Air Source Heat Pumps

  • air source heat pump diagram2Building Regulation Part L1A 2013 has now set even more stringent targets for CO2 reduction; hence many new houses are now built using Air Source Heat Pumps (ASHPs) to supply both space and water heating.
  • The technology for ASHPs has now truly come of age and is a viable alternative to gas or oil.
  • It is still possible to achieve Part L1A compliance using gas combination boilers provided special care is taken with insulation.
  • New dwellings built using oil boilers will not achieve Part L1A Compliance unless you also utilise some form of renewable energy, such as solar panels.

How do ASHPs work?
People often say it’s just like a fridge in reverse, which is true but not really a good explanation. The ASHPs consists of a closed loop of pipe which contains a refrigerant liquid. Inside the house the refrigerant is compressed to a high temperature and heat flows from the refrigerant into the house, outside the refrigerant is expanded and heat flows from the outside air into the refrigerant. A metaphor that I often use is

….. imagine you squeeze a sponge then go outside the house and release it and soak up some heat from the air --- then you go indoors and squeeze the sponge and the heat is released into the house …..

How much do ASHPs cost?

  • Installation Costs - In a standard 3 bedroom house a good well installed ASHP might cost between £3000 - £8000, which is a lot more than a standard gas combination boiler. However this extra cost can be mitigated by joining the Renewable Heat Incentive Scheme. The RHI is a government scheme (similar to the Feed-in-Tariff for solar panels) which pays you back money for the amount renewable heat you generate. You pay for your ASHPs up front and after about 7 years you will have got this money back.
  • Running Costs - ASHPs run on electricity, which is more expensive than gas or oil, however they are incredibly efficient. Whereas a gas or oil boiler might be 90% efficient ASHPs can often have efficiencies of over 300% which makes their running costs similar to a traditional gas boiler.

What ASHP shall I use?
All new houses require a SAP calculation in order to produce the mandatory Energy Performance Certificate (EPC). The output of the SAP calculation will give guidance on what ASHP to use.

  • The Products Characteristics Database (PCDB) contains a list of makes and models of ASHPs that are allowed to be used as part of the SAP calculation. The efficiency of the ASHP play a major role in whether the SAP calculation complies with Part L1A of the building regulation. Many makers of ASHPs make spurious claims about the efficiency of their products. It is only the makes and models that have been tested in government approved laboratories that appear in the Product Characteristics Database.
  • The Plant Size Ratio is a measure of the size of the ASHP compared to the size of the dwelling. ASHP have an optimum efficiency. If it’s too small then it cannot heat the space – if it’s too large then it runs at low efficiency. The Plant Size Ratio should be between 0.2 and 2.0

So we can enter various ASHPs from the PCDB into the SAP calculation and see which make and model gives the best Plant Size Ratio and optimum efficiency. This information can then be passed to your heating engineer.

A typical house requires an ASHP of about 4Kw – 6Kw which should deliver a Space Heating Efficiency between 300% – 400%

What does an ASHP consist of?

  • ASHP insideUnitASHP outsideUnitOutside Unit (Pump) - The outside unit needs to be installed in a place where there is plenty of air flow, and foreign objects such as boxes or containers need to be kept well away. ASHPs do make a small amount of noise (40-60 decibels) so it’s not a good idea to have it directly outside you bedroom window. Also think of the effect the noise might have on your neighbours. Planning permission can also sometimes be an issue.
  • Inside Unit (Storage Tank) - Inside the house you will have a storage tank and some other pieces of equipment. This will typically be installed inside a cupboard about the size of a traditional airing cupboard.

What sort of heating does an ASHP provide?

  • Space Heating – underfloor heating and/or radiators? - Because houses are now built with extremely high levels of insulation not much energy is required for space Heating.
  • ASHPs are most efficient when connected to underfloor heating because this runs at a lower temperature than conventional radiators. You can also use special larger size low temperature radiators with ASHPs. Many new dwellings are built with underfloor heating on the ground floor and radiators in the bedrooms and bathrooms.
  • Water Heating - ASHPs require a water storage tank to be installed inside the house and the size is usually determined by the number of bedrooms. A typical 3 bedroom house might have a 200 litre storage tank. The storage tank will also be fitted with an electric immersion heater in case the ASHPs cannot meet the demands for hot water.

What are ASHPs like to use?
This is often referred to as the 'end user experience'. People living in a house with an ASHP will not notice much difference than living in a house with conventional gas or oil systems. If the house feels a bit cold you merely turn up the thermostat, although it will take slightly longer to heat up than a house with conventional radiators. If you want hot water for a bath you simply turn on the taps, although if many people decide to use large volumes of hot water at the same time you run the risk of running out. However with a well-designed installation this should not happen.


If you would like more information, advice or a quotation, then please call

Richard Owen on 01485 544757 or click here for contact information  >>>    Contact Us