How Heat Pumps Work: A Plain-English Guide
Heat pumps are often described as a fridge working in reverse, and that is a helpful starting point. Instead of burning fuel to make heat, they use a small amount of electricity to move heat from one place to another. Understanding the basics helps you judge claims and ask installers the right questions.
The basic idea
Even cold outside air contains heat energy. An air source heat pump has an outdoor unit with a fan that draws in air and passes it over a coil containing a refrigerant, a fluid that boils at a very low temperature. The refrigerant absorbs the heat and turns to gas. A compressor then squeezes the gas, which raises its temperature. The hot gas passes through a heat exchanger, where it gives up its heat to the water in your heating and hot water system. The refrigerant cools, turns back to liquid and the cycle repeats.
Types of heat pump
- Air source heat pumps take heat from outside air. They are the most common type for UK homes because they are cheaper and simpler to install.
- Ground source heat pumps take heat from pipes buried in the ground or in a borehole. They are more efficient but need more space and cost more to install.
- Air-to-air systems heat the air directly and can work like air conditioning in reverse, but they do not usually supply hot water or radiators.
What efficiency figures mean
A heat pump's efficiency is described by its coefficient of performance, or COP. A COP of 3 means that for every 1 kWh of electricity used, 3 kWh of heat is produced. The seasonal figure over a whole year is the SCOP or SPF, and it is a better guide to real running costs. The efficiency varies with the weather and with the temperature of the water the system must supply, which is why design matters.
Why lower flow temperatures matter
A heat pump is most efficient when it supplies water at a lower temperature than a typical gas boiler. This works best with well-insulated homes, and with larger radiators or underfloor heating that can heat a room at lower temperatures. The system tends to run for longer at a steadier output, rather than firing on and off. Many people find this gives a more even and comfortable warmth.
Hot water
Most heat pumps need a hot water cylinder. They heat it to a suitable temperature for daily use and typically do a periodic higher-temperature cycle to control legionella bacteria. Make sure you have the space for a cylinder.
What to expect at home
- An outdoor unit that needs clear space and sensible positioning to limit noise.
- Possible changes to pipework and some radiators.
- A learning period to adjust the controls, since heat pumps work best when left on at a steady setting rather than turned up and down.
- Servicing, usually annually, though less intensive than for a boiler.
Is a heat pump right for you?
The answer depends on your home's insulation, your heating system, the available space and your electricity tariff. Read our comparison of heat pumps and gas boilers, check whether the Boiler Upgrade Scheme can help with the cost, and improve your insulation first. A proper heat loss survey from a certified installer is the best way to know what to expect in your own home.