How Does a Heat Pump Work? The Cycle Explained Simply
How a heat pump works: the refrigerant cycle, four core parts, heating and cooling modes, defrost and backup heat, all explained simply with real US numbers.
A heat pump does not make heat the way a furnace does. It moves heat from one place to another using a refrigerant, a compressor, and two coils, then runs the same loop backward to cool your home in summer. That single reversible cycle is why one box can both heat and cool, and why it can deliver 2 to 4 units of heat for every unit of electricity it uses.
The one idea behind every heat pump
Heat naturally flows from warm to cold. A heat pump forces that flow to run the direction you want by manipulating the pressure and temperature of a refrigerant. Compress a gas and it gets hot; let it expand and it gets cold. If you make the refrigerant colder than the outdoor air, that air, even at 30F, has heat to give up. If you make it hotter than your indoor air, it dumps heat inside. Everything else is plumbing and controls built around that trick.
This is the same physics your refrigerator and window air conditioner use. The difference is that a heat pump has a valve that lets it run the cycle in either direction, so it can heat in winter and cool in summer with the same hardware. That is worth understanding before you compare a heat pump versus a furnace or a heat pump versus an air conditioner, because the moving-heat idea is exactly what separates them.
A heat pump does not burn fuel to create warmth. It relocates heat that already exists, which is why it can beat 100 percent efficiency.
The four core parts
Almost every air-source system uses the same four components arranged in a closed loop. Get these straight and the cycle explains itself.
In a ducted system the indoor coil sits inside an air handler that blows conditioned air through your ductwork. In a ductless setup, that indoor coil lives inside a wall-mounted head in each room. The outdoor box, often called the condenser, holds the compressor and the outdoor coil. If you want the room-by-room version, our guide to mini split heat pumps walks through how the ductless layout works.
The heating cycle, step by step
Here is the loop in winter, following the refrigerant all the way around.
- Absorb. Cold liquid refrigerant flows through the outdoor coil, which now acts as the evaporator. Because the refrigerant is colder than the outside air, heat moves from the air into the refrigerant and boils it into a low-pressure gas.
- Compress. The compressor squeezes that gas, raising its pressure and its temperature until it is hotter than the air inside your home.
- Release. The hot gas travels to the indoor coil, now acting as the condenser. Your blower pushes household air across it, the refrigerant gives up its heat, warms your rooms, and condenses back to a liquid.
- Expand. The warm liquid passes through the expansion valve, its pressure drops, and it turns cold again, ready to head back outdoors and repeat.
Notice that at no point did anything burn. The electricity only runs the compressor and fans. That is why the physics ratio here, the COP, is commonly 2 to 4: you get several units of heat out for each unit of power in. As the outdoor air gets colder there is less heat to harvest, so the COP falls, but a modern cold-climate system still pulls useful heat well below freezing.
The cooling cycle is the same loop, reversed
In summer the reversing valve flips the refrigerant’s direction. Now the indoor coil becomes the evaporator: it gets cold, and household air blown across it gives up heat and drops in temperature, while moisture condenses out and drains away. The outdoor coil becomes the condenser and dumps that collected heat outside. In cooling mode a heat pump is, functionally, a central air conditioner. That is the whole reason a heat pump can replace both your furnace and your AC with one machine.
What happens when it gets really cold
Two things change in deep cold, and both are normal.
The defrost cycle
When the outdoor coil runs below freezing, moisture in the air frosts onto it and blocks heat transfer. Periodically the system runs a defrost cycle: it briefly flips to cooling mode to warm the outdoor coil and melt the ice, then switches back to heating. You may see steam off the outdoor unit and hear a whoosh. It lasts a few minutes and is not a malfunction.
Auxiliary and backup heat
On the coldest days the heat pump may not keep up with your home’s demand on its own. That is when auxiliary heat engages: electric resistance strips in the air handler, or in a dual fuel system a gas furnace that takes over below a set temperature. The “emergency heat” setting on your thermostat is a manual override that runs backup heat directly, useful only if the heat pump itself fails. For how much cold a heat pump truly handles, see do heat pumps work in cold climates.
Air-source versus ground-source
Everything above describes an air-source heat pump, which pulls heat from outdoor air. A geothermal, or ground-source, system runs the identical cycle but exchanges heat with the ground or groundwater instead. A few feet down, soil stays close to 50F year round, so the system never faces bitter air and holds a high COP through winter. The tradeoff is the buried loop field, which is why geothermal heat pumps cost far more to install than an air-source heat pump.
| Feature | Air-source | Ground-source (geothermal) |
|---|---|---|
| Heat exchanged with | Outdoor air | Ground or groundwater loop |
| Winter efficiency | Falls as air gets colder | Stays high and steady |
| Typical installed cost | ~$6,000 to $18,000 | ~$18,000 to $45,000+ |
| Main cost driver | Size, efficiency, ductwork | The buried loop field |
* Ranges are typical 2026 US installed prices and vary widely by region, home, and contractor. Get a written load calculation and itemized quotes before comparing.
Why sizing and refrigerant matter to the cycle
The cycle only runs efficiently when the equipment matches the home. Capacity is measured in tons, where one ton equals 12,000 BTU per hour. The right number comes from an ACCA Manual J load calculation, not a rule of thumb. An oversized unit short-cycles: it satisfies the thermostat fast, shuts off, and never runs long enough to dehumidify or hit peak efficiency. Undersized, it leans on expensive backup heat. Our guide on what size heat pump you need covers this, and installation covers who should run the numbers.
The refrigerant chemistry is changing too. Legacy R-410A is being phased down, and new systems ship with lower global-warming-potential refrigerants like R-454B or R-32. The cycle works the same way; the fluid inside it is simply cleaner. Once you understand the loop, the rest of the decision comes down to fit and money, which is where heat pump cost and available tax credits and rebates come in. If you are still deciding whether this technology suits you at all, start with what is a heat pump.
Frequently asked questions
How does a heat pump work in simple terms?
It moves heat instead of creating it. A refrigerant absorbs heat from one place, a compressor raises its temperature, and it releases that heat somewhere else. Run the loop one way to heat your home, reverse it to cool.
How can a heat pump heat a house when it is cold outside?
Even cold air holds heat. By making the refrigerant colder than the outdoor air, the system pulls heat from that air and concentrates it with the compressor. Cold-climate models keep working well below freezing, though efficiency drops as it gets colder.
Does a heat pump both heat and cool?
Yes. A reversing valve flips the refrigerant flow direction. In summer the indoor coil gets cold and removes heat and humidity from your home, exactly like a central air conditioner, so one machine replaces both furnace and AC.
What are the main parts of a heat pump?
A compressor, two coils (indoor and outdoor), an expansion valve, and a reversing valve, all connected in a closed refrigerant loop. The indoor coil usually sits in an air handler or a ductless wall head.
What is the defrost cycle?
In freezing weather, frost builds on the outdoor coil and blocks heat transfer. The system briefly reverses to warm that coil and melt the ice, then returns to heating. Steam and a brief whoosh are normal, not a fault.
What is auxiliary or emergency heat?
Auxiliary heat is backup that kicks in automatically on the coldest days, usually electric resistance strips or, in a dual-fuel system, a gas furnace. Emergency heat is a manual override you use only if the heat pump itself fails.