Sizing a heat pump: heat loss, flow temperature and rated output

A heat pump should be just big enough. Too small, and a backup electric heater has to cover the cold days, which is expensive. Too big, and it cycles on and off in mild weather, losing efficiency, wearing faster and making more noise than necessary. With a gas boiler, oversizing did little harm; with a heat pump, getting the size right matters.

Step 1: a room-by-room heat loss calculation

The starting point is the heat loss of your home: how many kW of heat escape through walls, roof, floor, windows and ventilation on a cold day. Under the MCS Heat Pump Design Standard, MIS 3005-D, installers must calculate this room by room using BS EN 12831-1:2017, with set internal temperatures for each type of room and external design temperatures for your location ¹.

The external design temperature is not the same everywhere. MIS 3005-D gives two options per location, based on temperatures exceeded for 99% or 99.6% of hours in a year; for London, for example, −1.7 °C or −3.0 °C ¹. Issue 3.0 of the standard has been mandatory for MCS contractors since 5 December 2025 ¹.

Your old boiler’s size is not a guide: boilers are usually far bigger than the real heat loss. Your gas use is a useful sanity check. Take the kWh for space heating from your bills and multiply by the boiler’s efficiency to get heat delivered. But two homes with the same annual use can need different peak outputs, for example because of ventilation or heating patterns, so this doesn’t replace the calculation.

Step 2: decide the flow temperature

How much heat a heat pump can deliver depends on the water temperature. That is why manufacturers declare rated output separately at 35 °C (underfloor, oversized radiators) and 55 °C (typical radiators) ². MIS 3005-D requires the designer to specify the design flow temperature leaving the heat pump and the emitter temperature needed in the worst-performing room ¹.

So first establish what flow temperature your radiators need on the design day, then compare the heat pump’s output at that temperature with the heat loss. Among the air-to-water models in our database, only about a quarter (27%) have the same rated output at 55 °C as at 35 °C; for about one in five (20%) it is more than 10% lower. Always check the right column.

Step 3: understand rated output

Rated output (Prated) on the datasheet is the heat output under the EU label’s design conditions ², which use a design temperature of −10 °C for the average climate ². That is colder than the design temperatures MIS 3005-D gives for London, so the figure is a standardised reference point rather than exactly what you’ll get on your own design day.

Two further ideas help:

• Full coverage: where the heat pump supplies all space heating, MIS 3005-D requires it to meet the calculated heat load at the design flow temperature without help from a supplementary electric heater ¹.
• Bivalent temperature: the outdoor temperature, declared by the manufacturer, below which the heat pump needs supplementary heat to meet demand ². For a correctly sized system it should sit below your design temperature.

Efficiency drops noticeably in the cold: for air-to-water models in our database the median COP is 4.83 at 7 °C and 3.00 at −7 °C, about 60% of the mild-weather value ³. An undersized system loses twice on cold days, with poorer COP and electric top-up heat, which drags down the system SCOP. For the Boiler Upgrade Scheme, air-to-water and ground source systems must reach at least 2.8 in the MCS calculation, and systems up to 45 kW are covered ⁴. See our guide to SCOP and COP for more.

What sizes are available?

Rated output at 35 °C of the heat pumps in our database, with data mainly from the German BAFA list and the Heat Pump KEYMARK ^3,5:

For air-to-water the middle half runs from 6.6 to 13 kW. Ground source and water-to-water models skew larger: about a third of ground source models (84 of 246 with a rating) and more than half of water-to-water models are rated at 20 kW or more. That describes the market, not your home: choose based on your calculated heat loss, even if it comes out below the most common sizes.

Modulation: the minimum output matters too

Most of the year is mild, and your home needs only a fraction of its peak heat. A heat pump that can turn down far runs steadily instead of cycling. Of the air-to-water models for which our data records it, 98% can modulate their compressor output; for ground source that is about 73% and for water-to-water about half. The minimum output varies too. Ask your installer for the minimum output at 7 °C and compare it with your heat demand in spring and autumn.

Hot water

A full heat pump usually heats a hot-water cylinder too. While it does, it isn’t heating the house. That rarely justifies a bigger heat pump, but the cylinder size and reheat schedule should be planned, for example reheating in the afternoon when outdoor temperatures are highest.

Questions to ask for your quote

• Has a room-by-room heat loss calculation to BS EN 12831 been done, and what was the total?
• What design flow temperature was used, and does the unit deliver enough heat at that temperature?
• Which external design temperature was assumed for your location?
• What is the unit’s minimum output, so it won’t cycle in mild weather?
• What SCOP does the MCS performance estimate show for your home?

If you are torn between two sizes, compare the most efficient 8–10 kW heat pumps and 6–8 kW heat pumps. Grant details are in our heat pump grants guide.