Your turn
Each lab is one step from the video. Press, slide and flip, and see what happens.
Each lab isolates one part of the cycle. The models are simplified to show the idea.
Tap the buttons and slide the sliders.
Slide the air temperature down. Do the molecules ever stop?
Molecular speed scales with the square root of absolute temperature. Where does the motion stop?
Slide it to cold. The tiny bits slow down, but keep wiggling!
The molecules are drawn huge and slowed down so you can see them.
Flip the machine. Which side gets the heat?
Same machine, reversed orientation: the cold side faces the heat source, the hot side the heat sink.
Tap the button. Where does the warmth go?
Step one blob of refrigerant around the loop. Where does it pick up heat, and where does it drop it?
Evaporator, compressor, condenser, expansion valve: follow the temperature and pressure at each stop.
Press the button. Where does the juice get cold, and where does it get hot?
A simplified loop: real units have fans, tubes of coil and valves not shown.
Switch seasons. Which coil is cold, and which way does the heat go?
Reversing the flow swaps the evaporator and condenser roles.
Tap winter, then summer. Where does the warmth go?
Drag the outdoor temperature down. How much heat do you still get for each unit of electricity?
COP falls as the temperature lift grows. ENERGY STAR cold-climate units must reach COP ≥ 1.75 at 5 °F.
The line is illustrative, drawn inside the sourced ranges; real units vary.
A heat pump is a machine that moves heat. Most have a box outside the house. It keeps the house warm, even on a snowy night. It does not make heat. It moves heat!
Air is made of teeny tiny bits that wiggle all the time. Wiggling is warmth. In cold air the bits slow down, but they never stop. So even snowy air has a little warmth in it.
Your fridge takes warmth out of the food and puts it out the back. That's why the back feels warm. A heat pump does the same thing, turned around: it takes warmth from outside and puts it in your house.
Inside the pipes, a special juice goes round and round. Outside, it is colder than the snow, so it soaks up warmth. A squeezer makes it hot. Inside, it gives its warmth to the room. Then it gets cold again and goes back for more.
In summer, the heat pump runs the other way. It takes warmth out of your house. One box keeps you warm in winter and cool in summer.
A heat pump moves heat instead of making it. It uses electricity to carry heat from a cooler place to a warmer place, just like your refrigerator does.1 In winter it gathers heat from the cold outdoor air and brings it inside. In summer it runs the other way and carries heat out of your home.13
Because moving heat takes much less energy than making it, an air-source heat pump can deliver two to four times more heat than the electrical energy it uses.2 A typical home heat pump gives about three or four units of heat for every unit of electricity.46
Heat is really the jiggling of tiny molecules. The faster they move, the hotter something is.9 When air cools down, its molecules slow down, but they don't stop. Heat energy is there at any temperature above absolute zero: minus 273 °C.6 So even on a day far below freezing, "there is still some amount of heat energy in the air."3
The trick is getting that heat to flow the "wrong" way. On its own, heat always flows from warmer things to colder things. To push heat from the cold outdoors into a warm house, you need to put in work, and that's the job of the heat pump.8
Your fridge is already a heat pump. It pulls heat out of the food compartment and gets rid of it through the coil on its back. That's why the back of a fridge feels warm.7 (See how a refrigerator works for the kitchen version.) A home heat pump does the same thing on a bigger scale. The outdoors plays the part of the inside of the fridge (the place heat is taken from), and your home plays the part of the warm back (the place heat is dumped).18
A typical heat pump has four main parts joined in a closed loop of pipe. A special fluid called a refrigerant flows around and around the loop.710 The whole trick depends on one fact: at low pressure a refrigerant easily soaks up heat and boils into a gas, and at high pressure the gas gives its heat away and turns back into a liquid.3
| Stop | Part (heating mode) | What happens to the refrigerant |
|---|---|---|
| 1 | Evaporator (outdoor coil) | Arrives as a very cold, low-pressure mix of liquid and vapour, colder than the outdoor air. Heat flows into it and it boils into a gas.10 |
| 2 | Compressor | Squeezes the gas, which raises both its pressure and its temperature. Now it's hotter than your room.10 |
| 3 | Condenser (indoor coil) | The hot gas gives its heat to the air or water in your home and condenses back into a liquid.710 |
| 4 | Expansion valve | The pressure drops suddenly, and so does the temperature. The cold liquid-and-vapour mix heads back outside for another lap.10 |
You can feel stop 2 for yourself. Pump up a bike tire quickly and the pump barrel gets warm, because squeezing air fast turns your work into heat.11 The compressor does the same thing to the refrigerant, powered by electricity.8
In summer a heat pump works just like a standard air conditioner. It soaks up unwanted heat indoors and releases it outside.3 What makes it different is a small part called a reversing valve, which changes the direction the refrigerant flows.2 Flip it, and the outdoor coil becomes the cold one that gathers heat, while the indoor coil becomes the hot one. One machine heats in winter and cools in summer. In winter the same valve also runs the cycle in reverse for a short defrost cycle, which clears frost off the outdoor coil.2
Engineers measure a heat pump's efficiency with its coefficient of performance, or COP: the heat it delivers divided by the electricity it uses.5
COP = heat delivered ÷ electricity used e.g. 3 units ÷ 1 unit = 3
A plain electric heater turns each unit of electricity into one unit of heat, so its COP is 1.8 In the U.S. Department of Energy's example, electric resistance heat gives about 3,400 Btu per kilowatt-hour, while a heat pump with an HSPF rating of 10.3 delivers about 10,300 Btu of heat per kilowatt-hour over a heating season.2 That's why a heat pump can cut the electricity used for heating by up to 75% compared with electric furnaces and baseboard heaters.1
This doesn't break the laws of physics. The extra heat isn't created. It's collected from outdoors.68
Yes. The colder it is outside, the harder a heat pump has to work to lift heat up to room temperature, so its COP and heating output go down.8 Older air-source models weren't always used where winters stay below freezing for long stretches, but newer technology now makes them a real heating option in cold regions.12
To earn ENERGY STAR's cold-climate label, a heat pump must still reach a COP of at least 1.75 at 5 °F (−15 °C) and deliver at least 70% of its mild-weather (47 °F) heating capacity.5 These units keep working below 5 °F, though pairing them with a backup heat source is most efficient on the very coldest days.4 The U.S. Department of Energy has also run a Cold Climate Heat Pump Challenge with manufacturers, aiming at units designed for 5 °F and even −15 °F.12
Every home has a balance point: the outdoor temperature where the heat pump's output exactly matches the heat the house is losing. It's usually well below 40 °F. Below it, backup heat helps out.2
A furnace makes heat by burning fuel or by running electricity through a hot element. A heat pump moves heat that's already there, so it delivers more heat for the energy it uses.14 An electric heat pump burns no fuel, so it gives off no carbon monoxide or other combustion fumes indoors, and there's no fuel tank to fill.4 Some homes use both. A "dual fuel" system lets a heat pump do most of the work and switches to a gas or oil furnace when that makes more sense.4
A heat pump moves heat instead of making it. A cold fluid called a refrigerant soaks up heat from the outdoor air, a compressor squeezes it so it gets hotter than your room, and an indoor coil hands that heat to your home. Then the fluid is cooled again and goes back for more.
Even freezing air still holds heat energy. The refrigerant in the outdoor coil is made even colder than the air, so heat flows into it. The compressor then raises its temperature so the heat can be released indoors. Cold-climate models are tested at 5°F and keep working below that.
It uses the same refrigerant cycle as an air conditioner. The difference is a reversing valve, which lets a heat pump run the cycle in either direction: it cools your home in summer and heats it in winter.
It doesn't create energy. The electricity runs the compressor and fans, and the rest of the heat is moved in from outdoors. That's why an air-source heat pump can deliver about two to four units of heat for each unit of electricity.
As it gets colder outside, a heat pump has to work harder, so its efficiency and heating output drop. In very cold places it is often paired with a backup heat source. Good installation also matters a lot: the U.S. Department of Energy warns that poor airflow, leaky ducts and the wrong refrigerant charge all hurt performance.
If you switched from gas or oil, heating now shows up on your electric bill instead of your fuel bill. Bills can also jump if backup electric resistance heat strips switch on more than they should; the U.S. Department of Energy lists badly wired backup heat as one cause.
An air-source heat pump swaps heat with the outdoor air. A ground-source (geothermal) heat pump swaps heat with the ground a few feet down, which stays around 40–70°F all year. Ground-source systems are more efficient but cost more to install.