A heat pump moves heat using refrigerant and a reversing valve, providing both heating and cooling from a single system by transferring thermal energy rather than generating it.
A heat pump isn’t a furnace—it doesn’t create heat through combustion or resistance. It works like an air conditioner that can run in reverse, pulling heat from outdoor air even in cold weather and delivering it inside. The same equipment that keeps your home cool in summer also heats it in winter, using the vapor-compression refrigeration cycle. Here’s exactly how that happens.
The Four-Step Refrigeration Cycle
Every heat pump—whether ducted, ductless, or ground-source—relies on the same closed-loop process where refrigerant changes pressure and state as it circulates. The cycle repeats continuously and uses electrical power almost entirely for the compressor, not for generating heat directly.
- Evaporation: Low-pressure liquid refrigerant enters the evaporator coil and absorbs heat from the surrounding air (indoor air in cooling mode, outdoor air in heating mode). The refrigerant boils into a low-pressure gas.
- Compression: The compressor pulls in the cool gas and compresses it, raising both pressure and temperature dramatically—the refrigerant becomes a hot, high-pressure gas.
- Condensation: The hot gas flows to the condenser coil, where it releases its heat to the opposite air stream (outdoor air in cooling, indoor air in heating) and condenses back into a high-pressure liquid.
- Expansion: The liquid passes through the expansion valve, which drops its pressure and temperature sharply, sending cold, low-pressure liquid back to the evaporator to restart the cycle.
Goodman’s explanation of this process names the compressor, condenser, expansion valve, and evaporator as the four core components, and both Daikin Applied and the IEA confirm the same vapor-compression architecture across system types.
How the Reversing Valve Switches Between Heating and Cooling
The component that makes a heat pump different from a standard air conditioner is the reversing valve. In cooling mode, the indoor coil acts as the evaporator (absorbing heat from your home) and the outdoor coil acts as the condenser (rejecting heat outside). When the thermostat calls for heat, the reversing valve flips the flow—the outdoor coil becomes the evaporator and pulls heat from outdoor air, while the indoor coil becomes the condenser and releases heat into your home. The NRDC describes heat pumps as a special type of air conditioner specifically because of this reversing capability.
Cold-Weather Operation: What Actually Happens
Even when outdoor temperatures drop below freezing, outdoor air still contains heat energy—just less of it. The refrigerant in the outdoor evaporator coil can be colder than the air, so heat naturally transfers from air to refrigerant. The compressor then raises that refrigerant’s temperature enough to heat your home. Multiple sources confirm that modern heat pumps can extract usable heat from outdoor air in below-freezing conditions, though efficiency drops as the temperature falls. This is not a device that “creates” heat in cold weather; it is still transferring existing thermal energy from one place to another.
When a Heat Pump Makes Sense
If your home already has ductwork and you need both cooling and heating, a single heat pump replaces both a furnace and an air conditioner. For homes without ducts, ductless mini-split heat pumps serve individual rooms. The thermodynamic cycle is identical across configurations, but the outdoor unit, indoor air handler, refrigerant piping, and reversing valve must all match the system design. If you’re evaluating specific models for your setup, our tested roundup of best AC heat pump units covers what actually performs well in real homes.
FAQs
Does a heat pump use more electricity than a furnace?
A heat pump’s compressor runs on electricity, but it moves three to four times more heat energy than the electrical energy it consumes. A resistance furnace converts nearly all its electricity to heat; the heat pump’s advantage is moving existing heat rather than generating it, so it typically uses less energy per unit of delivered heat in moderate climates.
Can a heat pump cool a house in summer like a regular AC?
Yes, in cooling mode a heat pump functions identically to a standard air conditioner. It removes heat from indoor air and rejects it outside while also dehumidifying the indoor air. The only mechanical difference is the reversing valve, which stays in the cooling position during summer.
Why do some people say heat pumps don’t work in very cold climates?
Early models lost efficiency sharply below freezing, and some older units had trouble maintaining comfort. Current cold-climate heat pumps use two-speed or variable-speed compressors and enhanced vapor injection to maintain heating output at lower outdoor temperatures, though their capacity and efficiency still drop as it gets colder.
References & Sources
- Goodman Manufacturing. “How a Heat Pump Works.” Explains the four core components and the refrigeration cycle in ducted system terms.
- Air-Conditioning, Heating, and Refrigeration Institute (AHRI). “What Is a Heat Pump / AC System?” Describes heat pumps as reversible air conditioners and the basic cycle.
- International Energy Agency (IEA). “The Future of Heat Pumps: How a Heat Pump Works.” Official explanation of the vapor-compression cycle and cold-weather operation.