How heat pumps work

A heat pump moves heat instead of making it. Natural Resources Canada (NRCan) defines it as "an electrically driven device that extracts heat from a low temperature place (a source), and delivers it to a higher temperature place (a sink)."^{[Source 1]} In winter the source is the outdoor air (or the ground, for geothermal systems) and the sink is your home. In summer it runs the other way and works like an air conditioner.

Because it moves heat rather than converting electricity directly into heat, a heat pump can deliver more heat than the electricity it uses. The DOE says an air-source heat pump "can deliver up to two to four times more heat energy to a home than the electrical energy it consumes."^{[Source 2]} That ratio is the coefficient of performance (COP), and it changes with outdoor temperature.

The parts

The DOE describes an air-source heat pump as consisting of "a compressor and two copper or aluminum coils (one indoors and one outside)."^{[Source 2]} NRCan lists the main components:^{[Source 1]}

Part	What it does
Refrigerant	The fluid that circulates, "alternately absorbing, transporting and releasing heat"
Compressor	"Squeezes the molecules of the refrigerant gas together, increasing the temperature of the refrigerant"
Evaporator coil	Where the refrigerant "absorbs heat from its surroundings and boils to become a low-temperature vapour"
Condenser coil	Where the refrigerant "gives off heat to its surroundings and becomes a liquid"
Expansion device	"Lowers the pressure created by the compressor," dropping the refrigerant's temperature
Reversing valve	"Controls the direction of flow of the refrigerant" and switches between heating and cooling

The outdoor coil and the indoor coil trade roles depending on the mode. In heating, the outdoor coil is the evaporator and the indoor coil is the condenser. In cooling, they swap.

The heating cycle, step by step

NRCan describes the heating cycle this way:^{[Source 1]}

1. Liquid refrigerant passes through the expansion device and becomes a low-pressure liquid and vapor mixture.
2. In the outdoor coil, it "absorbs heat from the outdoor air and boils, becoming a low-temperature vapour."
3. The vapor passes through the reversing valve to the compressor, which compresses it, "reducing its volume and causing it to heat up."
4. The reversing valve sends the hot gas to the indoor coil. Heat transfers to the indoor air and the refrigerant condenses back into a liquid.
5. The liquid returns to the expansion device and the cycle repeats.

The refrigerant is colder than the outdoor air when it enters the outdoor coil, so heat flows into it even on a cold day. NRCan notes that "the heat content of air at -18°C equates to 85% of the heat contained at 21°C."^{[Source 1]}

The cooling cycle

In cooling mode the reversing valve flips the flow. The indoor coil becomes the evaporator, picks up heat from the house, and the outdoor coil rejects it outside. NRCan notes that during cooling "the heat pump also dehumidifies the indoor air": moisture condenses on the cold indoor coil and drains away through a condensate line.^{[Source 1]}

The defrost cycle

In heating mode, when the outdoor temperature is near or below freezing, moisture in the air can freeze on the outdoor coil. The heat pump periodically reverses into cooling mode for a short time to send hot refrigerant to the outdoor coil and melt it.^{[Source 1]} The DOE notes the reversing valve "changes the direction of refrigerant flow for cooling and for the winter defrost cycle."^{[Source 2]} See Heat pump defrost cycles.

Why outdoor temperature matters

As the outdoor temperature drops, there is less heat to pull in and the compressor has to lift heat across a larger temperature difference. Both capacity and efficiency drop. NRCan says "the ability of the heat pump to transfer heat from the outside air to the house depends on the outdoor temperature."^{[Source 1]} Below a certain temperature, most systems need a second heat source. See Heat pumps in cold weather and Thermal and economic balance points.

Air-source, ground-source and air-to-water

• Air-source heat pumps use outdoor air. NRCan calls them the most common type on the Canadian market.^{[Source 1]}
• Ground-source (geothermal) heat pumps use the ground or ground water, which stays at a more stable temperature. NRCan calls them "the most energy efficient type of heat pump system," and notes they do not need a defrost cycle.^{[Source 1]}
• Air-to-water heat pumps heat water for radiant floors, radiators or fan coils rather than heating air directly.^{[Source 1]}

Air-source systems are further split into ducted and ductless. See Ducted vs ductless heat pumps.

How a heat pump feels different from a furnace

NRCan explains that heat pumps deliver "a higher volume of warm air, and heating that air to a lower temperature (normally between 25 and 45°C)," while furnaces deliver less air at 55 to 60°C.^{[Source 1]} That is about 77 to 113°F from a heat pump versus 131 to 140°F from a furnace. The air from a heat pump register can feel cooler to the hand even when the house is warm.

How much energy it saves

DOE's heat pump systems page says a heat pump "can reduce your electricity use for heating by up to 75% compared to electric resistance heating such as furnaces and baseboard heaters."^{[Source 3]} Actual savings depend on your climate, the heat pump, the system it replaces, and how it is sized and installed.