NATE Heat Pump Service cheat sheet

A heat pump moves heat against the temperature gradient; heating capacity equals condenser heat rejection, cooling capacity equals evaporator heat absorption plus compressor work.

Reversing valve (four-way valve) swaps evaporator and condenser roles; the solenoid is energized in cooling mode on most residential designs, so a failed coil typically leaves the unit in heating.

In heating, the outdoor coil is the evaporator and runs below ambient temperature; the indoor coil is the condenser and delivers heat to the airstream.

Balance point is the outdoor temperature where heat pump capacity equals building load; below it, supplementary or emergency heat must make up the difference.

Coefficient of performance = heat delivered / electrical energy input; rated HSPF2 and SEER2 values already account for cycling and defrost losses in the rating.

Accumulator protects the compressor from liquid return in heating; suction-line accumulator and crankcase heater are heating-season components.

Defrost termination is normally by coil temperature sensor or pressure switch, with time-temperature initiation; a stuck-open reversing valve yields no heat in heating mode.

Typical refrigerant glide matters for zeotropic blends such as R-410A; charge by subcooling in cooling and by weight or manufacturer chart in heating.

Pad or platform must be level and above grade with clearance per manufacturer; outdoor unit airflow recirculation reduces capacity and must be avoided.

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

Nitrogen purge while brazing is mandatory; flow dry nitrogen at low pressure to prevent internal oxidation scale that later plugs metering devices.

Evacuate to 500 microns or below and hold with the pump isolated; a rise above 500 microns within 10 minutes indicates moisture or a leak.

Trap and slope the condensate drain; in heating mode the outdoor coil produces condensate that must drain and not refreeze under the unit.

Charge verification at commissioning uses weigh-in for fixed-orifice systems and subcooling (cooling) or superheat (heating) for TXV systems.

Low-voltage thermostat wire size must be checked for voltage drop on long runs; 18 AWG is limited to roughly 100 ft for 24 V control circuits.

Record static pressure, delta-T, superheat/subcooling, and amp draw at startup; these become the baseline for later diagnostics.

Defrost initiation is typically demand-based: coil temperature sensor plus accumulated compressor run time; termination is by coil temperature or pressure.

During defrost the outdoor fan stops and the reversing valve shifts to cooling; auxiliary heat is energized to prevent cold blow indoors.

A failed outdoor fan motor causes rapid frost buildup and repeated short defrost cycles with high head pressure.

Defrost board output checks: verify 24 V at the reversing valve solenoid and at the auxiliary heat sequencer during the defrost cycle.

Crankcase heater must be energized whenever the compressor is off; it boils liquid refrigerant out of the crankcase to prevent slugging on startup.

Low-voltage transformer typically 24 V secondary; a blown 3 A or 5 A fuse points to a shorted thermostat wire or contactor coil.

Time-delay relay or board lockout prevents compressor short cycling; typical anti-short-cycle delay is 5 minutes.

High- and low-pressure switches are safety controls, not operating controls; a tripped high-pressure switch indicates airflow or overcharge, not a bad switch.

Superheat = suction line temperature minus saturation temperature at suction pressure; high superheat with low suction pressure indicates undercharge or restricted metering.

Subcooling = saturation temperature at liquid pressure minus liquid line temperature; low subcooling with bubbles indicates undercharge.

Delta-T across the indoor coil in cooling is typically 16-22 F (9-12 C); low delta-T with normal airflow suggests low charge or dirty coil.

In heating, check delta-T across the indoor coil (typically 25-40 F) and compare to manufacturer chart for outdoor temperature.

A frosted outdoor coil that does not defrost points to defrost sensor, board, or reversing valve; a coil that never frosts in cold weather may be undercharged.

Compare compressor amp draw to nameplate RLA; high amps with high head pressure indicates overcharge, restricted condenser, or noncondensables.

Measure superheat and subcooling simultaneously; a restricted liquid line drier shows a temperature drop across the drier and low subcooling downstream.

Always verify airflow before charging; low airflow mimics low charge by depressing suction pressure and raising superheat.

TXV systems are charged by subcooling in cooling; fixed-orifice (piston) systems are charged by superheat, adjusted for indoor wet-bulb and outdoor dry-bulb.

Weigh-in is the most accurate method when line set length and indoor coil volume are known; use manufacturer charge chart for added length.

Target subcooling is typically 8-12 F for TXV systems at design conditions; verify against the manufacturer's charging chart.

Static pressure: total external static pressure should be measured across the blower; typical residential target is 0.5 in. w.c. or less.

Airflow rule of thumb: 400 CFM per ton in cooling; heating airflow may be higher (450 CFM/ton) depending on manufacturer.

Delta-T multiplied by 1.08 times CFM estimates sensible capacity in Btu/h (English units).

Verify refrigerant charge only after airflow is corrected; dirty filters, coils, or duct restrictions invalidate superheat and subcooling readings.

Efficiency verification includes measuring actual SEER2/HSPF2 at installed conditions and comparing to the rated value on the AHRI certificate.