HVAC Excellence Heat Pump cheat sheet

Heating COP = heat delivered to space ÷ electrical energy input; cooling EER = BTU/h cooling ÷ watts input; SEER2/HSPF2 are seasonal metrics used on modern nameplates.

In heating mode the outdoor coil is the evaporator (absorbs heat from outdoor air) and the indoor coil is the condenser (rejects heat to the space); in cooling mode the roles reverse via the reversing valve.

Standard vapor-compression cycle sequence: compressor → condenser → metering device → evaporator → compressor; subcooled liquid leaves condenser, superheated vapor leaves evaporator.

Superheat = actual suction vapor temperature − evaporating saturation temperature; subcooling = condensing saturation temperature − actual liquid line temperature.

Balance point is the outdoor temperature where heat pump capacity equals building heat loss; below it, supplemental or auxiliary heat must make up the difference.

Defrost is required when outdoor coil temperature falls below roughly 32°F and frost accumulates; demand-defrost uses coil temperature and pressure sensors rather than fixed timers alone.

Cooling mode sensible heat ratio (SHR) = sensible capacity ÷ total capacity; typical residential SHR is about 0.70–0.80, and low airflow lowers SHR and coil temperature.

Refrigerant glide is the temperature difference between bubble and dew points of a zeotropic blend; for R-410A glide is small (about 0.2°F) but for R-407C it is roughly 10°F, affecting charging by temperature.

Low ambient heating capacity falls because evaporator pressure and refrigerant mass flow drop; capacity at 17°F is typically about 50–60% of rated 47°F capacity for a standard single-stage unit.

Reversing valve is a four-way solenoid pilot valve: de-energized in cooling on most residential units, energized in heating; a stuck valve shows equal pressures or wrong-mode temperatures.

Crankcase heater prevents refrigerant migration into the compressor sump during the off cycle; it must be energized before startup after long off periods.

Defrost control board terminates defrost by coil temperature sensor, time, or pressure; a failed sensor typically forces time-temperature defrost and can cause cold blow.

Compressor contactor coil is controlled by the low-voltage thermostat through the defrost board and high/low pressure switches; a pitted contactor causes high current and intermittent operation.

Start components (potential relay and start capacitor, or PTC) are required on many scroll and reciprocating compressors to overcome locked-rotor amps at startup.

Auxiliary heat sequencer or relay stages electric resistance heaters based on thermostat demand and outdoor temperature; check sequencer timing and heater element continuity.

Low-voltage transformer typically supplies 24 VAC control power; a blown low-voltage fuse or open transformer secondary disables thermostat and contactor operation.

Outdoor fan motor on a heat pump is usually a PSC or ECM; in defrost the fan is stopped to speed coil warming, so a fan that runs during defrost indicates a control fault.

High-pressure and low-pressure switches protect the compressor; low-pressure switch opens on loss of charge and high-pressure switch opens on condenser airflow restriction or overcharge.

Line set sizing follows manufacturer tables; undersized suction lines increase pressure drop and reduce capacity, while oversized liquid lines can reduce refrigerant velocity and oil return.

Suction line insulation is mandatory to prevent condensation and capacity loss; liquid lines should be insulated where exposed to heat or where code requires.

Brazing requires flowing dry nitrogen to prevent internal oxidation (scale); use a heat sink on valves and wrap TXV bulbs during brazing.

Evacuation to 500 microns or below with a standing vacuum test (rise less than 500 microns in 10 minutes) confirms a dry, leak-free system.

Filter-drier must be installed in the liquid line, in the direction of flow, and replaced after any compressor burnout or major repair.

Airflow target is typically 350–450 CFM per ton in cooling; in heating, airflow is often set higher to avoid high discharge temperatures and nuisance high-limit trips.

External static pressure includes filter, coil, duct, and grille losses; high ESP reduces airflow and can cause coil freezing in cooling or high-limit trips in heating.

Outdoor unit must be elevated above snow line and have clearance per manufacturer (often 12–18 inches) for defrost water drainage and airflow.

Refrigerant lines should be routed with traps or inverted traps per manufacturer for oil return on long vertical runs; horizontal runs need slight slope toward the compressor.

Weigh-in charging is the most accurate method for new installations or after recovery; use the nameplate charge plus adjustments for line-set length and indoor coil volume.

In cooling, charge by subcooling (typically 8–12°F for TXV systems) at 95°F outdoor and 80°F indoor dry bulb; in heating, use manufacturer charging charts or weigh-in because subcooling is not reliable.

Superheat target for fixed-orifice systems is typically 8–12°F at the evaporator; for TXV systems superheat is controlled by the valve and subcooling is the charging indicator.

Verify airflow before charging: measure static pressure and temperature rise/split; a dirty filter or coil skews refrigerant readings.

Record before-and-after readings: suction and liquid pressures, line temperatures, outdoor/indoor dry and wet bulb, voltage, amperage, and static pressure.

Heating mode performance check: measure temperature rise across indoor coil (typically 20–35°F depending on outdoor temperature and airflow) and compare to manufacturer charts.

Defrost initiation and termination should be verified by forcing a defrost cycle and observing coil temperature, fan operation, and reversing valve action.

Verify auxiliary heat operation with a call for second/third stage and confirm amperage draw of each heater element matches nameplate.

Thermostat configuration must match equipment type (heat pump, stage counts, auxiliary lockout, balance point) to prevent simultaneous compressor and aux heat or short cycling.

Before any electrical work, lock out and tag out the disconnect; verify with a meter that power is off at the contactor and capacitor terminals.

Capacitors store charge; discharge with a resistor before handling, and check capacitance with a meter against the rated microfarad value (typically ±6–10%).

Compressor won't start: check contactor, start components, run capacitor, winding resistance (balanced across terminals), and supply voltage under load.

Heating mode cold blow: check reversing valve solenoid, defrost termination, auxiliary heat, and refrigerant charge; a stuck reversing valve often shows equal high/low pressures.

Ice on outdoor coil in heating is normal during defrost; ice that persists or builds heavily indicates defrost control, sensor, or reversing valve failure.

High superheat with low suction pressure suggests undercharge or restricted metering device; low superheat with high suction pressure suggests overcharge or inefficient compressor.

Refrigerant recovery must follow EPA Section 608 rules; do not vent refrigerant, and recover to the required vacuum level before opening the system.

Electrical safety: verify grounding, check for burnt wire nuts and loose lugs, and measure voltage drop under load; a loose neutral can cause erratic control operation.

Document service with pressures, temperatures, superheat/subcooling, and amp draw before and after repair; compare to manufacturer charging charts for the specific model.