How fridges and air conditioners work: the heat pump

Heat only flows downhill on its own

Hot coffee cools and cold drinks warm up. On its own, heat always flows from warm to cold and never the other way. Yet a fridge takes heat out of its already cold interior and dumps it into the warmer kitchen. Just as you need a pump to lift water uphill, moving heat from cold to warm takes work. Fridges and air conditioners are heat pumps that do this work with electricity. In other words, they don't make cold and put it in; they scoop heat out of the inside and throw it away outside.

Evaporation takes heat away

Rub an alcohol wipe on the back of your hand and it feels cool. A liquid needs heat to evaporate, and it takes that heat from the skin it touches. Sweat cools the body the same way. Conversely, when a gas condenses into a liquid, it releases the same amount of heat. Another important fact is that the temperature at which a liquid boils depends on pressure: higher pressure raises the boiling point, lower pressure lowers it. A pressure cooker boiling water hotter than 100 °C is an example. A heat pump exploits both: the latent heat of evaporation and condensation, and the way pressure shifts the boiling point.

Four stages of the refrigerant loop

Inside a heat pump, a substance called refrigerant circulates continuously through pipes. At low pressure, refrigerant boils even at very low temperatures. The compressor squeezes the refrigerant gas, raising its pressure and temperature, so that in the outside condenser it is hotter than its surroundings, gives off heat and becomes liquid. As the liquid passes through a narrow expansion valve its pressure drops sharply and its temperature plunges, and in the inside evaporator it absorbs heat from its surroundings and evaporates back into gas. As the cycle repeats, heat keeps moving from inside to outside.

• Compressor: squeezes the gas and heats it
• Condenser: releases heat outside and turns liquid
• Expansion valve: drops the pressure and chills it
• Evaporator: absorbs heat inside and turns gas

Why the back or sides of a fridge are warm

A warm back or side panel on a fridge is normal, not a fault. Both the heat taken from inside and the electrical energy used to run the compressor come out into the kitchen through the condenser there. If this heat can't escape well, the refrigerant doesn't cool enough and the fridge uses more electricity for the same job. So leave the gap to the wall and sides that the manufacturer recommends, and now and then clean dust off the rear condenser or the bottom vents. A spot in direct sunlight or right next to the cooker also reduces efficiency.

Does an open fridge cool the room?

On a hot day it seems an open fridge should cool the room, but the result is the opposite. With the door open, the fridge just takes heat from the room and puts it back into the room, adding the electrical energy used by the compressor as extra heat. It may feel cool right in front of the door, but the room as a whole gets warmer. This is why an air conditioner's outdoor unit must be outside. If the side that dumps heat were indoors, it would just be moving heat from the room to the room, and nothing would cool. Portable air conditioners have an exhaust hose for the same reason.

Air conditioners and heat pumps for heating

An air conditioner works like a fridge, with the indoor unit as the evaporator and the outdoor unit as the condenser. When room air passes over the cold evaporator, water vapour condenses like dew, which is why air conditioners have a drain pipe and why they dehumidify as they cool. Reverse the refrigerant flow with a valve and it becomes a heater that moves heat from outdoors to indoors. Because a heat pump moves heat rather than generating it, under the right conditions it can move several times more heat than the electrical energy it uses. When it is very cold outside, though, there is less heat to collect and efficiency drops.

Using them efficiently

Knowing the principle shows you how to save electricity: let less heat in and help heat get out. Widely used food storage guidelines are 4 °C or below in the fridge and minus 18 °C or below in the freezer, so check your settings occasionally. Let hot food cool a little before putting it in, but don't leave it out at room temperature for too long.

• Don't pack the fridge so tight that cold air can't circulate
• Open the door less often and for less time
• Clean air conditioner filters regularly
• Block sunlight with curtains and circulate air with a fan
• Keep the outdoor unit clear and well ventilated

Common misconceptions and cautions

A fan does not make air colder. Its motor actually adds a little heat, but the moving air evaporates sweat from your skin faster, so you feel cooler. Running a fan in an empty room does nothing to cool it. Working on refrigerant pipes or topping up refrigerant involves high-pressure gas and should be left to qualified technicians. If a fridge or air conditioner smells of burning, makes unusual noises or leaks water, switch it off and ask the manufacturer's service to inspect it. Older appliances can lose efficiency, so if you are thinking of replacing one, compare official labels such as energy efficiency ratings.