Your heat pump is sized against a heat load calculation for your building, and the MCS design standard makes the chosen unit cover all of that calculated load on its own. The flow temperature at the heat pump counts in that selection too.
What the MCS standard requires of the size
The MCS heat pump design standard, MIS 3005-D, puts every heat pump meant to carry your whole space heating load down one route, clause 5.5.1, with air to air systems treated separately. Its requirements include these four.
- The heat load calculation comes first. It is performed on the building using internal temperatures not less than those in the standard's Table 1 and external temperatures from Table 2, column A or B, according to the MCS contractor's assessment of the building location, and it must otherwise comply with BS EN 12831-1:2017.
- The unit must cover all of it on its own. The standard says a heat pump shall be selected that will provide at least 100% of that heat load, taking into consideration the flow temperature at the heat pump and without input from any supplementary electric heater.
- A back-up heater is not a sizing allowance. Any supplementary electric heater shall be designed to not operate above the external temperatures in Table 2.
- Defrosting is part of the design. An air source heat pump system should be able to maintain the internal design temperatures across multiple defrost cycles.
Put plainly, the standard asks what your building loses at your location's design external temperature, then asks which unit covers all of that at the designed flow temperature with the electric heater switched off. For the emitter side of the same design, see our guide to radiators and heat pumps.
Hybrid systems are the one case with a lower bar. For a hybrid design, the standard says the heat pump power output in kW shall be selected to provide a minimum of 55% of the calculated heat load of the building, rated at a 55°C flow temperature. The trade-off is funding: gov.uk does not pay a grant for a hybrid heat pump system at all.
The sizes UK brands actually build
Mitsubishi lists the Ecodan R290 at 5kW, 6kW, 8.5kW, 10kW and 12kW, and Ideal publishes Logic Air specifications at 4kW, 5kW, 8kW and 10kW, so ask which of those sizes a design has chosen and why.
| Range | Published sizes |
|---|---|
| Mitsubishi Electric Ecodan R290 | 5kW, 6kW, 8.5kW, 10kW, 12kW |
| Ideal Heating Logic Air | 4kW, 5kW, 8kW, 10kW |
Those ladders have gaps in them, so if your calculated heat load lands between two rungs, ask which rung the design uses and what output it is credited with. Mitsubishi publishes a guaranteed operation range for the Ecodan R290 down to -25°C ambient and water temperatures up to 75°C, so ask for the output figure at the design external temperature for your area rather than the headline kW. We go through the model detail for both brands in our guides to the Mitsubishi Ecodan range and Ideal's Logic Air.
The grant rules set an upper limit too. Gov.uk states the maximum capacity for the Boiler Upgrade Scheme is 45kWth for individual systems, 70kWth if a heat pump is installed with one or more additional heat pumps, and 300kWth for shared ground loops.
Get the size calculated, not estimated
Ask MCS-certified installers for a quote built on a room-by-room heat load calculation, with the chosen unit's output shown against it. Tell us about your home once and we will pass your details to installers covering your postcode. The form takes about a minute to fill in.
What the installer measures on the survey
Energy Saving Trust describes the sequence plainly: your heat pump installer will assess your property, calculate your home's heat demand, and use that to inform what they recommend. Two outputs of that visit are worth asking for by name.
The first is the seasonal performance figure. Energy Saving Trust states that installers must calculate the seasonal performance factor based on the system design for your home, that the calculation reflects the average temperatures at your location and the size of your radiators, and that your installer should share it with you before they start any work. Ask for it in writing, with the flow temperature it assumes. For the wider efficiency case, Energy Saving Trust puts a heat pump at three to four times as much heat as the electricity it uses.
The second is what happens to your radiators and pipework. Energy Saving Trust notes that your existing radiators may already be big enough, and that the installer will tell you once they have calculated your heating requirements. Heat pumps typically send water to radiators at a lower temperature than a fossil fuel system, often between 35°C and 45°C. Energy Saving Trust also warns that microbore pipework, which has a smaller diameter than standard central heating pipes, can make heat pumps struggle to transfer water to the radiators fast enough to heat your home adequately.
The unit itself has to go somewhere. Energy Saving Trust puts outdoor units at as much as 1 metre by 1.5 metres, around the same space as two wheelie bins. Measure the spot you have in mind before you fix on a model, and ask the installer what clearance the unit they propose needs for airflow.
What size does to the price
Energy Saving Trust puts the typical cost of installing an air source heat pump at around £12,000, and lists the size of the heat pump, the size of your property, whether it is a newbuild or an existing property, and whether you need to upgrade your radiators as the things that move that figure. MCS says the cost of a heat pump depends on the size and age of your property as well as the heat pump type, and its MCS Data Dashboard puts the 2025 average for an air source heat pump at around £13,300. Ground source costs more again: Energy Saving Trust gives around £15,000, rising to around £57,000 where a borehole has to be drilled.
England and Wales. Gov.uk gives £7,500 towards an air source heat pump through the Boiler Upgrade Scheme, with an additional £1,500 until March 2027 if the property is heated by oil or LPG and has no mains gas connection. MCS states that figure as a £9,000 grant for off-gas-grid properties heated by oil or LPG, effective from 21 July 2026. It is one grant per property, and nothing at all for a hybrid heat pump system.
Scotland. Home Energy Scotland offers grant funding of up to £7,500 for heating systems such as heat pumps, with a further £7,500 available as an optional interest-free loan, plus a £1,500 uplift for rural and island homes. Home Energy Scotland also states that only heat pumps that provide 100% of heating and hot water requirements are eligible for funding.
Northern Ireland. Energy Saving Trust states there is no specific grant currently available, though you can still apply for insulation funding through NI Energy Advice.
The Boiler Upgrade Scheme grant is a fixed £7,500 towards an air source heat pump rather than a share of the bill, so ask each installer what their proposed size costs before the grant is taken off. For the full build-up of the installed price, including the cylinder and the emitter work, start with our UK heat pump cost guide, and for the running cost side of the decision, our heat pump versus gas boiler guide.
How to check the sizing on a quote you have been given
Energy Saving Trust recommends getting quotes from at least three installers accredited through the Microgeneration Certification Scheme. Ask each of them for the same four things.
- The room-by-room heat load calculation for your building, and which external design temperature it used.
- The output of the unit being proposed, shown against that calculated load.
- The design flow temperature the output figure is quoted at.
- The seasonal performance factor at that flow temperature, in writing, before work starts.
If a quote names a kW figure and cannot show you the calculation behind it, press that before you press the price. MCS also sets out what arrives at the end: your installer should provide a handover pack on completion, and an MCS certificate within 10 working days of completion. MCS puts the average life of an air source heat pump at up to 20 years.
Air-to-air systems are sized differently again. For those, the MCS standard requires the outdoor unit to have the capacity to provide at least 100% of the calculated heat load, taking into consideration the external design temperature, and the capacity of each indoor unit to be selected to provide at least 100% of the heat load of the habitable room in which it is located, noting that the total capacity of all the indoor units may therefore exceed the total heat load of the building. If you are being quoted for air-to-air, ask for the room-level figures as well as the headline one.
Frequently asked questions
Your installer works it out, and the published sizes to pick from are 5kW, 6kW, 8.5kW, 10kW and 12kW in Mitsubishi's Ecodan R290 range, and 4kW, 5kW, 8kW and 10kW in Ideal's Logic Air range. Which of them suits your home comes out of the heat load calculation the MCS standard requires on the building, with the chosen unit having to provide at least 100% of that heat load at the system's flow temperature.
The MCS standard sizes the heat pump against a heat load calculation performed on your building. Ask your installer for that heat load figure and the output of the unit proposed against it, rather than comparing the new kW number with the old one.
Not in an MCS design. The standard requires the heat pump itself to provide at least 100% of the calculated heat load without input from any supplementary electric heater, and adds that any supplementary electric heater shall be designed to not operate above the external temperatures in its Table 2. A quote that leans on an immersion or back-up heater to get the heat pump size down is not following the sizing procedure.
The standard requires the selection to take into consideration the flow temperature at the heat pump, so ask which flow temperature the quoted output refers to. Energy Saving Trust notes that heat pumps typically run radiators at a lower temperature than a fossil fuel system, often between 35°C and 45°C, and that installers must calculate the seasonal performance factor from the system design, reflecting your location's average temperatures and the size of your radiators. Ask which flow temperature the sizing assumes, and ask for the seasonal performance figure at that temperature.
In Scotland, yes. Home Energy Scotland funds heat pumps only where they provide 100% of heating and hot water requirements. In England and Wales the Boiler Upgrade Scheme sets a maximum capacity of 45kWth for individual systems, 70kWth where a heat pump is installed with one or more additional heat pumps, and 300kWth for shared ground loops, and gov.uk gives no grant at all for a hybrid heat pump system.
MCS says the cost depends on the size and age of your property as well as the heat pump type, and puts the 2025 average air source installation at around £13,300 from its MCS Data Dashboard. Energy Saving Trust puts a typical installation at around £12,000 and names the size of the heat pump and the size of your property among the four things that move it, with ground source at around £15,000 and up to around £57,000 where a borehole is needed. Gov.uk gives £7,500 towards an air source heat pump through the Boiler Upgrade Scheme, so ask each installer what their proposed size costs before the grant is deducted.
Energy Saving Trust puts outdoor units at as much as 1 metre by 1.5 metres, around the same space as two wheelie bins, so measure the position before settling on a model and ask the installer what clearance their proposed unit needs for airflow. The MCS standard also notes that, where planning requirements allow, multiple outdoor units may be combined to provide 100% of the calculated heat load.