Put your hand near the back of a fridge. It feels warm! A fridge doesn’t “make cold”. It carries heat out of its inside and lets it out at the back.
Air conditioners and heat pumps work in exactly the same way — just for a whole house. In summer they carry heat out of your home. In winter they carry heat in, even from freezing air outside. How is that possible? Let’s build it up step by step.
Heat flows downhill by itself
Heat always flows by itself from something warmer to something colder. A hot drink cools down in a cold room. An ice cube melts in your warm hand. Heat never flows from cold to warm on its own.
To move heat the “wrong” way — out of a cool room into the hot summer air — you need a machine that uses energy.
Even cold air holds heat
You might think freezing winter air has no heat at all. But it does! Everything is made of tiny particles that are always jiggling. As long as they jiggle, there is heat in them — and at −15 °C, the particles in the air are still jiggling a lot. A heat pump collects some of this heat and carries it inside.
The secret: a liquid that boils in the cold
Inside every air conditioner, heat pump and fridge flows a special liquid called a refrigerant. Its superpower is that it boils at very low temperatures — far colder than ice.
Two things happen when a liquid changes into a gas and back:
- When a liquid boils or evaporates, it soaks up heat from around it. That is why wet skin feels cold in the wind.
- When a gas condenses back into a liquid, it gives that heat away again.
The machine can also choose where the refrigerant boils, by changing its pressure. At low pressure it boils even in freezing cold. At high pressure it condenses even when it is hot. You can read why in why water boils at different temperatures.
A trip around the loop
The refrigerant travels round and round a closed loop of pipes. On every lap it passes four stops:
- Evaporator: the cold liquid boils and soaks up heat from the air around it.
- Compressor: a pump squeezes the gas. Squeezing makes it hot, the way a bicycle pump warms up when you use it. This is the part that uses the electricity.
- Condenser: the hot gas gives its heat to the air around it and turns back into a liquid.
- Expansion valve: the liquid squirts through a tiny opening. Its pressure drops, and it becomes very cold again. Then the lap starts over.
Summer: an air conditioner
When it cools, the cold evaporator is inside your home. It soaks heat out of the room air, and a fan blows the cooled air back into the room.
The hot condenser is outside, in the box on the wall with the big fan. That is why the air blowing out of that box feels warm: it is the heat from your rooms.
An air conditioner dries the air too. Water vapour from the room condenses on the cold evaporator, just like on a cold glass. The water drips out through a small pipe.
Winter: a heat pump
Now we want the heat to go the other way. A special valve simply turns the refrigerant’s flow around. The cold evaporator is now outside, soaking up heat even from freezing air. The hot condenser is now inside, warming the air, the radiators or the floor.
That is why many machines can do both jobs: heating in winter and cooling in summer.
Sometimes, on a cold day, a cloud of steam puffs out of the outdoor unit. Don’t worry: ice has formed on its cold coil, and the machine is melting it off.
Why is it so good at heating?
An electric heater turns 1 unit of electricity into 1 unit of heat. A heat pump uses 1 unit of electricity to move about 3 to 4 units of heat into your home. This number is called its COP.
Is that magic? No — and it doesn’t break the rule that energy can never be made from nothing. The electricity runs the compressor. The extra heat isn’t made; it is collected from the outside air.
The colder it is outside, the harder the heat pump has to work, so it moves a little less heat for each unit of electricity.
Heat pumps all around you
- Fridges and freezers are heat pumps too. They carry heat out of the food and release it at the back.
- Ground-source heat pumps collect heat from pipes buried in the garden. Deep in the ground, the temperature stays about the same all year.
- Water-source heat pumps collect heat from a lake or a river.
Go deeper: Pressures and temperatures in the loop for grown-ups and the extra curious
Take a heat pump filled with propane (R-290) on a 0 °C winter day. In the outdoor evaporator, the propane boils at about −8 °C, at roughly 4 bar. The compressor squeezes it to about 14 bar; at that pressure it condenses at about 40 °C, hot enough to warm a house. The gas leaving the compressor is hotter still, often 60–70 °C.
The bigger the difference between the cold side and the hot side, the harder the compressor has to squeeze, and the lower the COP. That is why heat pumps love underfloor heating: the water only needs to be about 35 °C, not 55 °C as in many radiators.
Go deeper: How much heat is in cold air? for grown-ups and the extra curious
Temperature measures how fast particles jiggle, and the jiggling only stops completely at absolute zero: −273.15 °C. Air at −15 °C is still more than 250 degrees above that, so it holds plenty of heat for a heat pump to collect. Heating the water, not finding heat, is the hard part.
Check yourself
In winter, where does most of the heat from a heat pump come from?
Show the answer
From the cold air outside — The electricity only runs the compressor. Most of the heat is collected from the outside air, even when it is freezing.
The electricity only runs the compressor. Most of the heat is collected from the outside air, even when it is freezing.