A heat pump heats or cools a building by moving heat rather than producing all of it directly. Refrigerant circulates through a closed system, absorbing heat as it evaporates and releasing heat as it condenses. A compressor raises the refrigerant’s pressure and temperature so heat can flow toward the chosen destination.
What happens in five steps
1. Low-pressure refrigerant absorbs heat
In heating mode, cold refrigerant enters the outdoor heat exchanger. It is colder than the surrounding air, ground, or water source, so heat flows into it. The refrigerant boils at low temperature because its pressure is low, changing from a liquid-rich mixture into vapor.
2. The compressor raises pressure and temperature
The compressor draws in low-pressure vapor and performs mechanical work on it. The resulting vapor leaves at higher pressure and a temperature above the indoor space. Electrical energy supplied to the compressor joins the heat collected at the outdoor exchanger.
3. Hot refrigerant releases heat indoors
In the indoor heat exchanger, heat flows from the hot refrigerant into indoor air or water. As it releases energy, the refrigerant condenses into a high-pressure liquid. A fan, pump, ducts, or hydronic loop distributes the delivered heat through the building.
4. An expansion device drops the pressure
The liquid passes through a metering device that restricts flow. Its pressure and temperature fall, and some refrigerant flashes into vapor. This prepares a cold, low-pressure mixture that can absorb heat again at the outdoor exchanger.
5. A reversing valve changes the direction for cooling
Many space-conditioning heat pumps can swap which coil acts as evaporator and which acts as condenser. In cooling mode, the indoor coil absorbs room heat and the outdoor coil releases it. The compressor still drives the same basic vapor-compression cycle.
How can it collect heat from cold outdoor air?
“Cold” does not mean zero thermal energy. Heat can flow from outdoor air into refrigerant whenever the refrigerant is colder than that air. The compressor then raises the vapor to a temperature useful indoors. As outdoor temperature falls, available capacity and efficiency can decrease, but modern cold-climate systems are designed and tested to operate well below freezing.
How can it deliver more heat than the electricity it uses?
A resistance heater turns electrical input into roughly the same amount of heat at the point of use. A heat pump uses electrical work to move additional heat from another place. If it delivers three units of heat for one unit of electrical energy during a particular condition, its coefficient of performance is three. This does not create energy: the delivered heat equals environmental heat plus electrical work, minus losses.
Performance changes with the temperature difference the machine must overcome, fan and pump energy, cycling, defrost, installation, and equipment condition. One advertised rating cannot describe every hour of operation or every building.
Why the outdoor unit sometimes makes steam
In heating mode, the outdoor coil can be colder than freezing. Water vapor condenses and freezes on it, blocking airflow if allowed to accumulate. The controller periodically runs a defrost cycle, often reversing the refrigeration circuit long enough to warm the outdoor coil. Meltwater drains away, and a brief cloud of water vapor can look like smoke. Persistent heavy ice or blocked drainage is not normal.
What auxiliary heat does
Some systems include electric resistance elements or another heating source for very cold conditions, rapid recovery, or defrost. Auxiliary heat can provide needed capacity but may cost more to operate than the compressor. Controls, sizing, climate, rates, and building heat loss determine when a backup source is useful.
Air, ground, and water sources
An air-source heat pump exchanges heat with outdoor air. Ground-source systems use buried loops or wells to exchange heat with relatively stable ground temperatures. Water-source systems use a suitable water loop. Ductless mini-splits distribute refrigerant to one or more indoor units; ducted systems move conditioned air through ducts. Heat-pump water heaters transfer heat from surrounding air into a storage tank.
Why sizing and the building matter
An oversized fixed-capacity unit may cycle frequently, while an undersized system may rely heavily on backup heat or fail to maintain comfort at design conditions. Variable-speed equipment can match a wider range of loads. Insulation, air sealing, ducts, airflow, climate, humidity control, and commissioning influence comfort and energy use as much as the equipment label.
A service safety boundary
Heat pumps contain mains voltage, capacitors, moving fans, high refrigerant pressures, hot and cold surfaces, and refrigerants that require specific handling. Some newer refrigerants are mildly flammable. Do not open the sealed circuit, bypass interlocks, or diagnose it by touching live components. Qualified technicians must follow the model instructions, codes, recovery rules, and electrical isolation procedures.
The useful mental model
A heat pump is a reversible heat ferry. Refrigerant picks up heat where it evaporates, the compressor raises its delivery temperature, and the refrigerant drops the heat off where it condenses. The expansion device resets the pressure for another trip.
