Heat emitters

Why size does matter when it comes to heat pumps

Grant air source heat pumps Training Academy
Selecting the correct size heat pump will help a home keep warm efficiently, sustainably and reliably.

As a heating engineer and installer, it is crucial to understand the importance of designing a well-functioning heat pump system. System design is an essential component of modern heating and designing a system that is tailored to a building's needs can significantly impact its efficiency and performance. In this blog, Grant UK’s Technical Design Manager, Stuart McWhinnie, talks through the processes involved in specifying and sizing a heat pump system.

Correctly designing a heat pump system is critical and installers who work with air source heat pumps need to consider all the factors which can contribute to the unit’s operation within a property.

How to design a heat pump system

As with most things in life, fully researching a potential purchase pays off in dividends. Specifying a heat pump is no different – I like to apply the ‘6 Ps’… Prior Preparation and Planning Prevents Poor Performance. If you take the time to comprehensively understand a property’s heating requirements and its physical build, correctly calculating heat loss within the home and projecting the heat demand, then the heat pump model most suitable for the job can be selected and installed. Here, I will take you through some of the key considerations to factor in when designing a heat pump system.

1. Do not cut corners

Prepare and plan the job well – you need to select the appropriate heat pump capacity needed to heat the rooms at the lowest temperatures and, once those room are heated, try to retain that heat without costing a fortune.

When sizing an air source heat pump, installers need to understand the term ‘drop off’ which is essentially referring to the difference between a heat pump’s potential output (eg. 17kW) and its actual operating output when installed and exposed to external temperatures. Manufacturers will test their heat pumps at extreme temperatures to gauge each unit’s heating capabilities at low winter temperatures such as -3ºC (mainland UK) and -7ºC (Scotland). It is these ‘drop off’ values that need to be fed into a system design because a 17kW heat pump will not always be able to deliver this output due to the changes in external temperatures.

If too small a heat pump is selected, it will not be able to sufficiently meet the property’s heating demands. Similarly, select too large a model and the heating system may not be as efficient. Either way, choosing the wrong size of heat pump will lead to an unhappy household and this in turn will make for an unhappy installer!

2. Do heat loss calculations

Full room-by-room heat loss calculations are a necessity with all heat pump installations (a good point of reference by the way is the Domestic Heating Design Guide). A heat loss accounts for the room sizes, the insulation levels of both the walls and windows, as well as the height of the rooms. The calculation brings together the amount of space that needs heating along with the building’s materials and their capability to reduce heat loss from the property.

Grant UK's in-house Design Team can assist with heat loss calculations and G1 Installers can download our handy calculator tool via the G1 Portal as well.

Let me delve a little deeper. Simple thermodynamics tell us that heat will flow from hot to cold. All materials will transfer heat and the material type will determine the rate at which heat transfers across it so this is the key to how we retain heat within a building and it is fundamental when specifying a heat pump. By measuring all the rooms individually and assessing all the building fabric ‘U’ values (technical jargon for how quickly and at what rate a building material will conduct heat), we can accurately build up a picture of the property’s total heat loss.

Other factors to consider

It is also important to factor in a number of other variables such as:

  • air change rates
  • expected comfort temperatures
  • the lowest temperature expected in winter for that area.

A heat loss calculation can ensure that, with certain performance figures for the heat pump, the correct selection of heat pump can be made. If short cuts are made, with a lack of attention to detail with these calculations, then it will be grief all the way with insufficient heating, cold rooms and high energy bills.

3. Plan your heat emitters

A heat loss calculation is how we kick start a project and the next step is to look at heat emitters. Air source heat pumps operate within low temperature heating systems so with this in mind, it is just as important to size the heat emitters correctly as it is to size the heat pump.

To effectively transfer the heat from the system into the occupied living spaces within the home, heat emitters with larger surface areas tend to be the best partners for air source heat pumps. This is why underfloor heating is a popular choice for heat pump systems, especially in new build and retrofit projects where this type of emitter is smoothly integrated into the initial designs during the project’s in-depth planning stages.

Installers should not feel restricted, however, when it comes to choosing the best solutions for their customers. A mixture of underfloor heating downstairs and then aluminum radiators upstairs could deliver the ultimate comfort in some circumstances.

Heat pump systems and radiators

You may be asked to install a heat pump in a property where radiators will be used. I would just like to point out that, although there is no reason why a heat pump cannot be used in this way, the Delta T across the radiator will be significantly lower and this will have an impact when radiator sizes are selected to meet the rooms’ heating needs.

As touched on above, heat pumps have a lower flow temperature and so the area of the surface providing heat to a room will need to be larger than one sized in gas or oil boiler system. With heat pump, as a general rule you can expect to fit a radiator two and a half times larger than normal to provide the same heat output. Oh, and one last point – the index circuit may have to be upgraded to cope with the lower flow temperature as well.

Grant UK are here to help with heat pump design and installation

Please do not worry if the above is becoming a bit daunting. Grant UK’s Design and Specification Team can help with all the above – from comprehensive heat loss calculations through to estimates, quotations and full underfloor heating system designs. To discover more, please contact your local Grant UK Sales Representative or contact the Design Team directly via

Why size does matter when it comes to heat pumps
Fully planning and designing a heat pump system is a critical phase of an air source heat pump installation
Stuart McWhinnie
Technical Design Manager at Grant UK
Return to top