HVAC Excellence Heat Pump key terms

The quantity of heat absorbed when a liquid changes to vapor at constant temperature and pressure, without any change in temperature. In a heat pump this heat is absorbed in the evaporator.

Heat that changes the temperature of a substance without changing its state. In air conditioning it is the portion of capacity that lowers or raises dry-bulb temperature.

The indoor coil in heating mode and outdoor coil in cooling mode where low-pressure refrigerant boils and absorbs heat from the surrounding air, producing the cooling or heating effect.

The coil where high-pressure refrigerant vapor rejects heat to the surrounding air and condenses to liquid. In heating mode this is the indoor coil.

The pump that raises refrigerant pressure and temperature, moving vapor from the low-side evaporator to the high-side condenser. It is the component that creates the pressure difference driving the whole cycle.

A fixed orifice, piston, or thermostatic expansion valve that drops refrigerant pressure between the condenser and evaporator and regulates how much liquid refrigerant enters the evaporator.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the coil is fully flooded with liquid without carrying liquid back to the compressor.

The number of degrees a liquid refrigerant is 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 temperature at which a refrigerant boils or condenses at a specific pressure. On a pressure-temperature chart, it is the temperature read straight across from a given pressure.

Absolute discharge pressure divided by absolute suction pressure. High ratios raise discharge temperature and reduce capacity, and are a common reason a compressor overheats or trips on overload.

A four-way solenoid-operated valve that swaps the evaporator and condenser roles by redirecting discharge and suction gas, allowing one refrigerant circuit to both heat and cool.

A timed or demand-initiated sequence that temporarily reverses the cycle or stops the outdoor fan to melt frost from the outdoor coil, then returns the unit to normal heating.

Supplemental electric resistance or fossil-fuel heat that energizes when the heat pump cannot meet the load, such as during defrost or very low outdoor temperatures.

The outdoor temperature at which heat pump capacity exactly equals the building heat loss. Below it, supplemental heat must make up the difference to hold setpoint.

A mode in which the compressor is locked out and only supplemental resistance heat is used, typically selected manually when the compressor or outdoor unit has failed.

A small resistance heater wrapped around or inserted in the compressor sump that keeps oil warm so refrigerant does not migrate and dilute the oil during the off cycle.

A vessel on the suction line of some heat pumps that traps liquid refrigerant and oil and meters them back to the compressor as vapor, protecting it from liquid slugging.

A heavy-duty relay whose coil is energized by the thermostat or control board to close high-amperage line-voltage contacts feeding the compressor or outdoor fan motor.

The electronic control that monitors coil temperature and run time, initiates and terminates defrost, and energizes the reversing valve and auxiliary heat during defrost.

A control operating typically on 24 volts that switches compressor, reversing valve, fan, and auxiliary heat circuits through the indoor unit's control board and terminal strip.

The pair of copper tubes connecting the outdoor unit to the indoor coil, consisting of an insulated larger suction line and a smaller liquid line.

The larger, insulated vapor line carrying low-pressure refrigerant gas from the evaporator back to the compressor. It must be sized and insulated to limit pressure drop and prevent condensation.

The smaller line carrying high-pressure liquid refrigerant from the condenser to the metering device. A liquid-line filter drier is installed here to remove moisture and debris.

A device in the liquid line containing a desiccant and screen that traps moisture, acid, and particulate matter to protect the metering device and compressor.

Flowing dry nitrogen through piping while brazing to displace oxygen and prevent internal oxidation, which would otherwise form scale that contaminates the system.

Pulling a deep vacuum with a pump to remove air, nitrogen, and moisture from the system before charging. A standing vacuum decay test confirms the system is leak-free and dry.

An electronic vacuum instrument that reads absolute pressure in microns, used to confirm evacuation below about 500 microns and to verify the vacuum holds.

Adding refrigerant by weight using a scale and the manufacturer's nameplate charge or line-set adjustment, the most accurate method for systems with fixed metering devices.

The manufacturer-specified subcooling value used to set charge on TXV systems. Measured subcooling above target means overcharge; below target means undercharge.

A value calculated from indoor wet-bulb and outdoor dry-bulb temperatures, used to set charge on fixed-orifice systems when conditions fall outside the charging chart's normal range.

Air temperature measured by an ordinary thermometer, unaffected by moisture content. It is used with wet-bulb to determine airflow and system capacity.

The temperature of air measured with a wetted thermometer bulb, reflecting moisture content and evaporative cooling potential. It is essential for calculating target superheat and airflow.

The resistance to airflow measured in inches of water column across the air handler, including ductwork and accessories. It is used with a fan table to determine delivered airflow.

The difference between return-air and supply-air dry-bulb temperatures in heating mode. Combined with airflow, it verifies that the unit is delivering rated heating capacity.

The combined sensible and latent heat a system moves, expressed in Btuh. It is verified in the field using airflow and the enthalpy difference across the coil.

A charging and diagnostic procedure that compares measured superheat with a target derived from indoor wet-bulb and outdoor dry-bulb, used on fixed-orifice systems and to check TXV operation.

A control state in which the unit is prevented from running, often after repeated safety trips such as high-pressure or low-pressure switches, requiring manual or automatic reset.

A safety control that opens the compressor circuit when discharge pressure exceeds a set limit, protecting against overcharge, restricted airflow, or a failed condenser fan.

A safety control that opens the compressor circuit when suction pressure falls too low, indicating loss of charge, a restriction, or low ambient operation.

The EPA credential required for technicians who service, repair, or dispose of appliances containing regulated refrigerants, with Type II covering most residential split heat pumps.

Equipment that removes refrigerant from a system into a recovery cylinder for reuse, recycling, or disposal, required before opening a circuit or scrapping equipment.