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Home & garden.

30 Mar 2022

An introduction to heat pumps

Find out all about heat pumps, how they work, the different types available, and which sort might be best for your home
Karen Lawrence
Heat pump

A heat pump is a low-carbon way to heat your home. It's a sustainable replacement for fossil fuel heating such as gas, oil or LPG, and can help reduce your household's carbon footprint.

Gas boilers and other carbon-intensive heating systems will be gradually phased out to help the UK reach its Net Zero target by 2050. Alongside other low-carbon and renewable heating systems, heat pumps will play a big part in heating our homes in future. 

Heat pump technology is well established, and they are widespread in Europe, including in cold countries such as Norway and Sweden. 

The government’s independent advisory body, the Climate Change Committee, has said that up to 15 million homes should be fitted with heat pumps, or hybrid heat pumps, by 2035.

So, could a heat pump soon be heating your home?

Read on to find out more about the pros and cons of heat pumps, and whether installing one now is the right choice for you.

What is a heat pump? 

In the simplest terms, a heat pump moves heat from one place to another. 

You almost certainly already have a heat pump in your home – your fridge or freezer. A heat pump moves heat from inside the box to outside, reducing the temperature inside the fridge.

When it comes to home heating systems, a heat pump does the same thing, but it moves the heat from outside to inside the box (your home), to increase the indoor temperature.

Heat pumps don't actually create heat, they simply move it from the outside air, ground or water, and transfer it inside your home, where it can be used in radiators, underfloor heating, or warm air heating, as well as supplying hot water for taps and showers. 

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Some heat pumps can also be used in reverse, to provide cooling in warm weather.

Although they are powered by electricity, heat pumps typically deliver three to four units of heat for each unit of electricity needed to run them. This is much more efficient than gas boilers, for example, which produce less than one unit of heat for each unit of gas used.

This makes heat pumps a highly efficient, low-carbon technology and, when they're powered by 100% renewable electricity, they can be zero carbon in operation.

How does a heat pump work?

The term 'heat pump' is commonly used to describe the entire system, though it's actually made up of several components that are installed both inside and outside your home.

They work by absorbing heat from the outside environment – air, ground or water – and transferring it to a refrigerant, which flows through a compressor, condenser, expansion valve and evaporator in a closed cycle. 

The refrigerant is compressed to increase its temperature, then moved to the indoor heat exchanger, where the heat is transferred into the heating system or hot water storage tank.

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Heat pumps work even when the outside temperature is below freezing, as there is always some heat present in any temperature above absolute zero (-273 degrees Celsius). They can operate effectively at -15 degrees, although their efficiency does drop in very cold weather. Ground and water source heat pumps are more efficient year-round, since ground and water temperatures are much more constant than the air temperature.

What types of heat pump are available?

There are a number of different types of heat pump, each with its advantages and disadvantages.

Air source heat pumps

There are two different types of air source heat pump; air-to-water and air-to-air.

Both types extract heat from the outside air, but they deliver it into your home in different ways. 
Two air source heat pumps at the back of a home

Air-to-water heat pumps are the most obvious replacement for a gas boiler, as they can circulate heat through a wet central heating system, including radiators and/or underfloor heating, and produce hot water too. 

Air-to-air heat pumps deliver heat through a fan or warm air circulation system, but these do not produce hot water, so you will need a separate system for water heating.

Read more, including the pros and cons, and how much they cost to install, in our guide to air source heat pumps.

Ground source heat pumps

Ground source heat pumps extract heat from the ground using buried collectors, known as ground arrays. Fluid is pumped through the ground array to absorb heat stored in the ground. This is compressed and goes through a heat exchanger, which extracts the heat and transfers it to your home heating system, similar to an air-to-water heat pump.

The ground array may be either a network of coiled pipes, also called 'slinkies', laid out in shallow trenches, or a vertical U-shaped pipe inserted into a very deep borehole. 

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Both types require a reasonably large garden or other outdoor space for installation. A larger area is needed for slinky pipes in trenches, but boreholes are generally a more expensive option and you will need access for a specialist drilling rig to dig up to 200m down.

Find out more about ground source heat pumps and whether they are a good choice for your home.

Water source heat pumps

Water source heat pumps are similar to ground source heat pumps. The pipe network or pond mat is submerged in water - a river, lake, large pond, or the sea - and extracts heat, which is compressed and transferred in exactly the same way.

Water source heat pumps are often very efficient because water temperatures are generally stable throughout the year and are warmer than the average air and ground temperature in winter. 

If you're within 100m of a suitable water source, then this option is worth considering.

Hybrid heat pumps

A hybrid, or dual fuel, heat pump system combines an air or ground source heat pump with a traditional gas, oil or LPG boiler. The hybrid system switches between the heat pump and the boiler, depending on which is the most efficient at any given time.

In very cold weather, if the heat pump cannot supply heat efficiently, it shuts down and the system switches to the fossil fuel boiler. This means that a hybrid system is less environmentally friendly, but it could be an option if you can't heat your home adequately with a heat pump alone.

High temperature heat pumps

High temperature heat pumps use the same technology as other heat pumps, but are designed to run at a higher temperature, usually 65-80 degrees.  

Standard, or low temperature, heat pumps are at their most efficient when running at 35-45 degrees, which is cooler than most gas central heating systems. To achieve a comfortable level of warmth with a standard heat pump, many current homes will need larger radiators, upgraded pipework, or improved insulation – or all three. A high temperature heat pump is an alternative option.

Although a high temperature heat pump may be a good choice for some homes, they are more expensive to fit and less efficient than low temperature models, and the external units are larger.

How efficient are heat pumps?

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Heat pumps are an energy efficient way to heat your home, because they use a small amount of energy to move heat around rather than burning fuel to create it.

The efficiency of heat pumps is usually expressed as the coefficient of performance, or COP. This describes how many units of heat you get out for every unit of energy you put in. A heat pump with an efficiency rating of COP 3.0 can produce three times more heat energy than the electrical energy it consumes. However, efficiency varies with the outdoor temperature, so the colder the air, ground or water source, the less efficient the heat pump will be.

The SCOP, or Seasonal Coefficient of Performance, is the average COP across the whole year. Energy labels for heat pumps show the energy efficiency rating according to their SCOP in heating mode. 

A more efficient heat pump will consume less energy and be more cost-effective to operate.

To get the best from your heat pump, you'll need to understand how to use it most effectively, as it may be different from the way you use your existing heating system. You may need to set your heating to come on for longer than with a traditional gas or oil boiler. Your installer should show you how to control your heat pump system.

Heat pump energy labels

All heat pumps must have an energy label, similar to the label used on gas boilers. It shows how energy efficient the pump is on a scale from dark green (most efficient) to red (least efficient). 

Related: Which gas boilers are the most efficient?

If your heat pump supplies both space heating and hot water then, like a gas combi boiler, the label will indicate how efficient the heat pump is for each function.

Heat pump energy labels also indicate how noisy they are, displaying the decibel level both inside and outside the home.

Why should you consider installing a heat pump?

  • To reduce your carbon footprint: Heat pumps themselves produce no carbon emissions, unlike gas, oil and LPG boilers. They do use electricity, so they are not entirely carbon-free, but as the UK's national electricity supply reduces its reliance on fossil fuels, they will become even lower carbon in future.
  • Use energy more efficiently: Heat pumps are highly efficient, so you'll be using less energy for heating and hot water.
  • Lower your energy bills: In a well-insulated home, a heat pump with an average annual efficiency of 3.0 or higher will cost around the same to run as a gas boiler. For off-gas homes using oil, LPG or electric heating, it could be cheaper to run a heat pump.

Why might you not install a heat pump?

  • No space for installation: All heat pumps need some outside space, so you may not be able to install one if you have none. You will also need space indoors for a water storage tank, pipework, controls, and perhaps larger radiators.
  • An uninsulated, draughty home: It may prove expensive to heat a poorly insulated home with a heat pump. The heat pump must work much harder to replace the heat that is constantly leaking out, so can't operate at maximum efficiency.
  • Costly to install: At the moment, heat pumps cost considerably more to install than gas boilers. There may also be more disruption, especially if you need extra insulation, new radiators and pipework, or other additional work.
  • Lower energy bills are not guaranteed: Per unit, electricity is considerably more expensive than gas, so your energy bills may go up if your heat pump does not run as efficiently as planned. It's expected that gas prices will continue to rise at a greater rate than electricity, so you may be better off in the long term but this is not guaranteed.   

Heat pump FAQs

Is my home suitable for a heat pump?

Not every home is suitable for a heat pump. All heat pumps need space for installation – read more about air source and ground source heat pumps to find out how much space is needed, inside and out. 

As they supply heat at a lower temperature, they work most efficiently in well insulated homes. 

If your home is not energy efficient, you can still install a heat pump, but you may need to add extra insulation, or replace your radiators and/or central heating pipework to increase the flow, to make sure you can heat your home adequately.

Will a heat pump save me money on my energy bills?

If designed, installed and operated correctly, a heat pump can be cheaper to run than a gas boiler. 

Before the latest energy price rises, many analyses showed that heat pumps were often more expensive to run than traditional gas boilers. However, the April 2022 price cap sees the unit price of gas rise by 84% while electricity rises by 35%, which shifts the calculation in favour of electric-powered heat pumps.

A heat pump with a COP of at least 3.0 may now be cheaper to run than a gas boiler, where previously a COP of around 3.7 was required to achieve the same running costs.

If you currently use oil, LPG or electric heating – which are generally more expensive to run than gas boilers – you are more likely to save money with a heat pump.

Read more about installation costs and potential savings in our air source and ground source heat pump guides.

Do I need planning permission for a heat pump?

Unless your home is a listed property or in a conservation area, you don't normally need planning permission for a heat pump as they qualify as a permitted development. 

You must not install an air source heat pump too near the property boundary, to avoid causing a noise nuisance to neighbours.

Can I get a grant to install a heat pump?

A new grant, called the Boiler Upgrade Scheme (BUS), is due to launch soon. 

Homes in England and Wales can get up to £5,000 towards the cost of an air source heat pump, or £6,000 for a ground source heat pump. To apply for the grant you must have a valid energy performance certificate (EPC) with no recommendations for loft or cavity wall insulation. 

More information will be available when the scheme officially launches, expected in April 2022.

If you live in Scotland, you can get a £2,500 loan plus up to £7,500 cashback towards a heat pump installation through Home Energy Scotland.

Read more: tips on saving electricity and money