NATE Heat Pump Service key terms

The ratio of heat delivered or removed to the electrical energy consumed, measured at a single rating point. Air-source heat pumps commonly reach COP values of 2 to 4 in mild weather.

Heating Seasonal Performance Factor, version 2, expresses seasonally averaged heating efficiency as total heat output in Btu divided by electrical input in watt-hours. It is the federally rated heating metric for residential heat pumps.

Seasonal Energy Efficiency Ratio, version 2, rates cooling efficiency using a revised test procedure with higher external static pressure. Higher numbers indicate more efficient cooling operation across a representative season.

A four-way solenoid valve that swaps the refrigerant paths between indoor and outdoor coils, changing the unit from cooling to heating. It is energized in heating mode on most residential systems.

A cylindrical vessel on the suction line that traps liquid refrigerant and oil and meters them back to the compressor as vapor. It protects the compressor from slugging during low-load or flooded-start conditions.

Liquid refrigerant entering the compressor crankcase and cylinders, which can wash out lubricating oil and damage valves or bearings. Symptoms include cold crankcase, oil foaming, and abnormal noise on startup.

The restriction that drops refrigerant pressure between the liquid line and evaporator, regulating flow to match load. Common types are fixed-bore orifices, thermostatic expansion valves, and electronic expansion valves.

A modulating metering device that adjusts refrigerant flow to hold a set superheat at the evaporator outlet, using a remote bulb and diaphragm. It is common on higher-efficiency and variable-speed heat pumps.

The temperature of refrigerant vapor above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the metering device is feeding the coil correctly.

The temperature of liquid refrigerant below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid is stacked in the condenser and verifies charge on TXV systems.

The manufacturer-specified subcooling value, often printed on the outdoor unit rating plate, used as the charging benchmark. Technicians adjust charge until measured subcooling matches this value at the stated indoor and outdoor conditions.

Adding refrigerant by weight using a scale and the nameplate charge plus adjustments for line-set length and indoor coil volume. It is the most accurate method, especially for systems with electronic expansion valves or long line sets.

The additional refrigerant charge required when the interconnecting tubing exceeds the factory-charged length, typically specified as ounces per foot of liquid line. It must be added or removed to keep charge correct.

A charging procedure for fixed-orifice systems in which refrigerant is adjusted until measured superheat matches a target derived from indoor wet-bulb and outdoor dry-bulb temperatures. It is used when subcooling is not the specified method.

The total resistance to airflow in the duct system, measured across the blower in inches of water column. It is compared to the blower performance table to set airflow and diagnose restrictive ductwork.

A manufacturer chart listing delivered airflow for combinations of external static pressure and blower speed or tap. Technicians use it to verify that the blower is moving the required cubic feet per minute.

The difference between supply and return air temperatures in heating mode. Comparing it to the unit's rating-plate range confirms whether airflow and charge are within specification.

The measured temperature difference across the indoor coil, used in both heating and cooling to sanity-check airflow and capacity. Values outside the manufacturer's range point to low airflow, duct loss, or charge problems.

A timed or demand-initiated sequence that temporarily switches the unit to cooling mode to melt frost from the outdoor coil. The outdoor fan stops and auxiliary heat often energizes to prevent cold supply air.

The condition that ends a defrost cycle, typically a coil temperature sensor reaching a set point or a maximum time limit. Proper termination prevents both residual frost and excessive heating-mode downtime.

The electronic module that initiates, monitors, and terminates defrost cycles and often controls the outdoor fan and reversing valve. It may store fault codes and accept a forced-defrost test input.

The outdoor temperature at which heat pump capacity equals the building's heat loss, below which supplemental heat is needed. It is set during commissioning to control when auxiliary or emergency heat stages engage.

Supplemental electric resistance or other heat that stages on automatically when the heat pump cannot meet demand. It is locked out above a set outdoor temperature to avoid unnecessary energy use.

A manual thermostat mode that disables the compressor and relies solely on backup resistance heat. It is used during compressor failure or severe weather and should be explained to the homeowner.

The 24-volt wiring path linking thermostat, control board, and contactors, including terminals such as R, C, Y, O, G, W, and E. Miswiring here is a frequent cause of no-cool, no-heat, or fan-only complaints.

A small resistance heater wrapped around the compressor base that keeps oil warm and drives refrigerant out of the crankcase during the off cycle. It prevents liquid slugging on startup in cold weather.

A start capacitor and relay assembly added to a single-phase compressor to boost starting torque. It is used where voltage drop, low ambient, or long line sets make startup difficult.

A capacitor that stays in the compressor or fan motor circuit to shift phase and maintain torque during operation. A bulged or out-of-tolerance capacitor causes high amp draw, slow starts, or motor overheating.

The current a motor draws when its rotor is stalled, listed on the rating plate. Measured startup current well below LRA can indicate a weak capacitor or failing start components.

A thermal or thermal-current device that opens the compressor circuit when temperature or current exceeds safe limits. Repeated trips suggest high head pressure, low voltage, or a failing compressor.

Condensing pressure above the expected range for ambient conditions, caused by dirty coils, restricted airflow, overcharge, non-condensables, or a failed fan. It raises amp draw and can trip the overload protector.

Evaporating pressure below the expected range, caused by undercharge, restricted metering device, dirty filter, or low indoor airflow. It often appears with high superheat and reduced capacity.

Air or nitrogen trapped in the refrigerant circuit that cannot condense at normal pressures. They cause elevated head pressure, high subcooling, and erratic gauge readings, and require recovery and evacuation to correct.

Pulling a deep vacuum on the refrigerant circuit to remove moisture and non-condensables before charging. A standing vacuum test confirms the system holds and that no leaks or trapped moisture remain.

Flowing dry nitrogen through tubing while brazing to prevent oxidation inside the lines. It stops carbon scale from forming and later clogging metering devices or compressor strainers.

The room-by-room method for sizing heating and cooling loads from envelope, orientation, and occupancy data. Proper sizing keeps the heat pump cycling correctly and avoids humidity and comfort problems.

The volume of air moved through the system in cubic feet per minute, typically set near 400 CFM per ton in cooling. Incorrect airflow skews charge readings and reduces capacity and efficiency.

Leakage that lets supply or return air escape the duct system or re-enter from unconditioned space, reducing delivered capacity. It is found with a duct-blaster test or by measuring pressure and temperature at registers.