All 116 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.

Evaporator = heat absorption. The low-pressure liquid refrigerant enters the evaporator and boils (evaporates) by absorbing heat from the surrounding air or product. This is how cooling occurs — heat moves from the warm space into the refrigerant.

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.

Superheat = vapour temperature above saturation temperature at that pressure, measured at the evaporator outlet. At the evaporator outlet, refrigerant should be fully vaporized with some additional heat (superheat). Measured superheat = actual suction temperature − saturation temperature at suction pressure. Proper superheat (8–12°F typical) ensures no liquid enters the compressor.

Subcooling = liquid temperature below its condensing (saturation) temperature at high-side pressure. Measured at liquid line = saturation temperature (at high-side pressure) − actual liquid line temperature. Typically 10–15°F. Ensures solid liquid (no flash gas) reaches the metering device — improves system efficiency. More subcooling = better capacity.

High discharge, normal suction = condenser problem. The condenser can't reject heat fast enough, causing discharge pressure to rise. Causes: fouled/dirty coil, failed condenser fan, high ambient temperature, air in system (non-condensable gas). Undercharge typically causes LOW discharge AND low suction. TXV issue = high superheat, normal/low discharge.

Non-condensables cause high discharge pressure and elevated condensing temperature. Air and other non-condensables collect in the condenser top (lighter than refrigerant). They cannot be condensed, so they occupy condenser space and capacity, raising discharge pressure and reducing efficiency. Identify: measure condensing temperature from pressure-temperature chart — if actual temperature is lower than chart value for that pressure, non-condensables are present.

Suction pressure in vacuum = severely low evaporator pressure. A running system should never pull into vacuum under normal operation. This indicates: 1) Very low refrigerant charge (barely any gas), 2) Completely blocked TXV or liquid line (nothing getting through), 3) The compressor is too large for the load. An extremely cold evaporator setpoint can also pull suction toward vacuum. Pull into vacuum risks drawing air/moisture through shaft seals.

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/temp), 3) subcooling (liquid cools further below saturation). Subcooling ensures that vapour bubbles do not form in the liquid line before the metering device. Normal subcooling: 8–15°F (4–8°C).

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.

Residual nitrogen = non-condensable gas — raises head pressure and reduces efficiency. Non-condensable gases (nitrogen, air) accumulate in the condenser, reducing the condenser surface area available for refrigerant condensation, reducing heat transfer, and raising head pressure — cutting system efficiency. Symptoms: higher-than-normal head pressure for ambient conditions, high subcooling (liquid refrigerant backed up behind non-condensables in condenser), shorter cycles. Always evacuate to the required vacuum after pressure testing before charging.

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.

Refrigerant leaving the condenser: high-pressure subcooled liquid. In the condenser, refrigerant changes state from superheated vapor to saturated liquid, then cools further below the saturation temperature (subcooling). Subcooling ensures no flash gas enters the metering device, improving efficiency and capacity. Typical subcooling: 10–15°F (5–8°C). Measuring subcooling: condenser outlet temperature vs saturation temperature at condenser pressure.

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.

Low suction pressure + high superheat + low discharge = undercharge (refrigerant shortage). With insufficient refrigerant, less mass flows through the system. Suction pressure drops because less refrigerant evaporates. The evaporator is not fully fed — superheat is high (starved). Discharge pressure is low or normal because there is less refrigerant to condense. Compressor runs hot from pumping hot vapor with little cooling mass. Diagnose by finding and repairing leak, then recharging to manufacturer specifications.

Non-condensable gases (air, nitrogen) in a refrigeration system: raise head pressure, reduce efficiency and capacity. Unlike refrigerant, these gases cannot condense in the condenser. They occupy condenser volume and partially block heat transfer, raising condensing pressure and temperature. Signs: discharge pressure higher than expected for outdoor temperature, condenser outlet warm, discharge/suction pressure ratio too high. Cause: improper evacuation, system opened without recovering refrigerant, leak on suction side pulling in air.

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.

High discharge temperature with normal head pressure = excessive suction superheat entering the compressor. Discharge temperature is a function of both compression ratio and suction gas temperature. With normal head pressure (ruling out high compression ratio), the cause must be the refrigerant absorbing excessive heat before reaching the compressor — compressing hot gas produces hotter discharge gas. Causes: refrigerant superheated excessively in the suction line (long uninsulated suction line in hot space — excessive suction line heat gain), or TXV under-feeding/starving (low charge but high suction temperature at compressor due to long warm suction run).

Subcooling = cooling liquid refrigerant below saturation temperature in condenser. Benefits: prevents flash gas at expansion device, ensures full liquid delivery to the TXV/EEV, maximizes cooling capacity and efficiency. If liquid refrigerant arrives at the TXV still at saturation temperature, any pressure drop in the liquid line causes flash gas (vapour). Flash gas reduces TXV efficiency and capacity. Subcooling provides a temperature margin — the liquid can absorb heat (from liquid line) or lose pressure without flashing. Typical subcooling: 5–10°C. More subcooling = more efficient but may indicate overcharge.

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).

Venting any refrigerant is illegal in Canada. Under the Environmental Protection Act and related regulations, all refrigerants (CFCs, HCFCs, HFCs) must be recovered before servicing, using approved recovery equipment. No exceptions based on system size. Use certified recovery equipment.

R-410A at a 35°C condensing temperature ≈ 290 PSI high side (option B, 280, is closest). R-410A operates at roughly 1.5× the pressure of R-22: at 35°C its saturation pressure is about 290 PSI versus about 195 PSI for R-22. (On a hot day the measured head pressure runs higher still, because the condensing temperature sits well above ambient.) This is why R-410A systems require components, hoses, and gauges rated for higher pressure.

R-22 phased out for new equipment since 2010. R-22 (HCFC) depletes the ozone layer and has moderate GWP — its phase-out was driven by ozone depletion under the Montreal Protocol, not by GWP. Canada phased it out of new equipment in 2010. It can still be used in existing equipment (using recovered/recycled R-22) but new production is banned. Current transition is also moving away from R-404A and R-410A (high GWP HFCs) to HFOs and natural refrigerants.

All refrigerant recovery requires certified equipment and approved DOT-rated cylinders. Releasing refrigerants to atmosphere is illegal under Canadian Environmental Protection Act (CEPA). Recovery machines pull refrigerant from the system and compress it into DOT-rated recovery cylinders. Never mix refrigerants. Recovered refrigerant must be reclaimed or properly disposed.

Zeotropic blend: temperature glide during phase change. R-410A is actually a near-azeotropic blend (very small glide). True zeotropic blends (like R-404A) have significant temperature glide — the components boil/condense at different temperatures, changing the composition as phase change occurs. Always charge zeotropic blends from the liquid phase of the cylinder.

A2L: lower toxicity (A) + lower flammability (2L = mildly flammable, very low burning velocity). ASHRAE 34 classification: first letter = toxicity (A=lower, B=higher), number = flammability (1=non-flam, 2L=mildly flam, 2=flam, 3=highly flam). R-32 burns slowly and requires specific conditions. Requires A2L-rated equipment (no open flames, proper ventilation, A2L compatible recovery equipment).

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.

R-410A zeotropic blend: always charge liquid-phase to prevent fractionation. R-410A is a near-azeotropic blend of R-32 and R-125. If allowed to leak or flash off as vapour, the components leave at different rates, altering the remaining blend composition. Charging liquid ensures the full blend ratio enters the system. Use a liquid valve (inverted cylinder) or ball valve to ensure liquid charging.

R-600a (isobutane): highly flammable A3 refrigerant — flammability is the primary hazard. Hydrocarbon refrigerants (R-600a, R-290/propane) have excellent thermodynamic properties but are flammable. Used in small-charge domestic appliances where the charge is small enough to be below flammability limits in room air. Servicing: ventilated areas only, no open flames or sparks, non-sparking tools. Recovery equipment must be rated for flammable gases. These fridges must not be serviced in enclosed spaces.

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 regulations under the Environmental Protection Act (aligned with the Kigali Amendment to the Montreal Protocol) require progressive reduction in high-GWP HFC use. Technicians must understand why refrigerants are transitioning to low-GWP alternatives (R-454B, R-32, R-1234yf, CO₂).

GWP (Global Warming Potential): relative greenhouse effect of a refrigerant compared to CO2 over 100 years (CO2 = GWP 1). R-134a has GWP of ~1,430 — meaning 1 kg released has the same warming effect as 1,430 kg of CO2. R-410A GWP ~2,088. Low-GWP replacements: R-32 (GWP 675), R-454B (GWP 466), R-1234yf (GWP 4). Regulatory trend: phase-down of high-GWP refrigerants (HFCs) under Montreal Protocol Kigali Amendment, Canadian regulations.

ASHRAE safety class: letter = toxicity, number = flammability. A1 = safest class. A = low chronic toxicity; B = higher chronic toxicity. 1 = no flame propagation; 2L = lower flammability, slower burning speed; 2 = flammable; 3 = highly flammable. Common: R-410A = A1 (safe), R-22 = A1, R-32 = A2L (mildly flammable), R-1234yf = A2L, R-290 propane = A3 (highly flammable). A2L refrigerants require special handling and equipment design.

R-410A operates at ~70% higher pressures than R-22 — requires dedicated high-pressure equipment. R-22 operating pressures: suction ~68 psig, discharge ~230 psig (typical summer). R-410A: suction ~120 psig, discharge ~400 psig. R-22-rated manifold gauges and hoses are NOT rated for R-410A pressures and can rupture. R-410A also uses different connection fittings (5/16" flare vs R-22 1/4" flare) to prevent cross-contamination. Dedicated R-410A recovery equipment required.

Fractionation: zeotropic refrigerant blends (R-410A, R-407C, R-404A) can separate into components during vapor leaks or vapor charging, changing the mixture composition and system performance. Zeotropic blends contain refrigerants with different boiling points and pressures. During a vapor-phase leak, lighter components escape faster, changing the remaining mixture ratio and thus the system's operating pressures and capacity. This is why zeotropic blends must be charged from the liquid phase of the cylinder (cylinder inverted or liquid valve). Azeotropic and near-azeotropic blends are less susceptible.

Canadian HPFCR (Halocarbons and Perfluorocarbons and their Substitutes Regulations): refrigerant venting is prohibited regardless of quantity. All refrigerants (HFCs, HCFCs, CFCs, HFOs) must be recovered using certified equipment before opening a system or disposing of equipment. Recovered refrigerant must be stored in approved cylinders, reclaimed, or destroyed. Technician certification required for purchasing refrigerants above threshold quantities. Violations: significant fines under CEPA (Canadian Environmental Protection Act).

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).

TXV calibrated for R-22 will 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 will 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. 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.

Canadian regulations require refrigerant handling certification for purchasing containers >2 kg. Under the federal Ozone-Depleting Substances and Halocarbon Alternatives Regulations (ODSHAR), persons purchasing HFCs, HCFCs, or CFCs in quantities over 2 kg must hold a recognized refrigerant handling certification. In Canada, this is typically the TECA (Technician Certification Authority) AC Level 1/2 or equivalent provincial certification. This also requires proper equipment for refrigerant recovery.

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 refrigerant safety classification, charge quantity, occupancy, and room volume. Each refrigerant has a Refrigerant Concentration Limit (RCL). If the full system charge could exceed the allowable concentration in the occupied space during a leak, the system must be isolated in a machinery room with detection and ventilation. Higher-risk occupancies (institutional, public assembly) lower the allowable limits. Compressor type, supply voltage, and condenser size do not drive this determination.

Filter-drier = moisture + contaminant removal from the refrigerant. Moisture in a refrigeration system is critical — it can form ice at the expansion valve (causing restriction) or mix with refrigerant to form corrosive acids. The desiccant in the filter-drier absorbs moisture. Replace whenever system is opened.

Liquid slug on startup = refrigerant migration during off cycle. Refrigerant migrates to the crankcase (lowest, coldest point) during the off cycle — liquid flood-back. On startup, liquid is pumped — compressors can't compress liquid. The knock is liquid slug damage. Prevention: crankcase heaters, pump-down cycle.

Filter-drier: removes moisture and contaminants from liquid refrigerant before it reaches the metering device. The desiccant (silica gel or molecular sieve) absorbs water that would otherwise freeze at the metering device or cause acid formation. The filter element captures particulate contaminants. Must be replaced after any system repair that opens the circuit to atmosphere.

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 + low discharge = worn scrolls. Worn or damaged scroll wraps reduce internal compression efficiency — the compressor can no longer build pressure effectively. Also check for refrigerant flood-back (liquid in scrolls accelerates wear). Low refrigerant alone causes low suction AND low discharge together.

Oil separator: recovers compressor oil from discharge gas and returns it to the compressor crankcase. All compressors pass some oil with discharge gas. Without separation, oil accumulates in evaporators (reducing heat transfer) and the compressor loses oil (lubrication failure). The separator uses velocity change and/or mesh to separate oil, returning it to the crankcase via a float valve.

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.

Crankcase heater: prevents refrigerant migration into and dilution of oil during off cycle. During shutdown, refrigerant vapour migrates toward the cooler compressor crankcase and dissolves into the oil. On startup, rapid pressure drop causes refrigerant to flash out of oil — "liquid slugging" that can break compressor valves and pistons. The heater keeps oil warm to prevent migration.

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.

EEV advantages: precise, multi-variable control enables variable capacity and optimization. A TXV responds only to suction superheat. An EEV is driven by the system controller using data from multiple sensors — suction and discharge pressures, multiple inlet/outlet temperatures, load demand, compressor speed. This allows: faster response, start-up optimization, defrost control, capacity modulation matching, and tighter temperature control. Essential for inverter-driven systems.

Sight glass: visual refrigerant flow inspection + moisture indicator. Bubbles or flash in the sight glass indicate insufficient refrigerant (vapour in the liquid line) — common cause: undercharge or restriction upstream. A colour element (usually green = dry, yellow = moisture present) changes colour when moisture contamination is detected. Note: some refrigerant blends (R-410A, R-407C) may show bubbles even when fully charged at certain conditions — verify with subcooling measurement.

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).

Crankcase heater: prevents refrigerant from migrating into compressor oil during the off-cycle, which would cause liquid slugging on start-up. During shutdown, refrigerant vapor migrates toward the coldest point — often the compressor crankcase. It dissolves in the oil. When the compressor starts, rapid pressure drop causes the refrigerant to boil violently out of the oil, foaming and carrying liquid into the compression chamber — liquid slugging. The crankcase heater keeps the compressor warm to prevent this migration. Must be energized during extended shutdown periods (especially winter).

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 past the damaged valve during the compression stroke, severely reducing compression efficiency and capacity. A broken suction reed: suction gas leaks back during compression stroke — low suction pressure, reduced mass flow. A broken discharge reed: compressed gas leaks back into cylinder during suction stroke — suction pressure rises, discharge pressure drops. Both reduce compression ratio drastically. Test: compressor amp draw will be lower than normal (less work done). Also: very hot discharge line (compressor recirculating hot gas). Verify with valve plate inspection after diagnosis.

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. The intermediate-pressure flash gas is injected into the compressor mid-stage (requires two-stage or economizer-port compressor). Capacity increases 10–15%. 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.

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 for proper oil return through the system. Mixing mineral oil with HFCs causes oil to separate out and pool in the evaporator, starving the compressor. POE oil is hygroscopic — keep containers sealed and minimize atmospheric exposure.

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.

High-pressure switch = compressor safety cutout. If discharge pressure rises dangerously past a safe limit (blocked airflow, dirty condenser, refrigerant overcharge), the HP switch opens the compressor circuit and shuts the compressor down before damage occurs. Manual reset types require investigation before restart.

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 EEV controller reads suction temperature, suction pressure (superheat calculation), and other inputs. It adjusts the valve position via a stepper motor with much faster response and wider control range than a mechanical TXV — allowing operation at variable conditions. Enables inverter-driven VRF systems to work efficiently across a wide range of conditions.

Time termination (not temp) means defrost is incomplete — the coil is not fully defrosting in most cycles. If the coil were fully defrosted, the termination thermostat would trigger (28°F) before the 30-minute limit. When time terminates, frost remains. Causes: thermostat positioned wrong (reads coil temp, not frost area), failed termination thermostat, undersized heaters, or too-frequent defrost schedule.

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.

Low ambient kit: maintains head pressure in cold weather by controlling condenser fan. Refrigeration systems need a minimum head pressure for proper TXV or metering device function. In cold weather, the condenser is over-efficient — head pressure drops too low, liquid refrigerant enters the TXV with insufficient pressure differential, causing poor evaporator and cooling performance and risking compressor damage. A low-ambient kit cycles or modulates condenser fans to keep head pressure within the required operating range.

Economizer: free cooling using outdoor air when conditions allow. When outdoor air enthalpy (or dry-bulb temperature) is below the space conditioning setpoint, the economizer opens outdoor air dampers and closes recirculation dampers — the outdoor air directly handles the cooling load, reducing or eliminating mechanical cooling. The compressor is unloaded or shut off, saving compressor energy. This provides significant savings during mild spring/fall weather and cool nights. ASHRAE 90.1 requires economizers on RTUs above certain sizes.

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. Also protects from operating below freezing point (air ingestion risk on R-22 systems). 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 (defrost termination thermostat — coil reaches ~55°F/13°C) or time. Adaptive/demand defrost systems use sensors to run defrost only when needed.

EEV (Electronic Expansion Valve): stepper motor-controlled valve that adjusts flow based on signals from the system controller — improving efficiency and enabling advanced system management. Unlike TXV (mechanical, responds only to suction superheat via sensing bulb), the EEV can respond to multiple inputs simultaneously: suction superheat, discharge temp, evaporator load, compressor speed, outdoor temp. This allows precise superheat control across a much wider operating range. Used in: VRF systems, inverter-driven systems, commercial refrigeration with digital controllers. Can be fully integrated into building automation.

Burned/pitted contactor contacts: caused by repeated arcing during opening/closing under high current. Compressor motors have high inrush current on start (6–10x running current). Each time the contactor opens or closes under load, arcing occurs. Excessive short cycling (frequent starts) accelerates contact wear. Also: low voltage causes contacts to close slowly (extended arcing). Symptoms of failed contactor: welded contacts (compressor won't shut off), open contacts (no start), high resistance contacts (voltage drop, 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.

Phase monitor relay: protects three-phase compressor motors from phase loss, reversal, and imbalance — which can cause motor overheating or reverse rotation. Phase loss (one phase fails): motor attempts to run on two phases, draws very high current, overheats rapidly. Phase reversal (phases swapped): three-phase motor runs backward — compressor pumps in reverse, no refrigerant flow, potential valve damage. Phase imbalance (unequal voltages): motor runs hotter, reduced life. Phase monitor detects these conditions and de-energizes the control circuit before damage occurs. Required for commercial refrigeration compressor protection.

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). Must balance: head pressure cannot go so low that the metering device cannot feed the evaporator. Typical minimum condensing temperature: 80–90°F (27–32°C).

In cold weather, low ambient temperature around the condenser reduces condenser heat rejection, causing condensing pressure (head pressure) to fall. A thermostatic expansion valve (TXV) requires a minimum pressure differential across it to meter refrigerant properly; when head pressure drops too low, refrigerant flow through the TXV decreases, suction pressure falls below the LPC setpoint, and the LPC trips the compressor. The correct remedy is a head pressure control (HPC) — typically condenser fan cycling or a fan speed controller — that maintains minimum condensing pressure (commonly 100-120 psig / 690-830 kPa for R-404A) regardless of outdoor temperature. Adjusting the LPC setpoint downward would mask the real problem and risk compressor damage from loss of lubrication; a restricted drier would cause high superheat, not an LPC trip under normal load conditions.

EEV advantages: precise, remotely adjustable superheat control from a central controller; fast response to load changes; diagnostic feedback for remote monitoring — improving system efficiency. EEVs use a stepper motor or pulse motor to control a needle valve with precise position control. The valve position (and thus refrigerant flow) is commanded by the controller based on measured superheat (suction temperature - saturation temperature). Benefits: optimal superheat across all load conditions (often lower superheat than TXV = more evaporator surface used = more capacity), faster response to load changes, reduced energy consumption, centralized monitoring.

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 or motor → motor cannot generate enough torque to accelerate, draws locked-rotor current, trips overload. Three-phase induction motors require all three phases to generate a rotating magnetic field. With one phase open, the motor produces a single-phase pulsating field — it can hold torque if already running (with greatly reduced capacity) but cannot start from standstill. The motor draws locked-rotor current (5-7× FLA) on two phases, heating the windings rapidly. Single-phasing causes: blown fuse (one of three fuses), open contactor contact, broken motor lead, open supply.

Both pressures high = too much refrigerant or poor heat rejection. Overcharge adds extra refrigerant that can't be stored in the condenser, raising both pressures. A dirty/blocked condenser or high ambient temperature prevents the condenser from rejecting heat efficiently — same result. Check condenser cleanliness and airflow first.

Icing at evaporator inlet = TXV starving the evaporator. A restricted, underperforming, or hunting TXV meters too little refrigerant into the evaporator. The small amount that enters boils immediately at the inlet, freezing that area while the rest of the evaporator is starved. Check TXV superheat setting and sensing bulb contact.

Evaporator iced up = airflow restriction most commonly (dirty filter, blocked return). Without sufficient air moving across the evaporator, the coil temperature drops below freezing and moisture freezes on it. Once iced, airflow is further reduced — a runaway condition. Check and clean the air filter first and clear any blocked return. Also possible: low refrigerant charge, dirty blower wheel, stuck indoor fan.

Normal pressures but can't pull down = excessive heat load or insulation failure — the refrigeration system is overwhelmed. If pressures are normal, the refrigeration system is working — but something is adding more heat than it can remove. Inspect: door gaskets (let in warm air), excessive door openings, insulation (settled, wet, or failing), door heater stuck on, unusually high product load, compressor undersized for current ambient.

Outdoor unit not running = no compressor/condenser = no cooling. Diagnose outdoor unit failure separately: check the thermostat call and control wiring, power (contactor coil energized?), fan capacitor (fan starts with push?), compressor capacitor, and safety control lockout (high-pressure/low-pressure control). The indoor fan circulating uncooled air produces room-temperature discharge — no cooling.

Proper evacuation: 500 microns or less (triple evacuation preferred, final pull to <500 microns). Micron gauge (electronic vacuum gauge) required — manifold gauges are not accurate enough. Pull vacuum to 500 microns (some specs require 200–300, or 100 microns or below). Isolate vacuum pump and hold — wait 5 minutes, or a full 30 minutes for complete confirmation. If vacuum rises significantly, there is a leak or residual moisture. Below 500 microns = water boils and is removed by vacuum.

Burnout cleanup is critical before installing a new compressor. A burned compressor leaves acid, carbon deposits, and metallic particles throughout the system. These contaminate the new compressor oil, degrade new windings, and block metering devices. Full system flush with dry nitrogen, oversized acid-removal filter-driers to remove all acid and contamination, and fresh oil is mandatory.

Confirm reversing valve position using multiple methods. 1) Solenoid voltage present = valve should be shifted (verify electrically). 2) Touch the reversing valve body — correct operation means specific tubes will be hot/warm vs cool. 3) System pressures should reflect heat pump mode (indoor coil at high side in heating). 4) Outdoor coil should be frost/cool (acting as evaporator in heating mode).

Condensing temp = saturation at discharge pressure = 110°F. Superheat = suction line temp − saturation at suction = 55°F − 40°F = 15°F. These values are used to assess system performance. 15°F superheat is in the normal range (8–20°F). Condensing at 110°F is high — check condenser performance.

Evaporator icing with good airflow: low refrigerant charge or TXV restriction. Normal evaporator temperature is 35–45°F (2–7°C). If suction pressure drops too low (undercharge, restricted or stuck-closed TXV), evaporator temperature drops below 32°F (0°C) and moisture in the air freezes on the coil. First: let ice thaw completely (airflow will not penetrate ice). Then measure suction pressure/superheat with gauges. Low suction + high superheat = undercharge. Low suction + low superheat = TXV restriction.

High suction superheat (>15°F target) = starved evaporator. If refrigerant fully vaporizes early in the evaporator, the remaining coil surface superheats the vapour instead of absorbing heat from the space — wasting evaporator capacity. Target superheat at TXV systems: 8–12°F. At 30°F, the evaporator is significantly starved — losing capacity and efficiency. High discharge superheat follows from high suction superheat. Investigate: refrigerant charge (undercharge), restricted TXV, restricted liquid line.

R-22 to R-422D retrofit: TXV must be resized/replaced for the new refrigerant. R-422D has lower refrigerating capacity per pound than R-22, and retrofit refrigerants have different thermodynamic properties (latent heat, density, flow characteristics). The original TXV is sized for R-22's specific properties. Using an R-22 TXV with a retrofit refrigerant typically results in incorrect superheat control, often flooding (too open) or starving (too restrictive) the evaporator. The TXV sensing bulb must be charged for the new refrigerant.

RTU not meeting setpoint with normal refrigerant pressures: look beyond the refrigerant circuit. If pressures are correct, the refrigerant system is likely working properly — but it may not have sufficient capacity to overcome extra load. Investigate: economizer damper stuck open (adding outdoor heat load), excessive outdoor air (fresh air damper malfunction), dirty condenser coil (reducing efficiency), dirty evaporator coil (restricting airflow), duct leakage, missing supply duct insulation, return-air bypasses, or building load exceeding equipment capacity on an extreme heat day.

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). 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).

Converging pressures (suction high, discharge low) = compressor not pumping effectively — internal compressor failure. A healthy compressor creates a large pressure differential between suction and discharge. When suction and discharge pressures converge (approach each other), the compressor is failing to compress: broken reed valves (gas bypasses), worn piston rings, or worn valve plates. Amp draw will be low (less work being done). The compressor runs but doesn't pump. Verify: compression ratio should be 3:1 minimum on most systems.

Compressor runs continuously: system cannot keep up with heat load, or capacity is reduced. Heat load causes: failing door seals/gaskets (warm moist air infiltration), excessive door openings (new usage pattern), failed evaporator fans (poor air circulation over coil), condenser not rejecting heat properly (dirty coil, high ambient), or a system undersized for the load. Capacity reduction: low refrigerant charge, dirty evaporator (frost build-up between defrosts), restricted metering device. Check: setpoint vs actual temperature, door gaskets, evaporator fans, defrost effectiveness, refrigerant pressures.

High head pressure with clean condenser and normal suction = non-condensable gases or refrigerant overcharge. Non-condensables: collect in condenser, increase total pressure without contributing to heat transfer — discharge temp also high. Overcharge: excess liquid in condenser, reduced condensing surface area — high head, possibly high subcooling, liquid slugging risk. Distinguish: check subcooling. Overcharge = very high subcooling. Non-condensables = high head pressure on warm day, subcooling may be normal or low. Recover system, evacuate, recharge to spec.

After compressor burnout: acid, carbon, and moisture contaminate the entire system — must be fully remediated before installing new compressor. Acid (from refrigerant/oil thermal breakdown) attacks new compressor copper windings and valves. Procedure: 1) Recover refrigerant. 2) Flush system with approved refrigerant flush agent or purge with dry nitrogen. 3) Replace filter drier with oversized burnout drier (suction side too). 4) Install new compressor. 5) Charge with new oil and refrigerant after triple-evacuating to below 300 microns. 6) Run system, then pull oil sample in 500 hours — check acid level. May need second drier replacement.

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).

Continuous run at setpoint = system is correctly maintaining temperature, but load exceeds design capacity. The thermostat is not calling for shutdown because it IS maintaining setpoint — but barely. Excess load sources: high product load (overloaded cooler), door left open or high door-open frequency, air infiltration through defective door gaskets, excessive ambient heat near the cooler. First check: door gaskets, door traffic, product load, evaporator fans (all operating). Also verify: discharge air temperature, superheat, and system pressures are normal. A correctly running but overloaded system is NOT a mechanical fault — it is an operational issue.

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: 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 (gauge) discharge pressure is higher than the refrigerant temperature alone would produce. Because subcooling is calculated as (P-T saturation temperature at measured discharge pressure) − (liquid line temperature), the inflated pressure makes the calculated subcooling appear larger than real. Diagnosis: compare measured condensing temperature to the P-T chart at measured discharge pressure — if the measured temperature is several degrees LOWER than the chart value, non-condensables are present. An overcharge also raises head pressure, but its measured condensing temperature agrees with the P-T chart.