How Heat Pumps Work

The Basic Idea

A heat pump moves heat in either direction

A heat pump uses the same basic refrigeration principles as an air conditioner, but it can reverse the direction of heat transfer. In summer it moves heat from the home to the outdoors. In heating mode it extracts available heat from outdoor air and transfers it indoors.

Because a heat pump moves heat rather than creating all of its heat through electric resistance or combustion, it can provide efficient heating under appropriate operating conditions.

Heat-pump performance depends on equipment selection, outdoor temperature, airflow, controls and the home's heating load.

Major Components

What makes a heat pump different

Reversing valve

Changes the direction of refrigerant flow so the refrigeration system can switch between heating and cooling.

Outdoor coil

Rejects heat during cooling and absorbs heat from outdoor air during heating.

Indoor coil

Absorbs indoor heat during cooling and releases heat into indoor air during heating.

Compressor

Circulates refrigerant and creates the pressure difference required for heat transfer.

Defrost controls

Periodically manage frost that can accumulate on the outdoor coil during cold-weather heating.

Auxiliary heat

Electric resistance heat or another heat source may supplement heat-pump capacity when needed.

Cold Weather

What homeowners should understand about winter operation

Capacity changes with temperature

An air-source heat pump's available heating capacity and efficiency change as outdoor conditions change.

Defrost is normal

The system may temporarily reverse operation to remove frost from the outdoor coil. Steam or water around the outdoor unit during defrost can be normal.

Balance point matters

The home's heat loss and the heat pump's available capacity determine when supplemental heat may be required.

System Design

Heat pumps have to be matched to the home

Equipment sizing

Oversizing and undersizing can both create comfort or efficiency problems. Heating and cooling loads should be considered.

Airflow

Heat pumps depend on proper indoor airflow just as central air conditioners do.

Controls

Thermostat setup, staging, auxiliary heat and equipment controls affect how the system operates in different conditions.

Common Questions

Heat-pump behavior that can surprise homeowners

Why does the supply air feel less hot?

Heat pumps often deliver air at a lower temperature than a gas furnace while still adding enough heat to warm the home.

Why does the outdoor unit run in winter?

The outdoor unit is part of the heating process and normally operates while the heat pump is extracting heat outdoors.

Why did auxiliary heat come on?

Supplemental heat may operate during high demand, recovery from a large thermostat setback, defrost or conditions where additional capacity is needed.

Continue Learning

Use system knowledge to make better HVAC decisions

These guides are intended to help homeowners understand equipment and system concepts. Actual repair, sizing and installation decisions should be based on the conditions in the home and the equipment being evaluated.

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