Guest Blog: How to correctly size a heat pump

Author: Paul Leedham, Matrix Energy Systems

Paul Leedham

Paul is an MCS certified Installer, his Sheffield based company established in 2009 has grown to become a leading provider of intelligent renewable energy, heating and cooling solutions in both the domestic and commercial markets. Paul authored the BPEC Heat Pump Installation Training Course and is currently developing the new MCS heat pump competent persons scheme. Recent accolades include Matrix scooping both Air Source Heat Pump Installer of the Year and Ground Source Heat Pump Installer of the Year at the national Heat Pump Awards 2020.

How to correctly size a Heat Pump

The ultra-low emission credentials of electrically driven heat pumps is becoming increasingly recognised. The Climate Change Committee recently placed their emphasis on heat pumps to help deliver the carbon reduction in the heating sector with a recommendation of the installation of 1 million heat pumps annually by the mid-2030s and 19 million in place by 2050.

Currently, one of the main frustrations of Installers is when a Heat Pump is sized incorrectly for the property it has been installed in. Paul, Managing Director of Matrix Energy Systems offers his guide to ensuring a Heat Pump is correctly sized.

“Heat pumps can be a very cost effective, efficient and low carbon way of heating your home or business but, they are not a magic silver bullet.

Firstly, you need to ascertain if the building is insulated, if it isn’t, its usually best to explain to the end user that a heat pump isn’t going to be a good fit for the building.  If there is the option to insulate as part of a renovation, then I always advise a ‘fabric first’ approach; no matter what heating system you use, insulating the building will always lower the running costs.  This then allows heat pumps to compete against other technologies using their strengths; low flow temperatures, high efficiency and no local emissions.

Whenever you are constructing anything the most important part is the foundations; and the heat loss of a property can be considered the foundations for the heating system.  For so many years, heat loss has been guessed at and taken a back seat due to the capacity and modulation rates of modern conventional technology such as gas and oil.

Heat pumps consume electricity and are therefore limited by the electrical supply which means they have a comparatively lower kW output than conventional options, it is therefore necessary to ensure that the heat pump is sized correctly against the property heat loss.  If the heat pump is sized incorrectly, whether that is undersized or oversized, then the end user will suffer from comfort problems, high running costs and possible faults and failures over time.

In order to correctly size a heat pump, it is necessary to complete a heat loss calculation for the building.  This works out the peak heat load of the property by measuring all of the fabric and ventilation heat losses for the coldest day of the year based on geographical location; so the same house could have a different peak heat loss in London compared to Glasgow; its colder in Glasgow.

The calculation works out all of the heat losses for each room and then allows you to size the emitters for those rooms, whether they be radiators, underfloor heating or fan convectors.

You can either use the free MCS heat loss tools yourself to calculate this or you can use another company to complete these for you, at a cost of around £100 per calculation; not much more than a SAP calculation that generates EPC’s.  It is important to note that a SAP calculation that generates EPC’s is not the same as a Heat Loss Calculation; SAP isn’t accurate enough to provide the data needed for correct sizing.


Usually projects have a set of plans and if they do, you need to input the data and measurements into the heat loss calculator software, once complete the calculator will give you the peak heat loss.  The data will comprise of the areas of the walls, floors, roof, windows and doors and their corresponding thermal efficiencies or U values.

A heat pump will be selected based on its kW rating compared to the kW heat loss of the building, always ensuring that the heat loss capability of the heat pump is higher to ensure comfort.  The output of a heat pump in kW will vary based on the temperature that will be fed to the emitters, where the higher output temperatures tend to cause the kW rating of the heat pump to drop.  All of this must be taken into account to ensure that the heat pump is still capable of providing comfort during peak heat loss and that the emitters are sized correctly for their designed flow temperature. The losses of each room will determine the sizes of the emitters so check the capability against your design flow temperature.

A correctly sized heat pump means that you have designed a system to provide comfort for your customer that is efficient and cost effective.  The customer will then be able to claim the Renewable Heat Incentive (RHI) to offset the cost of the installation.  Over time you will have a happy customer, a service schedule and a positive advocate both for your company and the wider industry.

A correctly sized heat pump means a system that will cost less to run, keep customers warm and won’t cost the earth. More confident consumers, happy and satisfied with their heat pump installation will contribute massively to raising positive awareness of the benefits of heat pumps and encouraging uptake.”

Since 2008, MCS (Microgeneration Certification Scheme) has become the recognised Standard for UK products and their installation in the small-scale renewables sector.

They create and maintain Standards that allows for the certification of products, installers and their installations. Associated with these standards is the certification scheme, run on behalf of MCS by Certification Bodies who hold UKAS accreditation to ISO 17065.

MCS certifies low-carbon products and installations used to produce electricity and heat from renewable sources. It is a mark of quality. Membership of MCS demonstrates adherence to these recognised industry standards; highlighting quality, competency and compliance.