What size heat pump do I need? The answer depends on how much heat your home loses on a cold winter’s day, not the size of your old gas boiler. A heat pump should be sized to replace that heat loss steadily and efficiently, so your home stays warm without the system constantly cycling on and off.
Most UK homes need a heat pump somewhere in the range of 4kW to 12kW, but the right size depends on the property, insulation, radiator setup, hot water demand and local design temperature. A proper room-by-room heat loss survey is the important bit. It tells the installer how much heat each room needs, and whether the heat pump, radiators and pipework can all work together properly.
Why heat pump sizing is different to boiler sizing
This is the bit that confuses most people, and it is worth getting straight before looking at any numbers.
Gas combi boilers are sized for hot water, not heating. A 30kW combi has that rating because it needs to heat a steady flow of cold mains water to bath or shower temperature on demand. Nothing about that 30kW figure tells you how much heat your house actually loses on a cold day. In most UK homes the answer is far less. A 30kW combi sits on the wall of plenty of houses that lose 5kW, 6kW, or 8kW at the coldest part of winter.
Heat pumps are sized the other way around. They are matched to the heat loss of the building, not to the flow rate at the kitchen tap. Hot water is handled separately by a hot water cylinder that the heat pump reheats over a period of time, so instantaneous output is not the limiting factor. That is why a 7kW heat pump replacing a 30kW combi is not a downgrade. It is the right answer to a different question.
If a number sounds suspiciously small compared to your old boiler, that on its own is not a red flag. What matters is whether the kW figure matches the heat loss the installer has actually calculated for your property.
How installers calculate the right size: the heat loss survey
A proper heat loss survey is a room-by-room calculation. The installer measures or estimates the dimensions of each room, the construction and insulation of the external walls, the glazing, the floor and ceiling, and how much air the room exchanges with the outside. They then work out how much heat the room would lose at a defined “design” outdoor temperature, usually around -2°C to -3°C in much of England, slightly colder in parts of Scotland and the north, and milder in coastal areas. Target indoor temperatures are typically 21°C in living rooms and around 18°C in bedrooms, with bathrooms a little warmer.
Each room’s heat loss is summed to give a whole-house heat demand in kW. The heat pump is then specified to match that figure, with a small margin and an allowance for reheating the hot water cylinder. The MCS standard (MIS 3005) defines how this calculation is performed, drawing on CIBSE and BS EN 12831 methods.
For any installation funded by the Boiler Upgrade Scheme, the MCS-accredited installer is required to carry out and document this calculation. That is one of the better consumer protections built into the grant: you cannot collect £7,500 and skip the maths.
Rule-of-thumb sizing by property type
These figures are ballpark only, useful for sense-checking a quote before a survey has been done. A real heat loss survey can land 2kW either side of these numbers depending on insulation, age, exposure, and how leaky the building is.
| Property type | Approximate floor area | Typical kW range | Key caveats |
|---|---|---|---|
| 1-bed flat | 40–55 m² | 4–5 kW | Lower if mid-floor with heated neighbours, higher if top-floor or corner |
| 2-bed terrace | 60–80 m² | 5–7 kW | Mid-terrace lower than end-terrace; insulation level matters a lot |
| 3-bed semi | 80–110 m² | 7–10 kW | Most variable category, hinges on insulation and glazing |
| 4-bed detached | 110–160 m² | 10–14 kW | Older uninsulated detached can push higher |
| 5+ bed or large detached | 160 m²+ | 12–16 kW+ | Often worth considering insulation upgrades first |
Indicative only, subject to heat loss survey.
A rougher shortcut some people use is floor area in square metres multiplied by a watts-per-square-metre figure. Very well-insulated modern homes can come in around 25–40 W/m². Pre-1980s housing with reasonable retrofit work is often closer to 50–70 W/m². Older, uninsulated solid-wall stock can be 100 W/m² or more. So a 100m² semi with patchy insulation at 70 W/m² lands at roughly 7kW. It is a useful first guess, no more than that.
What happens if the heat pump is oversized or undersized
Oversizing is the more common error in practice, partly because it feels like the safer choice and partly because some heat loss calculators default to worst-case assumptions that inflate the figure.
An oversized heat pump short-cycles. It produces more heat than the radiators or underfloor heating can shed, the return water heats up quickly, the unit switches off, the water cools, and it switches back on. The cycling hammers efficiency, drags the seasonal COP down, raises running costs, and shortens the lifespan of the compressor. You also pay more upfront for a bigger unit you did not need.
Undersizing is the opposite failure mode. The heat pump runs flat out on cold days and still cannot get rooms to target temperature, so the backup electric immersion kicks in. That is expensive electricity used inefficiently. Rooms feel cold, bills creep up, and trust in the system drops.
A small margin above the design heat loss is normal and healthy. A factor of two or three times the real heat loss is not.
Other factors that change the sizing answer
Hot water demand matters. A household of five with two bathrooms and a large cylinder reheating multiple times a day pulls more from the heat pump than a single occupant with a 150-litre cylinder. The kW figure does not change dramatically because of this, but cylinder size and reheat schedule do feed into the design.
Planned changes to the building should also feed in. If a loft conversion or rear extension is going in next year, it is often worth sizing for the future home rather than the current one, rather than paying to upgrade the unit later. The same applies in reverse for insulation. If cavity wall insulation, loft top-up, or new glazing is planned before the install, the heat loss figure shrinks and the system can sometimes drop into a smaller, cheaper model.
The emitters in your home affect the design flow temperature, which indirectly affects sizing. Underfloor heating runs at a lower flow temperature and gives the heat pump an easier job. Existing standard radiators that are too small for low-temperature operation may need to be upgraded. Larger emitters allow lower flow temperatures, which improves efficiency and gives the installer more headroom on sizing.
Regional climate plays a role too. A house in the Highlands is designed against a colder outdoor temperature than the same house in Cornwall, and that flows through to a slightly larger kW figure.
The Boiler Upgrade Scheme and MCS sizing requirements
As of 2026, the Boiler Upgrade Scheme in England and Wales provides £7,500 towards an air source or ground source heat pump. From July 2026, a temporary increase to £9,000 applies to homes and small businesses currently heated by oil or LPG, running until March 2027. The grant is paid directly to the MCS-accredited installer and deducted from the homeowner’s invoice. It is not paid to the homeowner.
Scotland, Wales, and Northern Ireland have separate schemes with different amounts and rules. Grant tiers and end dates can change, so the figures here should be sense-checked at the point of quoting.
The relevant point for sizing: because grant-funded installations must be carried out by MCS-accredited installers, and MCS requires a documented heat loss calculation, anyone taking the grant gets a properly sized system by default. That is a meaningful safeguard against guesswork.
FAQ
What size heat pump do I need for a 3-bed house?
A typical UK 3-bed semi usually needs a heat pump in the 7–10kW range. A well-insulated 3-bed could come in closer to 6kW, an older 3-bed with solid walls and single glazing can push past 11kW. A heat loss survey is the only way to land on the correct figure.
Is a heat pump smaller than my boiler?
Almost always, yes. Gas combi boilers are sized for instantaneous hot water demand and are routinely 24kW, 30kW, or higher. Heat pumps are sized for the heat the building actually loses on a cold day, which for most UK homes is in the 5–12kW range. A 7kW heat pump replacing a 30kW combi is not a mistake.
How is heat pump size measured?
In kilowatts (kW) of heat output, not in physical dimensions or in the wattage drawn from the electricity supply. The kW figure refers to the heat the pump can deliver to the heating system at design conditions.
Can a heat pump be too big?
Yes. An oversized heat pump short-cycles, runs less efficiently, costs more to run, and costs more to buy. Oversizing is one of the most common installation errors.
What happens if my heat pump is too small?
On the coldest days it cannot keep up with the heat loss of the building. The backup electric immersion kicks in to make up the shortfall, which is expensive, and rooms can fall short of target temperature.
Do I need a bigger heat pump for hot water?
Not usually. Hot water is handled by a cylinder that the heat pump reheats in batches, so peak space heating load is normally what drives sizing. Very large households with high hot water demand may see a small uplift in the design figure.
How accurate are heat pump sizing calculators?
Online calculators that ask for property type, floor area, and insulation level can give a reasonable first estimate, often within 1–2kW of the true figure. They are not a substitute for a room-by-room survey, especially in older or unusual buildings.
Does the Boiler Upgrade Scheme require a heat loss survey?
Yes, indirectly. The grant is only paid on installations carried out by MCS-accredited installers, and MCS requires a documented heat loss calculation as part of the design process.
What size heat pump for a 4-bed detached house?
Typically 10–14kW for a reasonably insulated 4-bed detached. Larger or older detached homes can push into the 14–16kW range. Insulation level is usually the biggest single variable.
Can I use my boiler’s kW rating to size a heat pump?
No. The kW rating of a combi boiler reflects hot water flow capacity, not how much heat the building loses. Using it as a proxy will almost always produce a heat pump that is much too big.