This is an oversimplified explanation of how heat pumps work. First, what is a heat pump?

A heat pump literally pumps heat from one place to another. You may open your fridge and witness that it’s colder inside than out, but in terms of physics, it’s actually lacking heat. The way that happens, is that the heat inside is pumped outside. When your older fridge hums or makes a motor noise, that’s the motor in the compressor making your room warmer by moving the heat from the inside.

How to pump heat

You may know that there are a few great ways to cool something down. Dropping a can of soda into a basin filled with ice and water will lower the temperature of the can. The water, actually, gets a little warmer as well, and between them they reach an equilibrium. Leave the can in there too long, though, and the ice will melt and the water, the can, and the basin, will all have the same temperature as the surrounding room. You also have the problem of making the ice in the first place.

Another way to cool something down is to blow over it. You may have done an experiment at school where you blew air over some ethyl alcohol to see that it cooled down due to evaporation. This is how sweating cools you down.

Another way, which is similar but different, is to reduce the pressure on a liquid. In a lower pressure environment, the boiling point reduces which, if the temperatures are right, means that the liquid will become a gas. As a gas it absorbs energy from the surroundings. When it’s then compressed by the motor, it contains heat that it can release as needed. This is how a heat pump works.

How residential heat pumps work

There are three typical examples of heat pumps in homes:

• Refrigerators;
• Mini splits (air conditioners); and
• Hybrid hot water heaters

We’re all familiar with refrigerators. They heat your room in order to cool the inside.

Air conditioners do something similar, when they’re cooling your home. They take heat from your room, and pump it outside by sending the lower-density refrigerant into the home-side of the mini split, and heating the refrigerant in your home before compressing the refrigerant in a machine outside. For a window air-conditioner, that all happens on one side of the other of your machine. (Source: US Department of Energy)

You can use air conditioners for heating by just reversing the compressor. It pumps heat from outside into your home. Modern heat pumps use the ‘heat’ outside to warm your home, even if the temperature outside is really low. They can do this because there’s always heat outside – absolute zero, or 0 Kelvin, does not occur naturally anywhere in the known universe. All that is required is that the refrigerant needs to be capable of absorbing the heat that is in the air outside.

Hybrid hot water heaters have a compressor on the top. This takes heat from the room they’re in, and pumps it into the water. They’re “hybrid” because they have a backup electrical-resistance method of heating the water if hot water is required urgently.

There are heat pump clothes dryers on the market, but they are horribly slow and expensive at the moment. Maybe something for the future?

Efficiency

An electric oven, a toaster, or a bar heater, get warm by pushing electricity through a wire that doesn’t quite have enough capacity. These ‘resistance’ heaters convert electricity into heat at a very high efficiency. The only loss is the glow of the wire. It is the heat that is generated in this process that makes incandescent light bulbs inefficient – the heat is a waste product when trying to make light.

A heat pump uses electricity to move the already-existing heat from where it is, to where it’s wanted. Because of this, they are more than 100% efficient – they move more heat in terms of joules of energy, than they take to run.

Conclusion

Heat pumps do not require the source of the heat to be in the air. You can get ground-source heat pumps which use the natural warmth of the soil to pump heat into the home. These are more expensive to install, but can work efficiently at any outside temperature because only a few centimeters down, the temperature of the soil varies surprisingly little over the course of a year.

Heat pumps are more efficient than the alternatives and the brief explanation above attempts to explain why. Let me know in the comments if there’s something I should clarify, of if there’s anything I missed.

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