How heat pumps work

Capturing heat

Heat pumps capture and ‘squeeze’ heat, from either air or the ground, to heat your home and provide you with hot water. They still work even when the temperature outside is well below freezing.

Diagram of an air source heat pump

These units are about the size of a small fridge, and sit outside your home quietly extracting heat from the air, even when it’s below freezing. If you have an air source heat pump, this will always be outside your home and doesn't connect to any pipes in the ground.

Circulating and storing

The pump heats water that is fed separately to:

1. Your radiators
2. A hot water storage cylinder for your taps and shower

Diagram of a ground source heat pump

Long pipes buried beneath your garden soak up the natural, constant warmth held underground (where it’s always about 10°C). This heat is concentrated and used to warm your home. There will be a small unit usually in a cupboard inside your home, but occasionally outside and housed in a specially made weather-proof cupboard. This unit transfers the heat from your underground pipes into your home.

Low and slow

Heat pumps are made to operate low and slow: they work best when they are on all the time, keeping your home at a constant and comfortable temperature.

Unlike with a traditional boiler, this means radiators won't feel burning hot – operating around 40°C rather than 70°C.

Set it and forget it

Keeping your radiator valves fully open will help to make the most of the heat circulating in your system. These are the knobs at the bottom of your radiator, with numbers that usually run from 1 to 5. They are fully open when set to the highest number.

Unlike a traditional boiler, a heat pump takes time to warm up or cool down, so you won't notice an instant effect when you change the thermostat.

'Set it and forget it' – turning the heating off when you leave the house (like you might have done with a gas or oil system) actually costs more with a heat pump. It’s cheaper and more effective to leave your heating on.

The whole system runs on electricity, so you won’t get a gas bill anymore (unless you have a gas hob).

If you have an air source heat pump, it’s important to let air circulate around the unit outside – make sure you keep it clear of obstructions.

Your immersion heater

If the water in your hot water cylinder runs out before the heat pump has had time to top it back up, you can turn on an extra heating element to warm the water.

This “immersion heater” has to be manually switched on, and works like a kettle using electricity, so it can be more expensive compared to heating your water with your heat pump.

If you want to minimise your use of the immersion heater and help to keep your bills low, make sure your heat pump is scheduled to heat water in a way that fits the daily needs and routines of your household. For example, you can set your hot water to be topped up following times of high use, like after everyone has showered in the morning, or after you’ve showered and washed up in the evening.

But don't forget – when your heat pump is heating hot water it won't be warming your radiators at the same time. So you might want to schedule your hot water to be topped up when your heating is less important, such as during the night.