Air source heat pumps absorb heat from the outside air. This heat can then be used to warm water for radiators or underfloor heating systems, or to warm the air in your home.

• How do air source heat pumps work?
• The benefits of air source heat pumps
• Is an air heat pump suitable for my home
• Costs and savings
• Find out more

How do air source heat pumps work?

An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. It can extract heat from the air even when the outside temperature is as low as minus 15° C.
There are two main types:

• An air-to-water system uses the heat to warm water. Heat pumps heat water to a lower temperature than a standard boiler system would, so they are more suitable for underfloor heating systems than radiator systems.
• An air-to-air system produces warm air which is circulated by fans to heat your home.

The efficiency of air source heat pump systems is measured by a coefficient of performance (CoP) - the amount of heat they produce compared to the amount of electricity needed to run them. A typical CoP for an air source heat pump is around 2.5.

The benefits of air source heat pumps

• Reduce your fuel bills: air source heat pumps run on electricity, so there's no need to pay for gas, oil or solid fuels to heat your home.
• Cut down on wasted electricity: heating your home with an air source heat pump is much more efficient than using electric radiators.
• Save space: an air source heat pump system is compact, and requires no storage space for fuel.

Is an air source heat pump suitable for my home?

To tell if an air source heat pump is right for you, there are a few key questions to consider:

• Do you have somewhere to put it? You'll need a place outside your house where a unit can be fitted to a wall or placed on the ground. It will need plenty of space around it to get a good flow of air.
• Is your home well insulated? Since air source heat pumps produce less heat than traditional boilers, it's essential that your home is insulated and draught proofed well for the heating system to be effective.
• What fuel will you be replacing? The system will pay for itself much more quickly if it's replacing an electricity, Liquid Petroleum Gas (LPG) or coal heating system than a gas or oil one.
• What type of heating system do you want? Air source heat pumps are much better at powering underfloor heating systems or warm air heating than radiator-based systems.
• Is the system intended for a new development? Combining the installation with other building work can reduce the cost of installing the system.

Read more about planning permission for renewable energy technologies
To find out more about whether an air source heat pump is suitable for your home, try the Energy Saving Trust Home Energy Generation Selector tool.

Costs and savings

Costs for installing a typical system suitable for a detached home range from about £5,000 to £9,000 including installation. Running costs for space heating and hot water for washing are likely to be around £790 per year. This will vary depending on a number of factors - including the size of your home and how well insulated it is.

Savings can be considerable - up to 5 tonnes of CO2 and £700 per year for a system that replaces an electric heating system.

All savings are approximate and are based on an air source heat pump providing 100% of space heating and up to 50% of hot water, with the additional 50% provided by electric heater, in a detached property.

To reduce your home's CO2 emissions further, consider installing solar electricity or some other form of renewable electricity generating system to power the compressor and pump.

Heating and cooling is accomplished by moving a refrigerant through the heat pump's various indoor and outdoor coils and components. A compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant from a liquid to hot gas and from a gas to a cold liquid. The refrigerant is used to heat or cool coils in a building or room and fans pull the room air over the coils. An external outdoor heat exchanger is used to heat or cool the refrigerant. This use of outside air has led to the term "Air Source" Heat Pump. The overall operation uses the concepts described in classic vapor compression refrigeration.

When the liquid refrigerant at a low temperature passes through the outdoor evaporator coils, the temperature of the outside air causes the liquid to boil. This change of state from liquid to a vapor requires a considerable amount of energy or "latent heat" which is provided by outside air passing over the coils.

This vapor is then drawn into the compressor where the temperature of the vapor is boosted to well over 100 degrees Celsius. At this point we have used heat from the outside air to change the liquid refrigerant to a gas and added an amount of compression "work" to raise the temperature of the vapor. The vapor now enters the condenser heat exchanger coils where it begins to transfer heat to the air being drawn across the coils. As the vapor cools, it condenses back to a liquid and in so doing releases and transfers considerable latent heat to the air passing over the condenser unit coils. We have used the heat energy of outside air to change the phase of the refrigerant and then released this heat for heating, a typical heat pump operation.

At this stage we now have a very cold liquid refrigerant compressed to a high pressure. The refrigerant is next passed through an expansion valve which turns it back to a low pressure cold liquid ready to re-enter the evaporator to begin a new cycle.

The heat pump can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.

The 'efficiency' of air source heat pumps is measured by the Coefficient of performance (COP). In simple terms, a COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. In mild weather, the COP of an air source heat pump can be up to 4. However, on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. The heat pump's performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases at around −18 °C (0 °F) outdoor temperature for air source heat pumps. However, heat pump construction methods that enable use of carbon dioxide refrigerant extend the figure downward to -30 °C (-22 °F). A Geothermal heat pump will have less change in COP as the ground temperature from which they extract heat is more constant than outdoor air temperature.

Seasonally adjusted heating and cooling efficiencies are given by the heating seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively.

Ground Source Heat Pumps

Heat water for your home with pipes buried in the garden
Ground source heat pumps use pipes buried in the garden to extract heat from the ground. This is usually used to warm water for radiators or underfloor heating systems. It can also be used to pre-heat water before it goes into a more conventional boiler.
See how a ground source heat pump can work in your home

Beneath the surface, the ground stays at a constant temperature, so a ground source heat pump can be used throughout the year - even in the middle of winter.

• How do ground source heat pumps work?
• The benefits of ground source heat pumps.
• Is a ground source heat pump suitable for my home?
• Costs and savings
• Air and water source heat pumps
• Find out more

How does a ground source heat pump work?
A ground source heat pump circulates a mixture of water and antifreeze around a loop of pipe - called a ground loop - which is buried in the garden. When the liquid travels around the loop it absorbs heat from the ground - used to heat radiators, underfloor heating systems and even hot water.
The length of the ground loop depends on the size of your home and the amount of heat you need - longer loops can draw more heat from the ground.
Normally the loop is laid flat, or coiled in trenches about two metres deep, but if there is not enough space in your garden you can install a vertical loop to a depth of up to 100 metres.
The efficiency of a ground source heat pump is measured by a coefficient of performance (CoP) - the amount of heat it produces compared to the amount of electricity needed to run it. A typical CoP for a ground source heat pump is around 3.2 without any reductions for the type of distribution system.
Ground source heat pumps

The benefits of ground source heat pumps

• Reduce your CO2 emissions: on average a ground source heat pump could save around 540kg of carbon dioxide every year when replacing an oil boiler.
• Eliminate your fuel bills: ground source heat pumps run on electricity, so there's no need to pay for gas, oil or solid fuels to heat your home.
• Cut down on wasted electricity: heating your home with a ground source heat pump is much more efficient than using electric radiators.

Is a ground source heat pump suitable for my home?
To tell if a ground source heat pump is right for you, there are a few key questions to consider:

• Is your garden suitable for a ground loop? It doesn't have to be particularly large, but the ground needs to be suitable for digging a trench or a borehole and accessible to digging machinery.
• Is your home well insulated? Since ground source heat pumps produce a lower temperature heat than traditional boilers, it's essential that your home is insulated and draught proofed well for the heating system to be effective. It could also make the system cheaper and smaller.
• What fuel will you be replacing? If you're replacing an electric, oil, Liquid Petroleum Gas (LPG) or coal heating system, a ground source heating system will pay for itself quite quickly. If you're replacing a new, more efficient heating system, your savings will be smaller.
• What type of heating system do you want? Underfloor heating systems or warm air heating will work much better than radiator-based systems.
• Is the system intended for a new development? Combining the installation with other building work can reduce the cost of installing the system.

Costs and savings
Costs of installing a typical system range from about £7,000 to £13,000. Running costs for a year, where all hot water and space heating can be provided by the system are likely to be around £650 per year, but will depend on a number of factors - including the size of your home and how well insulated it is.
Savings can be considerable - up to 540kg of CO2 and £160 if you're replacing an oil-fired central heating system.
To reduce your home's CO2 emissions further, consider installing solar electricity or some other form of renewable electricity generating system to power the compressor and pump.
 Carbon TrustCosts and savings
Costs of installing a typical system range from about £7,000 to £13,000. Running costs for a year, where all hot water and space heating can be provided by the system are likely to be around £650 per year, but will depend on a number of factors - including the size of your home and how well insulated it is.
Savings can be considerable - up to 540kg of CO2 and £160 if you're replacing an oil-fired central heating system.
To reduce your home's CO2 emissions further, consider installing solar electricity or some other form of renewable electricity generating system to power the compressor and pump.

Savings above assume ground source heat pump installed in a detached property which provides 100% of space heating and up to 50% of domestic hot water, the additional 50% is met through an electric heater.

A geothermal heat pump or ground source heat pump (GSHP) is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geosolar system with even greater efficiency. Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled, earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geoexchange" or "ground source heat pumps" because geothermal power traditionally refers to heat originating from deep in the Earth's mantle.[1] Ground source heat pumps harvest a combination of geothermal power and heat from the sun when heating, but work against these heat sources when used for air conditioning.[2]

Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat. Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia.[2] Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer. A ground source heat pump extracts ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.

The setup costs are higher than for conventional systems, but the difference is usually returned in energy savings in 3 to 10 years. System life is estimated at 25 years for inside components and 50+ years for the ground loop.[3] As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%.[4]