure/leak test with dry nitrogen to OEM spec (verify repair). 4) Release nitrogen. 5) Deep vacuum (500 microns or below) — removes all moisture and air. 6) Recharge with new/virgin or reclaimed refrigerant to manufacturer specification. Record refrigerant quantity. Document all work per regulations.

Three distinct terms: Recovery (remove to cylinder), Recycling (on-site oil/moisture cleanup), Reclaiming (certified facility to AHRI 700 virgin purity). Recovery: extracting refrigerant from a system into an approved recovery cylinder — mandatory before opening. Recycling: passing recovered refrigerant through filters and oil separators for reuse on the SAME system or same owner's equipment. Reclaiming: sending to a certified reclaim facility for processing to AHRI Standard 700 purity (essentially virgin quality, sold for any use).

Electronic leak detectors have limitations: cross-sensitivity to other chemicals, calibration for specific refrigerant groups, and possible misses of certain refrigerant types. Heated diode detectors: very sensitive but can trigger on alcohol, chlorinated solvents, and other halogens (false positives). Infrared detectors: more selective, fewer false positives. All detectors: sensitivity degrades over time (sensor element replacement needed). Most refrigerants are heavier than air (except ammonia) — check at low points. Always confirm finds with UV dye or soap bubble solution.

R-32 (GWP 675) and R-454B (GWP 466) are primary replacements for R-410A (GWP 2088), with similar performance characteristics. Both are A2L classified (mildly flammable) — requiring specific compressors and system design, updated components, leak detection in equipment, and technician training. R-32 is a single-component refrigerant with simpler reclaim. R-454B is a blend. Both use POE oil, similar to R-410A systems. Some manufacturers also use R-466A (non-flammable, higher GWP than R-32 but lower than R-410A).

A TXV calibrated for R-22 may not perform correctly with R-407C due to different thermodynamic properties (latent heat, pressure-temperature relationship). R-407C is a zeotropic blend (R-32/125/134a) with temperature glide — its saturation characteristics differ from R-22. A TXV calibrated for R-22 can pass incorrect refrigerant flow for R-407C operating conditions, and the sensing bulb charge must be compatible with R-407C. Also: check that TXV superheat setting is appropriate for R-407C, measuring superheat against the dew-point temperature from the R-407C pressure-temperature chart. Some TXVs are adjustable; others require replacement with R-407C-rated valves. R-407C is not a drop-in — mineral oil must be replaced with POE.

The seller has to be satisfied, before the sale, that the buyer holds a valid refrigerant handling certificate. In Canada this requirement is provincial, not federal. Ontario's page on the certificate to handle refrigerants states that an Ozone Depletion Prevention (ODP) certificate card is needed to purchase and handle refrigerants, under O. Reg. 463/10. British Columbia's Ozone Depleting Substances and Other Halocarbons Regulation, s. 6(2), lets a person buy for servicing air conditioning or refrigeration equipment only if the person first gives the seller satisfactory proof of being an approved person. The certificate is personal to the technician, so an employer's licence does not stand in for it, and there is no per-transaction federal permit. On the federal side, the Federal Halocarbon Regulations, 2022 under CEPA 1999 cover systems owned by federal departments and by federal works and undertakings; use and handling on an ordinary commercial system fall to the province.

CSA B52 machinery rooms require refrigerant detection plus emergency mechanical ventilation. A refrigerant vapour detector monitors the room and, at a set concentration, sounds an alarm and starts (or increases) mechanical ventilation to purge the leak. This guards against oxygen displacement/asphyxiation with any refrigerant and against ignition with flammable or A2L refrigerants. A CO detector is for combustion (fuel-burning) appliances, not refrigerant leaks. The 2023 edition (B52:23) adds dedicated A2L detection and ventilation provisions.

CSA B52 bases the requirement on the refrigerant's safety classification, the charge quantity, the occupancy, and the volume of the space. Each refrigerant has a Refrigerant Concentration Limit. If the full system charge released into the occupied space could exceed that allowable concentration, the system has to be isolated in a machinery room with refrigerant detection and mechanical ventilation. Higher-risk occupancies — institutional and public assembly, where people cannot readily leave — carry lower allowable limits than an ordinary commercial space. How the unit is arranged and powered does not enter into it: compressor type and count, the electrical supply, and how much floor the equipment takes up are installation questions, not the trigger for the machinery room.

On a pump-down system the thermostat controls the liquid line solenoid valve, not the compressor. When the box temperature is satisfied, the thermostat de-energizes the solenoid coil and the valve closes, cutting off liquid feed to the metering device. The compressor keeps running and pumps the refrigerant remaining in the evaporator and suction line forward into the condenser and receiver. Only once suction pressure has fallen to the low-pressure control's cut-out point does the compressor stop. That is the whole point of the sequence: with the evaporator and suction line emptied, there is very little refrigerant left on the low side to migrate into the cold compressor crankcase during the off cycle, so the compressor is not asked to swallow a liquid slug on the next start.

Dropping the compressor contactor and closing the solenoid at the same instant is the direct, non-pump-down control arrangement, where the thermostat breaks the compressor circuit itself. It is a real wiring method, but it leaves the evaporator flooded during every off cycle, which is exactly the migration problem pump-down exists to prevent. Having the low-pressure control open first and de-energize the solenoid reverses cause and effect: the low-pressure control does stop the compressor, but that is the end of the sequence, not the start of it, and it happens only because the solenoid has already closed. Cycling the condenser fan confuses two unrelated functions — that is head-pressure control for low-ambient operation, and it plays no part in shutting the system down. A useful field note on this circuit is that repeated pump-downs with no call for cooling point to refrigerant leaking past the closed solenoid seat or past the compressor discharge valves, refilling the low side and re-triggering the low-pressure control.

A capillary tube is a fixed restriction with no feedback: it does not meter, it simply passes whatever the pressure difference across it will push through a fixed bore. Flow depends on the tube inside diameter and length and on the difference between high-side and low-side pressure, and nothing in it senses superheat. Raise the load and the coil boils the refrigerant off early, so superheat climbs and capacity falls short of what the coil could do. Drop the load and the coil floods instead, superheat falls toward zero, and liquid can reach the compressor. Because there is no valve holding surplus refrigerant back, cap tube systems get a critical charge weighed in, run without a receiver, and often carry a suction accumulator as protection. This is precisely the behaviour a TXV exists to correct, since a TXV modulates on superheat and holds it roughly constant as the load moves.

The receiver is high-side liquid storage, sitting between the condenser outlet and the liquid line. How much refrigerant a system needs in circulation changes with load and ambient, and a pump-down cycle deliberately parks the whole charge on the high side; the receiver gives that refrigerant somewhere to sit while still feeding a solid column of liquid to the metering device. It also lets a technician isolate and hold the charge for service without recovering it. Do not confuse it with the two vessels it is often mixed up with: a suction accumulator sits on the low side and catches liquid before the compressor, and an oil separator sits in the discharge line and returns oil to the crankcase. A receiver is never a substitute for a correct charge either, because a receiver filled to the top backs liquid up into the condenser and drives head pressure up.

Check cause before replacing compressor. Refrigerant flood-back causes liquid slugging that hydraulically locks compressor pistons. Low voltage at the compressor terminals causes high current + thermal overload. Compressor may free itself after cooling — but replacing without fixing the cause means the replacement fails too. Check: liquid slugging (suction line frost/wet), power supply and voltage at the terminals, overload conditions.

Scroll compressor noise plus low discharge pressure equals worn scrolls. Worn or damaged scroll wraps let gas leak back across the flanks instead of being carried around and compressed, so the machine can no longer build pressure, and the loss of a proper seal is what makes the running sound rough. Also check for refrigerant flood-back, because liquid entering the scrolls accelerates exactly this wear. Normal subcooling is what rules out the tempting alternative: an undercharged system runs short of liquid in the condenser and shows LOW subcooling alongside its low discharge pressure, and it would show low suction pressure as well rather than a rough-running compressor. A failed condenser fan drives discharge pressure up, not down, and an over-feeding expansion valve shows itself as low superheat and a cold, sweating suction line.

A relief discharge goes outside, not into the building. CSA B52 requires the pressure-relief device on a refrigerant pressure vessel such as a liquid receiver to discharge to the outdoors, terminating clear of doors, windows and fresh-air intakes so a release cannot re-enter the building or an occupied space. The vent line must be sized and routed so it cannot throttle the device's rated relieving capacity, and it must not be trapped, capped or valved off. Venting into the machinery room is wrong: machinery-room vapour detection and mechanical ventilation are separate requirements that deal with leaks, and they are never a substitute for a relief vent. Piping the relief into the suction line gives no protection at all, because it discharges into the same system it is supposed to protect. Leaving the outlet bare dumps the charge into the plant room. Confirm the termination heights and clearances in the edition of B52 adopted by your province.

Bubbles in sight glass = flash gas = subcooling problem. Liquid refrigerant should arrive at the metering device as a solid liquid (no bubbles). Bubbles (flash gas) indicate: refrigerant is partially vaporizing before the metering device — caused by low charge (low subcooling), restriction in liquid line (pressure drop = boiling), or high liquid line temperature.

An internally equalized valve senses pressure at its own outlet, upstream of the distributor and the coil, so it never sees the pressure the bulb is reporting against. A TXV sits wherever three forces balance: bulb pressure pushing it open, and evaporator pressure plus the superheat spring pushing it closed. The distributor nozzle and its feeder tubes are a deliberate pressure drop, and the coil circuits add more, so pressure at the valve outlet is meaningfully higher than pressure back at the bulb. Feed that higher pressure into the closing side and the valve reads the coil as fuller than it really is, throttles back, and starves the coil at high superheat, which no amount of spring adjustment will fix. The external equalizer line, taken from the suction line just downstream of the bulb, gives the valve the true outlet pressure instead. A kinked, plugged or omitted equalizer line produces exactly the same starved behaviour.

TXV: maintains constant superheat at evaporator outlet by modulating refrigerant flow. The TXV has a sensing bulb on the suction line at the evaporator outlet. As suction superheat increases (evaporator starved, more load), the bulb pressure increases and opens the valve (more refrigerant). As superheat drops (evaporator flooding risk), the spring tension closes the valve. Target: typically 8–12°F (4–7°C) superheat at design conditions.

An unloader takes cylinders out of service by holding their suction valves open, so those pistons push gas back and forth into the suction manifold instead of compressing it. Capacity comes off in steps — a six-cylinder machine unloading two cylinders at a time runs at roughly two thirds and then one third of full capacity — and power falls with it, because an unloaded cylinder does very little work. Unloaders are pilot-operated, by a solenoid or by oil or suction pressure, and they earn their keep twice: matching a falling load without stopping the machine, and reducing the load the motor has to accelerate, which is why many compressors start unloaded and load up once they are turning.

Compare the alternatives. Hot gas bypass does reduce net capacity, but by feeding false load to a fully loaded compressor, so the machine keeps doing full work for less useful cooling — which is why unloading is preferred wherever it is available. Speed control is a third method again, done with a variable frequency drive or an inverter-driven compressor, not with an unloader. Throttling a suction service valve is never a capacity control: it starves the compressor of its own return gas and of the cooling that gas provides.

The colour element in a sight glass is a moisture indicator, and yellow means the refrigerant is wet. Green means dry and yellow means moisture above the safe level; the element reacts to moisture in the refrigerant and to nothing else, and it returns to green once the system has been dried out. Moisture matters because water with refrigerant, oil and heat produces acid, and acid produces copper plating, varnished valves and eventually a burnout — and where the system runs on POE oil the water is largely held in the oil, where a vacuum will not readily take it back out.

The response is to find how the water got in — a system left open, a low side running in a vacuum through a leak, careless charging, or oil taken from a container that had been opened before — then recover, fit a fresh liquid line drier (sometimes a larger one), evacuate deeply, recharge, run, and read the element again. What the element does not report is charge or oil condition: bubbles in the clear window, not colour, are the flow indication, and oil is judged with an acid test kit. Nor is the colour change an ageing effect; it is a reversible chemical indicator.

Hot gas defrost: discharge gas heats evaporator from inside — efficient and fast. During hot gas defrost, solenoid valves redirect superheated compressor discharge gas directly to the evaporator inlet (in some designs the coil is fed in reverse flow). The hot gas condenses inside the evaporator coil, releasing its heat of condensation directly to the frost accumulation. This is faster and more energy-efficient than electric defrost. Common in commercial walk-in coolers and display cases.

Scroll compressor: two interlocking scrolls — one fixed, one orbiting — compress refrigerant between them. Advantages over reciprocating: fewer moving parts (no pistons/connecting rods/valves), quieter, more efficient, smoother discharge pressure. Cannot tolerate liquid refrigerant easily (no valve to absorb shock — liquid can damage scroll tips). Very common in residential and light commercial HVAC. Failure mode: worn scroll tips, flooded start (liquid refrigerant migration during off cycle).

Copper heated in air grows a black, flaky oxide on the inside of the tube, and a slow trickle of nitrogen displaces the air so that scale never forms. The scale does not stay where it was made. Refrigerant and oil wash it downstream, where it plugs the metering device screen, blinds the filter drier, scores bearings and ends up in the compressor — one of the classic reasons a system never runs right after a repair, and a fault that is invisible from outside the tube. The flow has to be small: enough to sweep the air out, not enough to pressurize the joint and blow the molten alloy out of it. Regulate it down, vent it at an open end so it can actually flow through, and keep it going until the joint has cooled.

Nitrogen carries away very little heat at that flow, and quenching a brazed joint is the last thing wanted anyway. Brazing copper to copper with a phosphorus-bearing alloy needs no flux inside the tube at all, and flux is never carried by a gas. Leak testing is a separate step, done after the joint has cooled, with nitrogen brought up to the specified test pressure. And never apply a flame to a system that still holds refrigerant: refrigerant meeting a flame or a hot surface breaks down into toxic products.

TXV hunting: an overly sensitive valve constantly over/under-compensates, causing oscillating suction pressure and superheat. Causes: sensing bulb has lost partial charge (erratic signal), sensing bulb not firmly clamped to suction line (reads air temperature instead), incorrect superheat setting (adjustment spring set too close to system operating point), refrigerant/TXV mismatch (wrong TXV for refrigerant), or refrigerant cross-charged. Correction: inspect and re-clamp bulb, check bulb insulation, verify refrigerant type matches TXV, adjust superheat setting if accessible.

Check valve: allows flow in one direction only — prevents refrigerant flowing backward during the off-cycle. Used in discharge lines on heat pumps and tandem/parallel compressor systems. On heat pumps: discharge line check valves direct refrigerant correctly when the reversing valve switches modes. On parallel/tandem compressor systems: individual discharge line check valves prevent the running compressor from pushing hot discharge gas into the stopped compressor. Also used in liquid lines of multi-evaporator systems to prevent migration.

Broken compressor valve reed: gas bypasses the damaged valve instead of being compressed, severely reducing compression efficiency and capacity. A broken suction reed lets cylinder gas escape back into the suction line on the compression stroke — suction pressure runs high because the compressor can no longer pull it down, discharge pressure runs low, and mass flow drops. A broken discharge reed lets compressed gas leak back into the cylinder on the suction stroke and re-expand — again suction pressure rises and discharge pressure falls. Either way the two pressures converge, so a fault that pushes head pressure up is not this signature. Amp draw is below normal because the compressor is doing less work, while discharge temperature climbs because the same gas is compressed over and over. Confirm by comparing the measured compression ratio against the ratio the system's design evaporating and condensing conditions should produce — there is no single healthy number, since a low-temperature system normally runs a far higher ratio than a comfort-cooling system. A valved-out pump-down check (front-seat the suction service valve and watch whether the compressor pulls the low side down and holds it) separates a worn compressor from a system fault. A reed failure does not by itself cycle the compressor on and off, and it is not confined to the oil circuit — it shows in the refrigerant-side pressures long before an oil pressure switch reacts. Verify with valve plate inspection before condemning the compressor.

Refrigerant economizer: improves system efficiency and capacity by subcooling liquid using a flash tank or heat exchanger at intermediate pressure. A portion of liquid refrigerant is expanded to an intermediate pressure in the economizer. This cold refrigerant subcools the main liquid line, reducing flash losses at the metering device and increasing refrigerating effect per kilogram circulated. The intermediate-pressure flash gas is injected into the compressor mid-stage through the economizer port (requires a two-stage or economizer-capable compressor). Because flash losses grow as the pressure ratio grows, the gain is largest on low-temperature duty and in high ambient conditions. An air-side economizer, which brings in outdoor air when it is cool enough to carry the load, is a separate device on the air side and does nothing to the refrigerant circuit. Condenser heat reclaim for domestic hot water and simply cycling the compressor off at low load are also separate strategies, not the function of the economizer port. Common in commercial refrigeration and VRF systems.

Hot gas bypass: injects discharge gas into the suction line or evaporator inlet to maintain minimum compressor load at low demand, preventing short cycling and low suction pressure. When refrigeration load decreases significantly (e.g., walk-in cooler at night), suction pressure drops, risking low-pressure cutout and excessive short cycling. The hot gas bypass valve opens to inject warm discharge gas, artificially loading the compressor and stabilizing suction pressure. This is an efficiency penalty (wasted compression work) — used only when modulating capacity is not available.

Oil separator: catches compressor oil in the discharge stream and returns it to the compressor crankcase — preventing oil starvation and oil accumulation in the heat exchangers. Some oil inevitably leaves the compressor with discharge gas. Without separation, oil accumulates in the condenser and evaporator, reducing heat transfer efficiency and risking compressor oil starvation. The separator uses baffles, mesh, or centrifugal force to drop oil out of the gas stream. A float valve or capillary tube returns oil to the crankcase. Critical for: low-temperature systems (oil moves poorly), long line sets, and systems with multiple evaporators. A discharge muffler sits in the same part of the line but only smooths the pulsation from the compressor and separates nothing; solid particles are caught by the suction strainer or the liquid line filter drier; moisture is taken out by the desiccant in the filter drier, not by the separator.

HFC refrigerants require synthetic POE (polyolester) oil for proper lubrication and oil return — mineral oil is not miscible and will cause oil logging and lubrication failure. R-12 and R-22 (CFC/HCFC) systems use mineral oil or alkylbenzene oil. HFCs (R-134a, R-404A, R-410A, R-407C) require POE oil, which mixes with HFCs so the oil is carried around the circuit and returned to the compressor. Mixing mineral oil with HFCs causes the oil to separate out and pool in the evaporator, starving the compressor. POE oil is strongly hygroscopic — it takes up atmospheric moisture quickly and holds it, so containers stay sealed and open time is kept to a minimum.

Liquid refrigerant migration to compressor during off cycle causes liquid slugging on startup — grinding/rattling noise that clears as liquid is pumped out. The migrated liquid is often an oil-refrigerant mixture in the crankcase. Scroll compressors are less tolerant of liquid than reciprocating compressors — the scrolls momentarily separate but can still be damaged. Causes: low ambient temperature with refrigerant migrating to coldest point (compressor), crankcase heater failed/not installed, refrigerant overcharge. Prevention: crankcase heater, pump-down cycle on shutdown, check valve in suction line.

Net oil pressure is the difference the pump develops above the crankcase it is drawing from. The crankcase already sits at suction pressure, so a gauge on the pump outlet reads that suction pressure plus whatever the pump has added. Only the added part does the work of pushing oil through the bearings, so an oil pressure safety control connects one side to the pump outlet and the other to the crankcase and responds to the difference between them. A control watching pump outlet pressure alone would show a comfortable number on a system running high suction pressure while the bearings were being starved. These controls also carry a time delay so the pump can build pressure during start-up, and they lock out rather than cycle, because a compressor that has lost oil pressure must not be restarted until the reason is found.

LPC tripping = suction pressure falling below cut-out. Low refrigerant charge reduces suction pressure below the cut-out setpoint. Restricted liquid line, dirty evaporator coil, or low evaporator load can also cause low suction. The LPC protects the system from operating at pressures where evaporator freeze-up or compressor damage could occur.

HPC tripping repeatedly = condenser cannot reject heat fast enough. On hot days, condenser performance is already marginal. A fouled coil or failed condenser fan limits heat rejection further — discharge pressure climbs until HPC trips. After cool-down, it briefly works then trips again. Check: condenser coil cleanliness, fan operation, fan blade pitch, airflow obstructions.

EEV: electronic precision control based on multiple sensor inputs. The controller reads suction pressure from a transducer and suction line temperature from a sensor, calculates superheat from the two, and drives a stepper motor to reposition the valve — much faster response and a far wider control range than a mechanical TXV, which is why EEVs suit inverter-driven and VRF equipment. The other answers describe things an EEV is not. It is not a pressure-operated valve: without control power and a working controller it does not meter at all. It is not self-contained either — the valve is only one part of a package that includes the controller and its sensors, and the superheat target is a programmed setting that can be checked and adjusted. And it does not park at a fixed opening: superheat feedback is recalculated continuously, which is precisely the advantage being asked about.

A coil that comes out of defrost completely clear has had enough heat; if the cycle still ran to its time limit, the control never saw the coil warm up. Temperature termination is how an electric defrost is meant to end - once the ice is gone the coil metal climbs quickly past the termination setting and the cycle stops, which saves energy and stops the box temperature climbing any further. The 30-minute limit is a fail-safe, not the intended terminator. Here the ice is gone and the drain pan is hot, so heat input and frost load are not the problem; what has failed is the device that is supposed to notice. Either the termination thermostat has failed open or drifted out of calibration, or it is clamped where it never sees warm metal - on a return bend that stays cold, or away from the heater path. Test it for continuity as its temperature rises and confirm it is mounted where the equipment's installation instructions place it. Heaters that were undersized for the coil and its load would leave ice still on the coil when the timer ended, which is exactly what is not found here. An undercharge is a refrigeration-side fault and has no bearing on whether a defrost cycle terminates. And a defrost is not supposed to run a fixed length regardless of need - preventing that is precisely why temperature termination exists.

EPR valve: sets minimum evaporator pressure (warmer setpoint), preventing the evaporator from getting too cold. In multi-temperature systems, different evaporators need different temperatures. The EPR valve limits how cold a warmer evaporator can get by maintaining minimum suction pressure above the lower-temperature coil setpoint. Example: produce case at 35°F uses an EPR; freezer evaporator at -10°F does not.

High-pressure cutout: safety device that shuts down compressor at excessive head pressure. High head pressure can be caused by: dirty/blocked condenser, condenser fan failure, refrigerant overcharge, non-condensables in the system, or high ambient temperature. If head pressure reaches the cutout setpoint (typically 590–650 PSI for R-410A systems), the compressor is de-energized — protecting the compressor, condenser, and refrigerant circuit from over-pressure damage. The manual reset type requires a technician reset, preventing restart until the cause is investigated.

A crankcase pressure regulator throttles the suction line so that pressure at the compressor cannot climb above the valve's setting. The problem it solves appears on low-temperature equipment during a pull-down, or straight after a defrost, when the box is warm and the evaporator is producing far more gas, at far greater density, than the compressor was sized for. Denser suction gas means more mass through the machine, more work done, and motor current well past the compressor's rating. The valve sits in the suction line between evaporator and compressor, senses the pressure on its outlet — the compressor side — and closes down as that pressure rises, holding the compressor at a load it can carry. As the box pulls down the valve opens fully and stops interfering.

Do not confuse it with the valve at the other end of the line. An evaporator pressure regulator sits at an evaporator outlet and holds a MINIMUM pressure, so that a warmer coil in a multi-temperature system cannot run colder than its product needs. Holding head pressure up in cold weather is a condenser-side job, done by fan control or a flooding valve. Preventing off-cycle backflow is what a check valve or a pump-down cycle does.

Economizer: free cooling using outdoor air when conditions allow. An air-side economizer modulates the outdoor air, return and relief dampers so that outdoor air itself carries part or all of the cooling load and the mechanical cooling is unloaded or shut off. The changeover decision compares the outdoor air against the air returning from the space, either on dry-bulb temperature or, where humidity matters, on enthalpy, and against a high-limit setting: outdoor air is only worth bringing in while it is cooler, or lower in enthalpy, than the return air. That is why the savings show up in mild spring and autumn weather and on cool nights. In Canada the requirement comes from the National Energy Code of Canada for Buildings (NECB), Article 5.2.2.7. Cooling with Outdoor Air: a system that incorporates mechanical cooling and has an air-handling capacity of more than 1 500 L/s, or a cooling capacity of more than 20 kW, must be designed to use outdoor air to reduce mechanical cooling energy — by direct use of outdoor air (air economizer, Article 5.2.2.8.) or indirectly (water economizer, Article 5.2.2.9.) — with systems serving only dwelling units or hotel and motel rooms excepted. ASHRAE 90.1 is a United States standard and has no force of its own in Canada; it binds only where a province adopts it as a compliance path, as Ontario's Building Code Supplementary Standard SB-10 has done in offering ANSI/ASHRAE/IES 90.1-2013 with SB-10's own amendments as one of three paths, the NECB path being another. Work to the edition your provincial code adopts.

Note the name collision. The device described here sits on the air side: a damper arrangement that brings outdoor air into the building. A refrigerant economizer circuit is a different thing entirely — an intermediate-pressure circuit inside the refrigeration system that subcools the liquid and injects flash gas into a compressor economizer port.

Suction pressure transducer failed to 0 PSI: controller reads severe low pressure — likely shuts down or alarms. Modern refrigeration controllers use suction pressure for system protection (low pressure cutout), compressor management, and in some systems for EEV control. A transducer stuck at 0 PSI simulates a severe low-pressure or pump-down condition — the controller reacts as if the system is in a low-charge or restricted condition. It will typically trigger a fault code and shut down the compressor to "protect" from a non-existent problem.

Low-pressure control: stops compressor when suction pressure drops below setpoint. Protects against: 1) Low refrigerant charge (leak), 2) Loss of evaporator load (frozen/iced evaporator, closed damper), 3) Blocked metering device. Low suction pressure: compressor pumps vapor at very low density → reduced mass flow → overheating → possible valve damage. It also keeps the low side out of a vacuum: below atmospheric pressure, air and moisture are drawn in through any leak or loose joint, and that moisture and air lead to acid and sludge. Protecting against excessive discharge pressure is the job of the high-pressure control, and no pressure safety switch measures the refrigerant charge or limits liquid line pressure. Auto-reset allows restart; manual-reset requires technician intervention.

Defrost timer: initiates defrost at scheduled intervals to remove frost from evaporator coils, restoring airflow and heat transfer efficiency. Frost buildup on evaporator coils insulates the surface and restricts airflow, reducing refrigeration capacity. Defrost methods: electric resistance heaters, hot gas defrost, or reverse cycle (heat pump). The timer starts defrost at set times (usually early morning when load is low). Defrost terminates on temperature or on time. Temperature termination is the better of the two: a termination thermostat or sensor on the coil ends the cycle once the coil has climbed clear of freezing to its set termination temperature, which proves the ice is gone and stops the heaters before they needlessly warm the box. The termination setting is chosen for the application and the equipment, so it is read off the control or the equipment documentation rather than assumed. Time termination is only a backstop. Adaptive/demand defrost systems use sensors to run defrost only when needed.

The delay exists so the high side and the low side have time to equalize before the motor is asked to turn again. When a compressor stops, discharge pressure is still standing on top of the pistons or the scrolls. The motor develops only modest starting torque, and if it is switched back on before that pressure has bled off through the metering device it stalls at locked rotor, draws locked-rotor current and trips its overload — and does it again on the next attempt, until something is damaged. A fixed orifice equalizes in a couple of minutes; a thermostatic expansion valve holds pressure much longer, which is why those systems often carry a hard start kit as well. The timer also caps the number of starts per hour, and starts are where motor heating, contact arcing and mechanical wear are concentrated.

What the timer does not do is remove the reason the machine is cycling so quickly. Short cycling is itself a symptom: a control differential set too tight, a low charge cycling the low-pressure control, poor heat rejection cycling the high-pressure control, or equipment oversized for its load. As for the alternatives — oil does not drain out of a crankcase during an off cycle; the opposite happens, as refrigerant migrates into it. A few minutes without cooling does not frost a coil. And while contactor contacts do suffer from frequent starts, it is the contacts rather than the coil that pay, and protecting them is not what this timer is for.

Pitted, burned contacts are arc erosion: the damage is done in the instants when the contacts make and break under load, and on a compressor circuit the routine high-energy event is the starting inrush. A compressor motor draws many times its running current at the moment it starts, before the rotor is up to speed. Every make against that current, every bounce of the contacts as they close, and every break under load draws an arc across the contact faces, and each arc carries a little metal away, so the faces roughen, pit and discolour. Short cycling multiplies the number of those arcs, which is why a contactor that keeps burning up is usually telling you the machine is starting far too often. Fitting another contactor without finding out why it is cycling, whether that is a control differential set too tight, a low charge cycling the low-pressure control, poor heat rejection cycling the high-pressure control, or equipment oversized for its load, only buys time.

Voltage belongs on the check list, and it is a real second cause rather than a distraction. A coil that cannot pull the armature fully in, because the control voltage is low, lets the contactor chatter, and a chattering contactor makes and breaks load current over and over and burns its contacts doing it. On packaged equipment the control transformer is fed from the line, so a low supply voltage lowers the coil voltage with it; measure both at the moment of a start, not with the machine idle. But the question asks for the most likely cause, and inrush arcing is present on every start of every compressor, while a chattering coil is a specific fault that also announces itself with noise. The three alternatives each rest on a misunderstanding. Steady running current in closed contacts does not pit them: a clean, fully seated contact carrying current within its rating runs cool, and the contact that overheats while closed is one that has already been pitted and now has a high-resistance face, so heating is a consequence of the damage rather than its cause. Coil voltage sits in its own circuit, isolated from the load contacts, so raising it overheats or burns the coil and does not increase the current the contacts carry. And a contactor with more contact mass than the duty demands interrupts the inrush more easily, so oversizing reduces contact erosion rather than causing it.

Failure signatures worth knowing: welded contacts mean the compressor will not shut off, open contacts mean no start, and high-resistance contacts show up as voltage drop across a closed pole under load and local overheating.

NTC (Negative Temperature Coefficient) thermistor: resistance decreases as temperature increases. Common in HVAC/R for temperature sensing. Diagnosis: 1) Disconnect sensor from controller. 2) Measure resistance with ohmmeter at known temperature (e.g., room temp ~25°C). 3) Compare to manufacturer resistance-temperature table (e.g., 10kΩ at 25°C is common). 4) Check for open circuit (infinite resistance = failed sensor or broken wire) or short circuit (near-zero resistance = failed sensor or shorted wiring). Also check connector for corrosion.

VFD (Variable Frequency Drive): adjusts motor speed by varying AC frequency and voltage proportionally (V/Hz control). In HVAC/R: used for condenser fans (high-head pressure control), evaporator fans, and compressors. Installation mistake: using a standard induction motor (not VFD-rated) with long cable runs. PWM (Pulse Width Modulation) output creates voltage spikes. Long cables amplify these spikes (voltage reflection/standing wave effect), which can break down motor winding insulation. Fix: use inverter-duty motors (enhanced insulation), add output reactor or dV/dt filter.

A phase monitor relay watches the three-phase supply and drops the control circuit when that supply is not fit to run a motor: a lost phase, two phases swapped, or phases too far out of balance. Phase loss is the worst of the three. A three-phase motor needs all three phases to make a rotating field, so with one phase open it cannot start from rest at all: it sits and hums and draws locked-rotor current through the two live phases. If it was already running when the phase opened it keeps turning on a pulsating single-phase field at greatly reduced capability, while current in the two remaining phases climbs and the windings heat quickly. Single-phasing is the leading cause of burnout on three-phase motors.

Phase reversal turns the motor backwards. Refrigerant flow does not reverse, it stops. A scroll compressor compresses in one direction only, so a scroll run backwards is very loud, delivers no cooling and is damaged within minutes; a screw compressor is direction-sensitive in the same way. A reciprocating compressor is far less sensitive, because its pistons reciprocate whichever way the crankshaft turns, but anything else fed from the same supply, condenser fans and pumps included, also runs backwards and loses its output. Phase imbalance is the quiet one: unequal phase voltages produce a considerably larger imbalance in the currents, and that extra current shows up as winding heat and shortened motor life.

The relay is a protective device, not a power supply. It cannot make a third phase out of a single-phase service, it does not redistribute load between the phases to save energy, and it senses nothing on the refrigerant side, so it has no way to know whether a system is undercharged. Phase protection is commonly specified on three-phase commercial equipment, and manufacturers normally call for it on direction-sensitive scroll and screw compressors precisely because reversed rotation ruins them. Confirm rotation after any supply work upstream of the unit.

Floating head pressure: condenser fan speed or staging is controlled to allow condensing pressure to drop to the lowest stable level based on ambient conditions — improving system efficiency. Traditional systems maintain high fixed head pressure year-round. Floating head control reduces fans (or varies their speed) to allow the head pressure to drop as ambient drops in winter, reducing compressor work (lower pressure ratio = less energy per unit of refrigeration). The float is not unlimited, though: every floating head strategy is given a floor, because the metering device still has to be left with enough pressure difference across it to feed the evaporator, and refrigerant velocity has to stay high enough to carry oil back. The floor is worked out from the system design data for that refrigerant and evaporator temperature, and below it the same head pressure control described elsewhere in this bank takes over and holds condensing pressure up.

In cold weather the air passing over the condenser is colder, the condenser rejects heat too easily, and condensing (head) pressure falls. A thermostatic expansion valve is driven by the pressure difference across it, so it needs a minimum condensing pressure to push its rated flow. When head pressure sags, flow through the valve drops, the evaporator is starved, suction pressure falls below the low-pressure cut-out, and the compressor trips — even though the box is still holding setpoint, because the trips are short and the load is light in winter. The remedy is a head pressure control that holds condensing pressure up as ambient falls: condenser fan cycling on pressure or temperature, a fan speed controller, or a flooding valve that backs liquid into the condenser to reduce its active surface. The minimum condensing pressure it is set to hold is whatever keeps enough differential across the metering device for that refrigerant and that evaporator temperature, so it is taken from the system design data rather than from a rule of thumb. Lowering the low-pressure cut-out only hides the symptom: the evaporator stays starved, refrigerant velocity stays too low to return oil, and the compressor runs short of both cooling and lubrication. A restricted drier is a year-round restriction, not a seasonal one, and it shows up as a temperature drop across the drier with high superheat; an overcharge raises head pressure rather than lowering it.

A water regulating valve is a head pressure control that happens to act on water. Its power element senses compressor discharge pressure: as head pressure rises the valve opens further, as head pressure falls it closes down, so the condenser is given only as much water as the load actually calls for. Two useful things follow. Head pressure is held steady enough for the metering device to keep feeding the evaporator even when the supply water is very cold — the same problem a low-ambient control solves on an air-cooled machine. And because the valve is spring-closed and pressure-opened, it shuts when the compressor stops and the pressure falls away, so water is not run through an idle condenser all night.

Set it too low and the metering device loses the pressure difference it needs and the coil starves; set it too high and water is wasted while the compression ratio stays wider than it needs to be. Note what the valve does not sense: it responds to refrigerant pressure, not to water temperature, so it neither holds the leaving water temperature constant nor trips on high water temperature. Nor is it a minimum-flow device — closing off completely at shutdown is one of the things it is there to do.

Demand defrost: defrosts only when needed (based on measured frost accumulation) vs fixed-time schedule. Time-initiated/time-terminated defrost runs defrosts at fixed intervals (e.g., every 6 hours) regardless of frost buildup — wastes energy when little frost is present, and may not defrost often enough under heavy conditions. Demand defrost senses frost via air pressure differential across the coil, a capacitance sensor, or temperature-rise sensing, and initiates defrost only when buildup is sufficient. Benefits: fewer unnecessary defrosts and energy savings (up to 30% reduction in defrost energy), improved product quality (less product temperature cycling), longer equipment life.

Single-phasing: one phase open at the contactor, the fusing or the motor leads, so the motor cannot generate enough torque to accelerate, draws locked-rotor current, and trips the overload. A three-phase induction motor needs all three phases to produce a rotating magnetic field. With one phase open, what is left is a single-phase pulsating field — the motor will keep turning if it was already running, at greatly reduced capacity, but it cannot start from standstill; it hums and sits there. Meanwhile it draws locked-rotor current (5-7 times FLA) through the two live phases and heats the windings fast, which is what the overload is reacting to. Causes: one blown fuse of three, a burned or open contactor pole, a loose or broken motor lead, or an open in the supply.

Condenser split is the condensing saturation temperature minus the dry-bulb temperature of the air entering the condenser. An air-cooled condenser can only push its heat into the air it is given, so the entering air is the reference the condensing temperature has to be judged against. A condensing temperature on its own means nothing until you know whether the unit is sitting in cool morning air or in full afternoon sun. A split much wider than the unit was designed for says the coil cannot move its heat: fouled fins, a failed fan or one turning the wrong way, discharge air recirculating around the unit, blocked airflow, or non-condensables occupying condenser volume. A split narrower than design, on a unit that is cooling poorly, points the other way and suggests there is not enough refrigerant in the coil to load it. Note what the other comparisons actually give you: condensing saturation temperature against liquid line temperature is subcooling, a different measurement answering a different question, and wet-bulb belongs to evaporative equipment rather than a dry air-cooled coil.

Low suction pressure with high superheat and frost only at the coil inlet means the evaporator is being underfed with refrigerant. A restricted or underperforming expansion valve meters too little liquid into the coil. The small amount that does get through flashes and boils immediately at the inlet, frosting that one spot, while the rest of the coil face runs dry and warm. Because the refrigerant is fully boiled off long before the outlet, the vapour keeps picking up heat all the way to the compressor, which is why superheat is high and the suction line is warm and dry rather than cold and wet. Check the valve for a restriction, confirm the sensing bulb is tight, insulated and on a clean horizontal run, and verify the bulb still holds its charge before condemning anything. A dirty air filter also ices an evaporator, but it starves the coil of air rather than of refrigerant, so the ice spreads across the whole face and superheat falls instead of rising. A dirty condenser coil and an overcharge both drive head pressure up; neither produces low suction pressure with a warm suction line.

A motor that hums at locked-rotor current has power and continuity but no starting torque, so look at the components that produce that torque. Voltage confirmed at the compressor terminals means the supply, disconnect and contactor have all done their job, which puts the fault in the motor or in what starts it. On a capacitor-start hermetic the starting torque comes from the start winding together with the start capacitor and the start or potential relay that switches it in and back out again; a PSC compressor has no start capacitor and no start relay at all, and on that machine the equivalent check is the run capacitor. A failed start capacitor, a relay whose contacts have burned open, or an open start winding leaves the rotor standing still while the run winding draws locked-rotor amps until the overload opens. Those parts are external, inexpensive and a common failure, so test them before condemning the compressor. Keep in mind that a mechanically seized compressor produces the same symptom, so if the start components test good the compressor itself becomes the suspect. Discharge any capacitor through a proper resistor before handling it.

Voltage only tells the truth under load, and a drop measured between two points is made in the length of circuit between them. Standing still, a corroded lug, a loose terminal or a burnt contactor contact carries no current and drops no voltage, so everything measures perfectly. Ask the compressor to start and it draws locked-rotor current for a moment; that current through a high-resistance joint produces a large voltage drop, and what reaches the motor is no longer enough to develop starting torque. The rotor stays put, locked-rotor current continues, and the overload does its job. Because the two readings were taken at the same instant, they localize the fault: full voltage at the disconnect with low voltage at the terminals puts the loss between them - pitted contactor contacts, a loose or corroded connection, an undersized or badly spliced conductor. Look for discolouration and heat damage, and measure the drop across each connection and across the closed contactor while the machine is trying to start.

If the supply itself were weak, the disconnect reading would sag along with the terminal reading instead of holding at 240 V. A failed start capacitor leaves full voltage standing at the terminals while the motor hums, so it puts no drop between the two measuring points. A seized compressor also draws locked-rotor current, but through a sound supply path the terminal voltage would stay close to the disconnect reading rather than falling 50 volts below it.

Evacuation is judged by a micron reading that holds, not by gauge position or pump run time. The point of evacuation is to remove air, non-condensables and water vapour. Water only boils out of the system once the absolute pressure is driven well below the vapour pressure of water at the system temperature, and the usual industry target is 500 microns or lower before charging. Reaching that number once is not enough: the pump is valved off and the reading watched. A stable reading means the system is dry and tight; a reading that climbs and levels off means moisture is still boiling; a reading that keeps climbing means a leak.

The three wrong answers all share one flaw — none of them can actually resolve a deep vacuum. A manifold compound gauge is graduated in inches of mercury, and its whole scale from 29 inches to a perfect vacuum covers everything below about 25,000 microns, so it cannot tell 25,000 microns from 500. Zero psi on the low-side gauge is simply atmospheric pressure, which is where evacuation starts, not where it ends. And run time proves nothing on its own: a large system, a restricted hose, a wet system or a tired pump can all still be far above target after 30 minutes, while a small dry system may be there much sooner.

Frost on the outdoor coil is normal in heating — in that mode the outdoor coil is the evaporator and it runs below freezing and below the dew point of the outdoor air. What is abnormal is that the control never acted on it. A heat pump permits defrost either on time and temperature (a timer that runs only while the compressor runs, gated by an outdoor coil sensor showing the coil is cold enough to be frosting) or on demand, from the spread between coil temperature and outdoor ambient or from the pressure drop across the coil. If that coil sensor is open, shorted, drifted or clamped where it does not read coil temperature, the board never sees the condition that allows a defrost, and the machine ices itself solid while every other reading simply follows from the blocked coil. Check the sensor's resistance against the manufacturer's resistance-temperature table, check where and how it is clamped, and force a defrost at the board to prove the reversing valve shifts and the outdoor fan stops.

Low suction pressure and lost capacity here are consequences of an ice-blocked evaporator, not evidence of an undercharge; the charge cannot be judged at all until the coil has been thawed and the machine run clean. A reversing valve stuck part-way would show poor capacity in both modes with the pressures converging, not a defrost that never begins. Staging the auxiliary heat would hide the complaint while the outdoor coil stayed frozen.

Which coil is rejecting heat tells you which way the valve is set. In heating, the reversing valve sends discharge gas to the indoor coil, so the indoor coil is the condenser (hot, warm supply air) and the outdoor coil is the evaporator (cold, often frosted). If the valve is stuck in the cooling position during a call for heat, those roles are swapped: the outdoor coil runs warm because it is still acting as the condenser, and the indoor coil is boiling refrigerant, so the supply air comes off cool. That reversal is the confirmation.

A frosted outdoor coil with warm supply air is what correct heating operation looks like, not a fault. Discharge pressure being higher than suction pressure is true of every running system in either mode, so it distinguishes nothing. Continuity through the solenoid coil only proves the winding is not open — the coil can read fine while the valve body is mechanically stuck or while the coil is never energized by the control circuit, so it must be backed up by measuring voltage at the coil during the heat call and by feeling the valve body tubes.

Condensing temperature is the saturation temperature at the discharge pressure = 110°F. Superheat = suction line temperature − saturation temperature at the suction pressure = 58°F − 43°F = 15°F. Superheat is measured against the saturation temperature at the coil, so subtracting the coil saturation temperature from the 75°F return air is not superheat, and the 58°F suction line reading on its own is not superheat either. The saturation temperature at the suction pressure is the evaporating temperature, not the condensing temperature. Judge the condensing temperature against the measured outdoor ambient rather than against a fixed number — it normally sits roughly 20–30°F above ambient, so 110°F is ordinary on a warm day. A thermostatic expansion valve system is normally set for 8–12°F of suction superheat; a fixed-orifice system runs higher and swings with load and ambient.

Evaporator icing with good airflow points to a low refrigerant charge or a restricted metering device. Normal evaporator temperature is 35–45°F (2–7°C). If suction pressure drops too low — undercharge, or a restricted or stuck-closed TXV — the evaporator falls below 32°F (0°C) and moisture in the air freezes on the coil. First let the ice thaw completely, because airflow will not penetrate ice. Then take gauge and superheat readings. Low suction with high superheat means a starved evaporator: undercharge, a restricted or stuck-closed metering device, or a liquid line restriction. Subcooling separates those — an undercharge reads low subcooling, while a liquid line or drier restriction reads high subcooling with a temperature drop across the restriction. Low suction with low superheat points the other way, to low airflow or low load. Worn compressor valves raise suction pressure rather than lowering it, so they do not ice a coil.

Suction superheat of 30°F against an 8–12°F target means a starved evaporator. When refrigerant fully vaporizes early in the coil, the remaining coil surface superheats vapour instead of absorbing heat from the space, so evaporator capacity is wasted. On a thermostatic expansion valve system, a sustained reading above roughly 15°F is worth investigating; at 30°F the coil is badly starved. Investigate refrigerant charge (undercharge), a restricted or stuck-closed expansion valve, and liquid line restrictions. An overcharge does the opposite — it floods the coil and drives superheat down — and high subcooling on its own does not raise suction superheat.

Resistance between terminals proves the windings are continuous; only an insulation test says whether they are still isolated from the shell. Common-to-start and common-to-run readings that sit in spec confirm the copper is intact and the windings are balanced, which is exactly why the meter looks reassuring on a motor that is finished. An insulation resistance tester applies a high test voltage between the windings and the earthed shell, and a very low reading means the insulation has broken down and current is leaking to ground. That is a grounded motor: it cannot be repaired in the field, and the compressor is replaced. Treat it as a burnout rather than a simple swap, because the same event that destroyed the insulation loads the circuit with acid and carbon. Check the oil for acid, clean the circuit, fit the appropriate driers and evacuate properly, or the replacement will fail the same way. For contrast, an open internal overload reads as an open circuit between terminals rather than as a path to ground, and a hermetic motor is never intended to conduct through its shell. Isolate, lock out and confirm the circuit is dead before applying an insulation tester, and discharge the windings afterwards.

An RTU that will not hold setpoint while its refrigerant pressures read normal is telling you to look beyond the refrigerant circuit. Normal pressures mean the refrigerant system is working; it simply cannot overcome the load it is being handed. Investigate: an economizer damper stuck open or a fresh-air damper out of adjustment, either of which drags outdoor heat into the return; a dirty condenser coil; a dirty evaporator coil restricting airflow; duct leakage; missing supply duct insulation; return air bypassing the space; or a building load beyond the equipment's capacity on an extreme heat day. Normal pressures on both circuits also rule out a low-pressure-control cutout, and adding refrigerant to a system that already reads normal only overcharges it.

Evaporator icing: most commonly caused by insufficient airflow across the coil (dirty filter, blocked return, or failed fan). Normal operation: evaporator coil surface stays above 32°F (0°C) as warm air flows across it, continuously melting any frost. Restricted airflow (clogged filter, failed blower, blocked return): coil surface temperature drops below freezing, moisture freezes faster than it can drain. Ice eventually insulates the coil completely, stopping heat transfer. Also can be caused by low refrigerant charge (low suction pressure = low coil temp). An overcharge does the opposite — it raises suction pressure and coil temperature, so it floods the compressor rather than icing the coil. A weak run capacitor does not slow the compressor either: a single-phase induction motor runs at essentially fixed speed, and a weak capacitor shows up as hard starting, high amp draw, or overload trips — and any loss of compressor capacity leaves the coil warmer, not frozen. Start diagnosis with: check filter, check blower operation.

Indoor running but outdoor unit not starting: check power supply to the outdoor unit first. The indoor blower fan and outdoor compressor/condenser fan run on separate circuits. A tripped circuit breaker, open disconnect switch, or blown fuse at the outdoor unit disconnect kills the outdoor circuit while leaving indoor running. After confirming power: check the contactor (coil energized? contacts closed?), capacitor (start/run capacitors fail frequently — multimeter capacitance test), and check for safety control lockout (high/low pressure switch).

Superheat is heat added to vapour beyond saturated vapour, so on a blend with glide it is measured from the dew temperature: 32°F - 21.9°F, about 10°F. R-407C is a zeotropic blend: at one pressure it starts boiling at the bubble point and finishes at the dew point, and on this chart the two are 11.5°F apart at 40 psig. The refrigerant is only fully vapour once it reaches the dew temperature, so that is the reference for superheat. Subtracting the bubble value gives about 22°F, overstating superheat by the whole glide; a technician who believes that reading may diagnose a starved coil or low charge and add refrigerant to a system that does not need it. The average value still counts part of the boiling range as superheat, giving about 16°F. The bubble column is the right one for subcooling at the condenser outlet, but the liquid line temperature has nothing to do with suction superheat.

Low suction with high superheat means the evaporator is not getting enough refrigerant — a capacity problem, not a load problem. Read the two numbers together. High superheat says the liquid is boiling off early and the last part of the coil is doing nothing but warming vapour, so the coil is being underfed. Low suction pressure says the compressor is pulling on a coil that cannot supply it. Underfeeding comes from too little refrigerant in the system or from something in the way of it: an undercharge or a leak, a restricted filter drier or liquid line, a plugged metering device screen, or a TXV that has lost its bulb charge. The compressor runs on and on because the reduced capacity barely matches the load.

This is the mirror image of a load problem. When the box is simply asked to remove more heat than the equipment is sized for — bad gaskets, constant door traffic, warm product loaded in — the coil is fully fed, so suction pressure sits normal or high and superheat measures normal; the compressor runs continuously because the load never lets it catch up, and nothing about the refrigerant circuit is faulty. An overcharge does the opposite of what is measured here: it floods the coil and drives superheat down, with high subcooling and high head pressure. And a thermostat that could not sense setpoint would drive the box below setpoint, not leave it struggling to reach it.

A restriction in the liquid line starves the coil in exactly the way an undercharge does, and subcooling is what separates the two. Low suction pressure with high superheat says one thing only: the evaporator is not getting enough refrigerant. Both faults produce it, so look upstream to tell them apart. An undercharge leaves too little liquid in the condenser, so subcooling reads LOW. A restriction leaves the compressor backing liquid up behind it, so subcooling reads HIGH while the coil downstream goes hungry.

The temperature drop across the drier is the direct evidence. Refrigerant is expanding through it as though it were a second metering device, losing pressure and therefore losing saturation temperature, which is why the outlet is cold enough to sweat — on a badly plugged drier, cold enough to frost — while the inlet stays warm. Recover, replace the drier, and treat the restriction as evidence rather than simply as a failed part: a drier that plugs and then plugs again says the system is dirty, so look for moisture, acid or burnout residue and check the metering device screen while you are in there. The remaining answers do not fit the readings. An overcharge does raise subcooling, but it raises suction pressure and lowers superheat rather than starving the coil, and it puts no temperature drop across the drier. A valve stuck wide open floods the coil, giving low superheat with high suction pressure — the opposite of what is measured.

After a compressor burnout, acid, carbon, and moisture are spread through the whole system and all of it must be removed before the replacement compressor runs. Acid formed by the thermal breakdown of oil and refrigerant attacks the winding insulation and plates copper onto bearing and valve surfaces, so a system left dirty destroys the new compressor. Procedure: 1) Recover the refrigerant. 2) Clean the lines and coils — flush with an approved flushing agent or blow through with dry nitrogen to carry out acid and carbon. 3) Fit an oversized burnout filter drier in the liquid line and a suction line drier ahead of the compressor. 4) Install the new compressor and charge it with the oil specified for that machine. 5) Triple evacuate — pull a vacuum, break it with dry nitrogen, repeat, then pull the final vacuum and prove it with a decay test, watching that the reading holds steady instead of creeping back up. 6) Charge and run the system, then acid-test the oil after the first day or two of operation; if acid or discolouration remains, change the drier cores and retest, and check the system again about two weeks later.

Oil logging: oil circulates out of the compressor but cannot return — inadequate oil circulation lets it accumulate in low points and the evaporator. Refrigerant velocity carries oil through the system. If suction line velocity is too low (system running at partial load, oversized or incorrectly sized/routed piping), oil drops out of the refrigerant stream and pools. Risers need oil traps (P-traps) sized to maintain velocity even at minimum load. Wrong oil type/viscosity: too thick to flow at evaporator temperature. Low refrigerant charge also reduces velocity. Correction: verify line sizing and re-pipe as needed, add traps/risers, correct refrigerant velocity, verify oil specification and compatibility.

Repeated compressor failure: symptom of an unresolved underlying problem — requires systematic root cause analysis. Audit: 1) Electrical: voltage quality (sags/harmonics), single-phasing, phase imbalance, improper motor protection settings. 2) Refrigerant circuit: operating pressures vs design (high head/low suction = high compression ratio), superheat/subcooling, liquid slugging (low superheat, flooded start), contamination and oil acid levels (acid, moisture). 3) Lubrication: wrong oil, oil logging, oil returning contaminated. 4) Operating conditions: high ambient, overloaded system, defrost effectiveness. 5) Installation: improper piping, incorrect electrical protection. Review operating logs for patterns, and inspect each failed compressor (failure mode determines root cause).

Normal readings with the box at setpoint point at the load, not at the equipment. The refrigerant circuit is telling you it is healthy: normal suction and discharge pressures with normal superheat and subcooling mean the coil is properly fed and the compressor is doing its rated work. A machine in that condition that still cannot get ahead of the box is simply being asked to remove more heat than it was sized for, so it runs continuously and just holds the line. Look at load sources: door gaskets and door alignment, door traffic, warm product loaded in, an evaporator fan out, a condensing unit sitting in a hot corner, or equipment undersized for how the box is now used.

Low charge is ruled out by the readings, not by the running time — an undercharged system shows low suction with high superheat and low subcooling, and this one does not. A thermostat failed closed or a welded contactor would keep the compressor running past setpoint and pull the box down below it, which is exactly what is not happening here; if the space had drifted below setpoint, those would be the first things to check. Continuous running with normal readings at setpoint is an operational finding, not a mechanical fault.

Blocked condensate drain (algae, mold, debris) is the most common cause of indoor water leakage — water backs up, the pan overflows, and it spills from the unit. The condensate drain pan collects water that drips off the evaporator coil. A blocked drain causes the pan to fill and overflow. Algae growth in the drain line (common in humid environments) is the most common blockage cause. Additional causes: incorrect drain pan slope (should slope toward drain), cracked/leaking drain pan, drain line installed without adequate slope, condensate pump failure.

Non-condensable gases give high discharge pressure, near-normal suction pressure, and inflated apparent subcooling. Air or nitrogen collects in the condenser and adds its own partial pressure to the refrigerant's, so the total pressure the gauge reads is higher than the refrigerant's temperature alone would produce. Because subcooling is calculated as (P-T saturation temperature at the measured discharge pressure) minus (liquid line temperature), that inflated pressure makes the calculated subcooling look larger than it really is. Diagnosis: read the tube surface temperature part-way along the actively condensing section of the coil and compare it with the P-T chart value for the measured discharge pressure — several degrees low means non-condensables are present. An overcharge also raises head pressure and subcooling, but on an overcharged system the condensing temperature still agrees with the chart, which is what separates the two.