Explainer #10
How do a fridge and a heat pump move heat?
A fridge doesn't make cold: it moves heat from inside the box to the kitchen. A closed loop of pipe holds a fluid that boils even far below freezing. A pump squeezes the gas so it gets hot, a coil on the back gives that heat to the kitchen air and the gas turns back into liquid, a tiny gap drops the pressure so the liquid boils and turns ice cold, and a cold coil inside soaks up heat from the food. A heat pump is the very same loop, with the cold coil outdoors and the hot coil inside the house.
The principle
A fridge doesn’t make cold. It moves heat. Heat is taken out of the box and dumped into the kitchen, using a closed loop of pipe filled with a special fluid.
Step 1: A fluid that boils very easily. The fluid in the loop boils at a very low temperature, even far below freezing. A pump pushes it round and round the loop, so the same fluid is used over and over.
Step 2: Squeezed gas gets hot. The pump squeezes the gas into a much smaller space. Just like a bike pump gets warm when you pump up a tyre, squeezed gas heats up. Now it is hotter than the kitchen.
Step 3: The hot coil gives heat away. Heat always flows from something hot to something cooler. So as the hot gas flows through the coil on the back of the fridge, its heat flows out into the kitchen air (that is why the back of a fridge feels warm). As it cools, the gas turns back into a liquid.
Step 4: A tiny gap makes it ice cold. The liquid is then pushed through a very narrow gap (often a long, thin tube). Behind the gap the pressure is much lower. At low pressure the fluid boils at a much lower temperature, so part of it starts to boil straight away, and that makes the rest ice cold, much colder than the inside of the fridge.
Step 5: The cold coil soaks up heat. The ice-cold fluid flows through the coil inside the fridge. The food and the air in there are warmer, so heat flows into the coil. That heat makes the rest of the fluid boil into gas, and the gas goes back to the pump to start again. The fridge stays cold because heat keeps being carried out.
Step 6: A heat pump is the same loop. A heat pump uses exactly the same loop. The only difference is where the coils are. The cold coil sits outdoors. Even freezing air still holds some heat, and because the coil is colder still, heat flows from the air into it. The pump squeezes the gas so it gets hot, and the hot coil gives the heat to the air inside the house. A fridge pumps heat out of a box, and a heat pump pumps heat into a house. Many heat pumps can also run the loop the other way in summer, to move heat out of the house and cool it down.
Common myth
Leaving the fridge door open cools the kitchen
It doesn’t. The fridge only moves heat from one side of the loop to the other, and the coil on the back dumps it right back into the same kitchen. On top of that, the pump itself turns electricity into extra heat. So with the door open, the kitchen slowly gets a little warmer, not cooler.How this video was made: Rendered 3D animation of a simplified loop. The fluid is drawn as dots that bunch up where it is squeezed, coloured blue where it is cold and red where it is hot. Sizes, pipe lengths and speeds are simplified.
