The short answer
The right heat pump size is set by your home's heat loss — how much heat it loses to the outside on a cold day — not by the rating of your old boiler. Boilers are usually heavily oversized, so copying their output would lead to a heat pump that is too large. Instead, an installer carries out an MCS heat loss survey that calculates, room by room, how much heat the property loses at a cold design temperature. The pump is then sized to match that peak heat demand in kilowatts (kW). For UK homes this commonly lands somewhere in the region of 5 to 12 kW, with smaller, well-insulated homes at the lower end and larger or older properties higher. Getting the size right matters: an undersized pump cannot keep the home warm in deep cold, while an oversized one cycles on and off inefficiently and costs more.
Sizing is the single most important design decision in a heat pump installation. The kW figure that matters comes from measuring how your specific home loses heat, which is why a proper survey, not a rule of thumb, is essential.
Sizing basics
- Sized onHome's heat loss, not old boiler
- Survey usedMCS heat loss calculation
- Typical UK rangeAround 5 to 12 kW
- Undersized resultCold home on coldest days
- Oversized resultInefficient cycling, higher cost
What heat pump sizing actually means
"Size" in heat pump terms means the unit's heat output in kilowatts (kW) — how much heat it can deliver at the design condition. The goal is to match that output to the home's peak heat demand: the most heat the property loses to the outside on the coldest day the system is designed for.
This is a fundamentally different exercise from picking a boiler. A boiler's job is to burn fuel fast, so being generously rated barely matters. A heat pump moves heat steadily and efficiently, and it performs best when its output closely matches the building's need. Too big and it cannot run gently; too small and it cannot keep up in deep cold. The sizing therefore has to be derived from the building itself — its floor area, construction, insulation, glazing and ventilation — rather than from any existing appliance. That is the whole purpose of a heat loss survey, and it is why two homes that look similar from the street can need different-sized pumps depending on what is behind the walls.
Why your old boiler size is the wrong guide
Gas boilers are routinely installed far larger than a home actually needs, partly to give rapid hot water and partly because oversizing a boiler carries little efficiency penalty. A typical combi might be rated at 24 to 30 kW or more, yet the home's actual heat loss might be only a fraction of that.
A heat pump is different. It is designed to run steadily at a low flow temperature, and oversizing causes real problems. So sizing a heat pump from the old boiler's rating would almost always produce a pump that is too big. The correct approach ignores the boiler and measures the building.
The reason boilers are oversized without harm is that a boiler simply modulates down or cycles off once the home is warm, and the wasted capacity costs little. A heat pump, by contrast, is most efficient when it runs long and gently, matching its output to the slow, steady rate at which the house loses heat. An oversized pump cannot do that — it heats the water quickly, satisfies the demand, switches off, then repeats. Each of those cycles is less efficient than continuous running and adds wear. So the very habit that is harmless with boilers — fitting something with plenty of headroom — actively undermines a heat pump. This is the single most common sizing mistake, and it is why the old appliance's rating is the wrong starting point.
How the heat loss survey sets the size
An MCS heat loss survey calculates how much heat each room loses through its walls, windows, roof, floor and ventilation, at a defined cold design temperature for your area. Adding up the rooms gives the home's total peak heat demand in kilowatts. That figure is the basis for the pump's size.
The same survey also informs radiator sizing and the design flow temperature. A heat pump emits heat at a lower temperature than a boiler, so each room needs emitters large enough to deliver its heat demand at that lower flow temperature. The survey ties all of this together so the system is comfortable and efficient.
One nuance worth knowing is how hot water demand factors into the size. The kilowatt figure that sizes the pump is driven by the home's space-heating loss, but the system also has to reheat the hot water cylinder. A well-designed installation handles this by briefly prioritising the cylinder and then returning to heating, rather than by fitting a larger pump — so a busy household's hot water needs rarely push the pump into a bigger size band. This is another reason the heat loss figure, not a guess about peak usage, governs the sizing. The surveyor balances space heating, hot water and the chosen flow temperature into a single coherent design, which is something no rule of thumb based on floor area or boiler rating can replicate.
| Home type (illustrative) | Insulation | Typical heat pump size |
|---|---|---|
| Small, modern, well-insulated | Good | Lower end of range |
| Average 3-bed semi | Moderate | Mid range |
| Large or older, poorly insulated | Poorer | Upper end of range |
Illustrative only — actual size depends on the heat loss survey. Sources: Energy Saving Trust; MCS.
The cost of getting the size wrong
Sizing is a balance, and errors in either direction cause problems:
- Undersized: the pump cannot meet the home's peak demand, so the house feels cold on the very coldest days and any backup heating has to make up the shortfall.
- Oversized: the pump produces more heat than needed for most of the year, so it short-cycles — switching on and off frequently — which wears components and reduces efficiency. An oversized pump also costs more to buy.
Because the consequences run in both directions, sizing should never be guessed. A reputable installer produces a documented heat loss calculation and shares the assumptions, so you can see how the kW figure was reached. This is a core part of the MCS design and is your assurance that the system will perform.
Frequently asked questions
Can I just match my old boiler's kW rating?
No — that usually leads to an oversized heat pump. Boilers are commonly installed much larger than a home's real heat loss, so copying their output produces a pump that short-cycles and runs inefficiently. The correct size comes from a heat loss survey that measures how your specific home loses heat, which is almost always a lower figure than the boiler.
What kW heat pump does a typical UK house need?
It varies widely with size, age and insulation, but many UK homes fall somewhere around 5 to 12 kW. A small, well-insulated property sits at the lower end, while a large or poorly insulated home needs more. The only reliable way to know your figure is a room-by-room MCS heat loss survey.
Why is an oversized heat pump a problem?
An oversized pump produces more heat than the home needs for most of the year, so it switches on and off frequently — called short-cycling. This reduces efficiency, increases wear on the compressor, and pushes up both the purchase cost and the running cost. Correct sizing keeps the pump running steadily and economically.
Sources & further reading
- Energy Saving Trust — Air source heat pumps
- MCS — Heat pumps for consumers
- Heat Geek — Heat loss and sizing
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.