Heat Pump glossary

Heat pump glossary

The heat-pump-specific parts and concepts, defined in plain English. For cross-system terms like HSPF2, SEER2, and refrigerant, see the master glossary.

Auxiliary / backup heat

The secondary heat — electric resistance strips or, in a dual-fuel setup, a gas furnace — that carries the load below the balance point. "AUX HEAT" on the thermostat means it's running.

Balance point

The outdoor temperature where a heat pump's output stops matching the home's heat loss, so backup heat takes over. Central to sizing and to dual-fuel systems.

Cold-climate rating

A designation for heat pumps engineered to hold heating output in deep cold, some rated to around −13°F. It's what lets a heat pump carry a northern winter without much backup.

COPCoefficient of Performance

The ratio of heat delivered to energy consumed at a given moment. A COP of 3 means the heat pump moves three units of heat per unit of electricity — the reason it beats resistance heat.

Defrost cycle

A brief reversal in winter that melts frost off the outdoor coil, producing steam and cooler air from the vents for a few minutes. It looks alarming but is completely normal.

Dual-fuel

A heat pump paired with a gas furnace on one thermostat, switching to the furnace below the balance point. The full explainer is on the dual-fuel page.

Inverter compressor

A variable-speed compressor that ramps up and down to match demand instead of cycling on and off. It's behind the efficiency and steady comfort of modern heat pumps and mini-splits.

Reversing valve

The component that flips the refrigerant's direction so a heat pump can heat as well as cool — the single part that makes it more than an air conditioner. See heat pump vs AC.