How to specifiy air source heat pumps integration

Sizing the ASHP
There are a lot of misunderstandings when it comes to sizing a heat pump.  Many suppliers will have you believe that for some magical reason the heat pump only needs a small output.  This may be because they only produce small heat pumps or more likely, they just don’t understand how to correctly size the unit.

Sizing the heat pump starts in just the same way as sizing a boiler; you have to work out the true heat loss of the building.  There are many ways to do this and if you need help then we can quote you for this.  Once you know the heat loss you can then look to match this to a heat pump.  However, there is the additional factor of defrosting that must be added to correctly size the heat pump.

An ASHP will automatically switch to defrost mode during normal operation.  This happens because the air will contain varying degrees of moisture.  As this ‘moist’ air is drawn across the evaporator by the fan, the moisture begins to freezes on contact.  This results in a slow build-up of frosting or ice on the evaporator that must be removed to prevent a loss of performance.  The reason this happens is because the refrigerant gas inside the evaporator is many times colder than the air temperature.

The defrosting of the evaporator can take quite a while depending on the air temperature.  Whilst the heat pump is in defrost mode there will be no heat going in to the building. 

To compensate for this defrost cycle an additional capacity is required from the heat pump.  Radiant Heating Solutions recommend that this additional capacity is 25 to 35% more than the true heat loss of the building. 

For those who are unsure and don’t want to get it wrong then we can quote you for this.

A quick guide to sizing





Building Type (thermal insulation)








New - High U Value

38 W/m2

42 W/m2

48 W/m2





New - 2009 Building Regs

48 W/m2

50 W/m2

58 W/m2





Retrofit - Upgrade/Improved

72 W/m2

79 W/m2

85 W/m2





Retrofit - Original (no upgrade)

96 W/m2

107 W/m2

115 W/m2

Guide note and example;
The best form of heat emitter is a low temperature delivery system such as UFH or fan coils (or fan assisted radiators).  For areas with high ceilings that are not using UFH then the total volume of the space needs to be factored in which will add extra kW capacity.

UFH should always be installed with the pipes embedded in screed or similar to give high thermal mass.  The UFH pipework should be spaced at 150mm centres for current building regulations (and upgraded retrofit) and 200mm centres for high U value buildings.  For original retrofit buildings with poor insulation it is advisable to space the pipes at 100mm centres.

e.g. A 200m2 building in the midlands that has a high U value would have a 8.4kW heat loss.

Now add on the addition defrost capacity (35%) to arrive at the total output requirement which would then be 11.34kW.
Selecting the right heat pump
Once you have the total output requirement for your project you can select the Radiant air source heat pump that is nearest to this.  Radiants single phase heat pumps have single compressors for outputs up to 11kW and twin compressors from 14 to 24kW.  The twin compressor units can operate on one or both compressors depending on the load. Please note that the 9kW models are also available as a split system and the 20kW models are still under development.


kW Output

Heat only Model

kW Output

Heat/Cool Model

































The above table is for air temperature at 20oC and water temperature at 35oC.

For the example we gave of the 200m2 building, the required heat only model would be the RBB11AH.  This takes into account the defrost factor of 35%

What to do next
You can either but it now or for a full package, request a quote which would include such things as the buffer tank, the circulation pump and anti freeze etc.  If you want help adding the ASHP to your existing system or to be part of multi energy design then go to system integration.

  • buy it
  • get a quote
  • design service

The additional energy required to off-set the lack of heat when the machine is in defrost mode has to be added to the actual heat load. This then becomes the total output that the machine has to deliver.
In its simplest form a fan coil is a radiator with an electric fan to circulate air over the radiator. There are floor, wall and ceiling mounted units and these can be used for both heating and cooling applications. Because of the fan, the water temperature can be relatively low when used for heating and is therefore ideal for use with a heat pump.
Underfloor Heating is a warm water based heat distribution system of embedded pipes in a floor. The average temperature of the heating water is 35 - 40oC and is therefore ideal for using with a heat pump.
The evaporator is the name given to the heat exchanger than transfers energy from the source (air or water) to the refrigeration gas circuit. It is so called because the liquid gas will boil and therefore 'evaporate' in to a gas.