Sizing Is the Most Important Decision
Get the size right and your heat pump is quiet, cheap to run and keeps the whole house evenly warm. Get it wrong — in either direction — and you pay for it every winter. Sizing is arguably the single most important decision in the whole installation, and it is one you cannot judge from a brochure.
Forget "Bedrooms" — It Is About Heat Loss
A common shortcut is to size a heat pump by the number of bedrooms or the old boiler's output. Both are unreliable. A 3-bed 1930s semi and a 3-bed new build can have completely different heat demands.
The correct method is a room-by-room heat loss calculation: measuring how much heat your specific home loses on a cold day, based on wall construction, glazing, floors, room sizes and ventilation. That figure — in kilowatts — is what the heat pump is sized to match.
Why Bigger Is Not Better
It is tempting to think a larger heat pump is "safer". It is not. An oversized unit short-cycles — switching on and off repeatedly — which reduces efficiency, increases wear and can make the system noisier. It also usually costs more to buy.
A correctly sized heat pump runs steadily at a low output for long periods, which is exactly how a heat pump is most efficient. Right-sizing is what delivers a high seasonal COP of 3.5 to 4.9.
Typical Sizes — But Only as a Rough Guide
As a very rough illustration, many UK homes end up with a heat pump somewhere between 5kW and 12kW. A well-insulated smaller home might need 5–7kW; a larger or older solid-wall property might need 10–16kW.
Treat those numbers as context only. The whole point of this guide is that your figure comes from a survey of your home, not from a table — and a good installer will show you the calculation.
Flow Temperature and Cylinder Size Matter Too
Sizing is not just the heat pump's kW rating. A proper design also sets the flow temperature low (which lifts efficiency), sizes some radiators to suit, and specifies a correctly sized hot water cylinder so you never run short of hot water.
These elements work together. That is why a design-led installer looks at the whole system, not just the box on the wall.
How to Get Your Number
The only reliable way to know what size heat pump you need is a proper heat loss survey of your home. Be wary of any quote that skips it or sizes from the old boiler.
Start with a free, no-obligation estimate. We survey properly, show you the heat loss figures, and size a system that is efficient, quiet and right for your house.
Frequently asked questions
How do I know what size heat pump I need?
The reliable method is a room-by-room heat loss survey of your specific home, which produces a heat demand figure in kilowatts. The heat pump is then sized to match. Sizing by number of bedrooms or by your old boiler's output is unreliable.
Is a bigger heat pump better?
No. An oversized heat pump short-cycles, runs less efficiently, wears faster and can be noisier — and it usually costs more. A correctly sized unit runs steadily at low output, which is how a heat pump achieves its best efficiency.
What size heat pump does an average UK home need?
As a rough guide many homes fall between 5kW and 12kW, but this varies hugely with insulation, age and size. Your actual figure should always come from a heat loss survey rather than a rule of thumb.
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