Introduction to heat pump systems

A heat pump is a highly efficient alternative to a central heating system powered by fossil fuels - such as a gas boiler. A heat pump produces no pollution at the point of use, as they do not burn anything to produce heat but work instead by moving heat from one place to another. 

Heat pumps draw the sun's heat energy from  environmental sources to use as heating energy for our buildings and hot water using the principle of transferring heat through a heat exchanger. 

How a heat pump works

Rather than burning fuel to generate heat energy, heat pumps transfer environmental heat from outside air, water and earth to a heat sink inside a building for use in a heating system. 

Heat transfer

At the heart of a modern air source, ground source or water source heat pumps is a refrigeration system powered by electricity. This is essentially a heat transfer system.

Operating principles of a heat pump

There are two principle locations in the transfer of heat; the place where heat is absorbed, (the source), and to where it is transferred, (the destination).

The mechanical refrigeration cycle in a heat pump consists of an arrangement of heat exchangers in a loop. One side of the heat pump loop is set to stay colder than the ambient temperature and contains an exchanger that absorbs heat. This heat is then transferred to the hot side of the heat pump loop where it is then used to heat your home and hot water.

In order to absorb and release the heat into and from the refrigerant, the system exploits the ability of the refrigerant fluid to boil from a liquid to a vapour and then to condense back into a liquid. This is a continual process while the compressor is running and circulating the refrigerant.

The cold side of the loop is where heat from the environment is collected. First the refrigerant gas is expanded and then evaporated exploiting two principles:

1. A gas cools down when it expands (blow on the back of your hand to test it!)

2. Evaporation from a liquid to a gas robs takes heat from the environment (lick the back of your hand and feel it cool as your saliva evaporates)

Similarly, on the hot side, the two opposite principles are exploited:

1. A gas heats up when it is compressed (so the warmed gas transferred from the cold side is made even hotter). You can feel this at the end of a bicycle pump as you pump up a tyre.

2. Condensing a gas back into a liquid returns the heat gained during evaporation plus the extra heat gained during compression allowing water to be heated to high temperatures for hot water and heating systems.

Although some heat pumps are configured for heating only, in others it is possible to alter the direction of refrigerant flow within the system so that the system is able to deliver both heating and cooling as desired.

How heat pumps work

Does a heat pump work when it is cold?

A heat pump works in cold weather as long as the differential between your heat source and target temperature is not too great. Modern heat pump units made specifically for heating in European climates are typically effective and efficient down to low temperatures of minus 25 degrees. If you have heard that they don't work when outside air is cold,  this likely stems from trying to use cheap air conditioning units in reverse to provide heat; unsurprisingly they can struggle when used to provide heating as they are not designed for it! 

Our heat pumps are designed for maximum efficiency when the outside temperature is 5 degrees, when most heating energy is required in the UK. 

A hybrid system

It is possible to have a hybrid heat pump system for heating where heat demand is very high, using another heat source with a heat pump (such as a gas boiler) just for the the very coldest days of the year.

Hybrid heat pump systems are, however usually unnecessary with modern heat pumps that can supply high flow temperatures. 

Types of heat pump systems

There are a variety of different types of heat pumps, each with its own pros and cons. 

How does an air source heat pump work?

Air source heat pumps extract heat from the air. There are two types: air-to-air and air-to-water.

Air-to- air heat pumps transfer the heat they gather to a building as hot air for heating.

An air-to-water unit transfers heat to a store of water for use in radiators, underfloor heating or for hot water and is the more commonly seen type in the UK.

Air source heat pumps are usually outdoor units in the UK, although units for indoor installation do exist.

Air source heat pumps usually have lower running costs than electric heaters or a fossil fuel heating system.

Most heat pumps installed for domestic heating and hot water in the UK are air source heat pumps. 

An air source heat pump is typically a little noisier than a ground source heat pump as this type of heat pump has a fan to move air across the heat exchanger as they work by absorbing heat from the air. 

You can find out more detail about air source heat pumps here.

How does a ground source heat pump work?

A ground source heat pump extracts heat from the ground via a collection loop. These loops of pipe can also be laid in water to make a water source heat pump. 

Ground loops can be horizontal arrays of pipes laid in trenches or vertical boreholes. They are filled with a medium called brine - a mixture of water and anti freeze solution. 

A ground source heat pump is typically more efficient that air source heat pumps and offers the greatest reduction in carbon footprint and highest potential annual savings on bills.

You can find out more details about how ground source heat pumps work here.

Advantages of Heat Pump Systems

Heat pumps are powered by electricity. As electricity is increasingly generated by renewable sources and they themselves are highly energy efficient, they have very low carbon emissions and can reduce your energy bills. 

Heat pumps can not only produce hot water and power a heating system, but, unlike other heating systems, can also provide sufficient cooling in the summer months for our climate, removing the need for an air conditioner. 

Incentives such as the Boiler Upgrade Scheme are available for financial support of the installation costs of heat pumps in England and Wales.

Heat pumps are low maintenance.

Considerations for installing a heat pump

One of the myths about heat pumps is that they can only be installed in homes with the very highest levels of insulation, triple glazed windows and underfloor heating. 

In fact heat pumps work even for older homes when the system is well designed and can replace gas boilers in the majority or homes. Heat pumps do work with lower flow temperatures than boilers and whilst underfloor heating is ideal, sometimes all that is required is larger radiators. 

You will need space for an outdoor unit for an air source heat pump system and outdoor space for a ground collector for a ground source heat pump. Both heat pump types will need indoor room for hot water cylinder, particularly worth considering if you are replacing a combi boiler. 

The size of heat pump and heat pump system you need to keep your home warm and provide sufficient hot water will depend on both the size of your property (how many bedrooms and bathrooms is a good first proxy for exact floorspace), its age and insulation values and the local average air temperature. 

Start your thinking with our guide to writing your own brief for your system

To help you start thinking about what you want from your project we have produced a booklet on writing a brief for a renewable heating system. This guide for property owners and installers is by Chairman of the Domestic Heat Pump Association, John Felgate

Download the guide

Where to buy a heat pump

Our heat pumps are supplied through installers and wholesalers. Because proper installation is critical to the efficient running of a heat pump, and indeed all renewable systems, we recommend that you use installers trained on our products. We are happy to help you find a STIEBEL ELTRON  trained installer in your area.

Find local installers

Request a specification now

We offer a free design service to help ensure that you get the best advice on heat pump technologies to your home. Because we offer such a wide range of heat pump technologies we can offer advice on which standard heat pump options will give the best return on investment, lowest running costs and highest carbon savings and performance.

Our local specification managers will provide you with costs, initial drawings to show how your heating system will work, calculations showing how much energy a system can harvest from the environment, how much electrical energy it will use and how it can save money on energy bills.


Request a specification now


Both air source heat pumps and ground source heat pumps work well for heating homes in the UK and are an excellent choice for most properties given a well designed system.