A heat pump heats and cools your home by moving heat through a refrigerant loop.
The central idea is heat transfer. A compressor, two heat exchangers, an expansion device, refrigerant, fans or pumps, and controls work together so heat can be absorbed in one place and released in another.
For an air-source heat pump in winter, the outdoor coil is the heat-absorbing side of the circuit. Refrigerant leaves that coil as a vapor, the compressor raises its pressure and temperature, and the indoor coil releases that heat into the home. After the refrigerant passes through an expansion device and drops in pressure and temperature, it can return outdoors and absorb heat again.
Cooling uses the same vapor-compression principle. The system changes which coil absorbs heat and which coil releases it, so indoor heat is moved outdoors much like a central air conditioner.
Swipe or scroll sideways to see the full loop.
- 1Absorb heat
Cold, low-pressure refrigerant passes through the outdoor coil and absorbs heat from outdoor air.
- 2Compress
The compressor raises refrigerant pressure and temperature so heat can be released at the other coil.
- 3Release heat
Hot refrigerant passes through the indoor coil and transfers heat to indoor air or another distribution medium.
- 4Drop pressure
The expansion device lowers refrigerant pressure and temperature before the cycle begins again.
In winter, the outdoor coil absorbs heat and the indoor coil releases it.
The outdoor air does not have to feel warm for heat transfer to occur. The refrigerant entering the outdoor coil is colder than the surrounding air, so heat can move from the air into the refrigerant. As the refrigerant absorbs heat, it evaporates into a vapor.
The compressor then raises the vapor’s pressure and temperature. That hot, high-pressure refrigerant travels to the indoor coil. Indoor air moving across the coil receives heat from the refrigerant, and the refrigerant condenses back toward a liquid state.
After the refrigerant passes through the expansion device, its pressure and temperature fall. It is then ready to return to the outdoor coil and repeat the process.
Electricity runs the compressor, fans, controls, pumps where applicable, and any auxiliary electric heat. The heat pump’s defining job is to transfer environmental heat through the refrigerant circuit.
Cooling is the same basic cycle with the indoor and outdoor coil roles reversed.
A reversing valve changes the refrigerant path. The indoor coil becomes the heat-absorbing side, while the outdoor coil becomes the heat-rejecting side. That is why a heat pump can replace the cooling function of a conventional central air conditioner.
| Coil | Heating | Cooling |
|---|---|---|
| Outdoor coil | Absorbs heatRefrigerant evaporates as it collects heat outdoors. | Releases heatRefrigerant condenses as the captured heat is rejected outdoors. |
| Indoor coil | Releases heatRefrigerant condenses as heat is transferred into the home. | Absorbs heatRefrigerant evaporates as it collects heat from indoor air. |
Compare a heat pump with a central air conditionerBack to the heat-pump definition guide
The names of the parts matter less than the job each one performs.
- 2Compressor
Raises refrigerant pressure and temperature
The compressor drives refrigerant around the circuit and creates the pressure relationship that lets heat be absorbed at a lower temperature and released at a higher temperature.
- 13Coils
Transfer heat between refrigerant and air or another medium
One coil acts as the evaporator and absorbs heat while the other acts as the condenser and releases heat. Their roles switch when a reversible heat pump changes between heating and cooling.
- 4Expansion device
Drops refrigerant pressure before the heat-absorbing side
Reducing pressure lowers the refrigerant temperature so it can absorb heat again when it reaches the evaporator side of the circuit.
- Reversing valve
Changes the refrigerant path between heating and cooling
This is the feature that lets a reversible air-source heat pump use the same basic refrigeration hardware for both seasons.
- Fans and blowers
Move air across the heat exchangers
The outdoor fan moves ambient air across the outdoor coil. A ducted indoor blower or ductless fan moves room air across the indoor coil so the transferred heat reaches the occupied space.
- Controls
Coordinate capacity, mode, backup heat, and defrost
Thermostat commands and equipment controls determine when the system heats, cools, changes capacity, starts backup heat, or enters a defrost cycle.
Cold outdoor air can still be a heat source, but available capacity and efficiency change with conditions.
The outdoor coil operates at a temperature low enough to absorb heat from the surrounding air. As outdoor temperature falls, an air-source system generally has a harder operating job and model-specific heating capacity becomes more important.
That is why the question “does a heat pump work in the cold?” cannot be answered from the category name alone. The useful comparison is the home’s heating load at the relevant outdoor design condition against the candidate equipment’s published low-temperature performance.
Cold-climate equipment is designed and rated to maintain useful performance at lower outdoor temperatures, but exact suitability still depends on the house, the selected model, distribution, controls, and any backup strategy.
Frost on the outdoor coil is a normal operating problem the system has to clear.
In heating mode, the outdoor coil can run below freezing. Under the right combination of outdoor temperature and humidity, moisture can freeze on that coil. Too much frost restricts airflow and reduces heat transfer.
Many air-source heat pumps clear frost with a reverse-cycle defrost. For a short period the refrigerant circuit is switched so the outdoor coil becomes warm enough to melt accumulated frost. System controls determine when defrost starts and stops.
Some ducted systems use supplemental heat during defrost so indoor supply air does not become uncomfortably cool. Defrost behavior varies by equipment and conditions, so a brief defrost event is not by itself evidence of a fault.
Auxiliary or backup heat is a separate heating source, not another stage of the refrigerant cycle.
Some systems include electric resistance heat, a furnace, or another source that can supplement the heat pump. Backup may be used because of equipment design, very cold conditions, a rapid thermostat recovery, defrost support, or a hybrid control strategy.
The presence of backup heat does not automatically mean the heat pump is undersized or failing. What matters is whether the backup behavior matches the intended system design, the home’s load, the equipment’s available capacity, and the controls.
A dual-fuel system is a distinct configuration in which a heat pump and furnace share the heating job. The changeover strategy is a controls and equipment decision, not a universal outdoor-temperature rule.
Ducted and ductless heat pumps move heat in the same way but deliver heated and cooled air differently.
- Ducted
Central heat pump
An indoor coil and air handler deliver conditioned air through ducts. The outdoor unit, refrigerant circuit, indoor equipment, ducts, and controls work as one system.
Air-source heat pumps- Ductless
Mini-split heat pump
Refrigerant lines connect an outdoor unit to one or more indoor units that serve rooms or zones directly. There is no central duct distribution requirement.
Ductless and mini-split heat pumps- Ground-source
Geothermal heat pump
The source and sink change from outdoor air to the ground or water loop, but the system still uses a heat-pump cycle to move thermal energy into or out of the building.
Geothermal heat pumps
Common questions about how the cycle behaves.
Does a heat pump make heat?
The system uses electricity to run the refrigeration equipment, but its primary heating process is heat transfer. Refrigerant absorbs heat at one heat exchanger and releases it at the other.
How can it get heat from cold outdoor air?
The refrigerant at the outdoor coil can be colder than the surrounding air. That temperature difference lets heat move into the refrigerant even when outdoor air feels cold to a person.
Is a heat pump basically an air conditioner run backward?
That is a useful starting point for a reversible air-source heat pump. In cooling mode it operates like an air conditioner. In heating mode a reversing valve changes the refrigerant path so the indoor and outdoor coils exchange roles.
Why does the outdoor unit sometimes frost up in winter?
The outdoor coil can be below freezing while it is absorbing heat. Moisture can freeze on the coil, so the system periodically needs to remove that frost.
Why would auxiliary heat come on?
Depending on system design, controls may call for supplemental heat during very cold conditions, rapid recovery from a large thermostat setback, defrost, or other conditions where additional heating is needed.
Does a ductless mini-split work differently?
The heat-transfer cycle is fundamentally similar. The main difference is how conditioned air is delivered: ductless indoor units serve rooms or zones directly instead of sending air through a central duct system.
Do all heat pumps use outdoor air?
No. Air-source systems exchange heat with outdoor air. Ground-source or geothermal systems exchange heat with the ground or water loop. Other heat-pump equipment families use different heat sources and sinks.
Does understanding the cycle tell me what size system to buy?
No. The refrigeration cycle explains how the equipment moves heat. Equipment size and selection require a house-specific load basis plus model-specific heating and cooling performance.