What it costs
Real installed prices for 2026, before and after grant, and the six things that actually move the number on your property.
CostsSizing comes from a heat loss calculation, never from floor area or your old boiler's output. Here is what that involves and why oversizing is as damaging as undersizing.
Tell us about your property and an MCS-certified engineer will be in touch, usually the same working day.
Most UK three and four bedroom houses land somewhere between 6kW and 12kW. But that range is so wide as to be almost useless, because a well-insulated 1990s four-bed can need less than a solid-wall Victorian three-bed. The number comes from your property, not from a table.
Rules of thumb like "70 watts per square metre" persist because they are easy, and they are wrong often enough to matter. Two houses of identical floor area can differ by a factor of two in heat demand depending on wall construction, glazing, insulation, air tightness, orientation and exposure.
Sizing off your existing boiler is worse. Most UK boilers are enormously oversized, frequently 24kW or 30kW on a house needing 8kW, because boilers are sized for hot water flow rate rather than heat demand. Copying that number gives you a heat pump twice the size you need.
An engineer measures every room and works out how fast each one loses heat at a design outside temperature. That means recording:
The output is a heat loss figure in watts for every room. Sum them and you have the whole-house demand that determines the unit. Individually, each room's figure determines whether its radiator is big enough at your design flow temperature.
MCS requires this calculation. If an installer cannot show you one, they have not done it.
People assume a bigger unit is safer. It is not. An oversized heat pump spends most of the year cycling on and off rather than running steadily, which is exactly the condition in which they are least efficient and most prone to wear. It also costs more to buy, is physically larger, is noisier and is harder to site within the permitted development noise limit.
Undersizing leaves you cold on the coldest days and forces the system to run flat out at high flow temperatures, which is expensive. There is a correct answer and it is worth getting right.
Space heating and hot water are different loads. A household of five showering daily needs substantially more hot water capacity than a couple, and that affects both cylinder sizing and, sometimes, the unit. A good survey asks how many people live there and how they actually use hot water, which surprises people who expected a purely technical conversation.
| Property | Well insulated | Solid wall / period |
|---|---|---|
| Flat or small 2 bed | 4 – 5kW | 6 – 8kW |
| 3 bed semi | 5 – 7kW | 8 – 12kW |
| 4 bed detached | 7 – 9kW | 12 – 16kW |
| Large period property | 10 – 14kW | 16 – 25kW+ |
Use this to sanity check a quote, not to specify a system. If an installer's proposed size sits far outside the relevant column, ask them to walk you through the calculation.
Most UK three and four bedroom houses need between 6kW and 12kW, but the figure comes from a room-by-room heat loss calculation rather than floor area. A solid-wall Victorian three-bed can need more than a well-insulated modern four-bed.
No, and it is one of the commonest mistakes. Boilers are sized for hot water flow rate, not heat demand, so most UK boilers are massively oversized. A house with a 30kW boiler often needs an 8kW heat pump.
No. An oversized unit cycles on and off instead of running steadily, which is when heat pumps are least efficient and wear fastest. It also costs more, takes more space, is noisier and is harder to site within the permitted development noise limit.
A room-by-room assessment of how fast your property loses heat at a design outside temperature, accounting for wall construction, glazing, insulation, air changes and your regional climate. It determines both the unit size and which radiators need upsizing. MCS requires one.
Yes. Space heating and hot water are separate loads, and a household of five uses far more hot water than a couple. It affects cylinder sizing and sometimes the unit selection, which is why a good survey asks about occupancy and habits.
Real installed prices for 2026, before and after grant, and the six things that actually move the number on your property.
CostsThe Boiler Upgrade Scheme in England and Wales, Home Energy Scotland north of the border, and what applies in Northern Ireland.
GrantsSurvey, design, grant, installation and commissioning, with honest timescales and a clear account of the disruption.
ProcessWhy sizing comes from a room-by-room heat loss calculation and never from floor area or your old boiler's output.
SizingWhich radiators need upsizing at a lower flow temperature, and why this is the single biggest swing factor in your quote.
RadiatorsWhy heat pumps need a cylinder, what happens if you are on a combi, and how much space you actually need to find.
Hot waterUK coverage
We install air source heat pumps nationwide through MCS-certified regional teams. Grant support differs between England, Wales, Scotland and Northern Ireland, and the regional pages set out what applies where you are.
Get a free, no-obligation quote for heat pump installation in the UK, with the grant deducted so you see the real figure. We come back to you the same working day.