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Air-to air or air-to-water: which air source heat pump is best?

Posted by Graham Hazell on 13 February 2014 at 1:26 pm

Air source heat pumps, which have been used extensively for many decades, are an increasingly popular form of heating. They take the heat from ambient, external air.

There are two main choices for emitting the heat from an air source heat pump:

  1. via air - warm air is blown within the building, or
  2. via water - forming a heating loop around the building which provides underfloor heating or heats radiators.

The most obvious advantage in favour of air-to-water heat pumps is financial. They are eligible for the Renewable Heat Premium Payments scheme, the domestic Renewable Heat Incentive (RHI) and will be included in the non-domestic market RHI this spring.

However air-to-air units are popular in their reversible form. These are units that can both cool (‘air conditioning’) and heat. They provide one less heat exchange process and so are often slightly more efficient than their wet counterparts.

However, the heat/coolth emitter system is more complex in cooling mode. This means that air-to-air units have indoor units designed as well as the unit outside. These can either be mounted on the wall or floor, or concealed in the ceiling or another suitable void.

They are not eligible for support from government incentives for a range of reasons, including the difficulty in measuring the heat produced and the risk of encouraging the use of cooling.




Air-to-Air (general)

Often marginally more efficient, operating at lower heat emission temperatures

Impractical  to heat domestic hot water directly


Quicker response times



Some qualifying systems can be financially supported by government incentives (RHPP/RHI)

Requires careful , often bespoke design of water distribution system- flow rates are critical


Can heat domestic hot water



Flexible heat emission systems (underfloor, radiator or fan convector)





















More information about air source heat pumps on YouGen

The YouGen guide to ASHPs

From the blog

Sizing a heat pump correctly is vital for good performance

10 questions to ask when choosing your air source heat pump installer

YouGen's search tool to help you find a local heat pump installer

About the author: Graham currently works as a freelance specialist consultant and as a consultant to the Heat Pump Association.

If you have a question about anything in the above blog, please ask it in the comments section below.

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