The short answer
The size of heat pump you need is set by your home’s heat loss: how many kilowatts of heat escape through the walls, roof, floor, windows and draughts on a cold winter day. A kilowatt (kW) is a measure of heating power. An installer works this out with a room by room heat loss calculation, then picks a heat pump that can cover that loss at your local design temperature. For many UK homes the answer lands somewhere between about 4kW and 12kW, but the range is wide on purpose. Floor area, bedroom count and the size of your old boiler are poor guides, and relying on them is the most common reason heat pumps disappoint.
How a heat loss calculation works
An MCS-certified installer follows a standard method. MCS is the certification scheme for heat pump installers, and it is required for the Boiler Upgrade Scheme grant. In outline:
- Measure each room. Floor area, ceiling height, and the size of every window and external door.
- Record the construction. Solid or cavity walls, insulation depth in the loft, floor type, glazing type. Each has a known rate of heat loss, called a U-value.
- Allow for ventilation. Older, draughtier homes lose more heat through air leakage.
- Set the target temperatures. For example, around 21°C in living rooms and a little cooler in bedrooms.
- Use the local design temperature. The outside temperature the system must cope with, based on where you live. Colder areas need more capacity.
- Add it up. The total heat loss in kW becomes the minimum output the heat pump must deliver.
The same calculation tells the installer how much heat each room needs, which decides whether your radiators are big enough.
Why rules of thumb mislead
| Rule of thumb | Why it goes wrong |
|---|---|
| “Watts per square metre” | Two homes of the same size can lose very different amounts of heat depending on walls, glazing and draughts |
| “Match the old boiler” | Boilers are usually oversized, and combis are sized for hot water, not heating |
| “One kW per room” | Ignores room size, which walls are external, and how warm each room needs to be |
| “Go bigger to be safe” | Oversizing costs more and can make the system less efficient |
| “Use the EPC figure” | EPCs use standard assumptions and are not designed for sizing a heating system |
These shortcuts are fine for a very rough budget conversation. They are not fine for choosing the unit you will live with for the next fifteen years or more.
Hot water changes the sum a little
A heat pump also heats a hot water cylinder. The installer sizes the cylinder for your household, usually around how many people live there and how many bathrooms you use at once. The heat pump does not normally need to be bigger to cope with hot water, because the cylinder stores heat and the system can switch between heating and hot water. A larger household may need a bigger cylinder rather than a bigger heat pump.
What changes the size
Things that push the size down:
- Loft insulation topped up, cavity walls filled.
- Draught proofing, especially around floors and chimneys.
- Being a mid-terrace, with shared walls on both sides.
- Double or secondary glazing.
Things that push it up:
- Solid walls with no insulation.
- High ceilings and large rooms.
- An exposed, colder location.
- Large areas of glass, such as a conservatory open to the house.
This is why it can pay to do cheap insulation work before the survey. A smaller unit can mean a lower price and lower running costs. Our ECO4 and insulation grants guide covers help with insulation.
Ask every installer for their heat loss figure in kW and the heat pump output at your design temperature. If two quotes differ wildly in size, ask each one to explain how they got there.
How size affects price
Larger units cost more, but size is only one part of the bill. Radiator changes, the cylinder, pipework and the position of the outdoor unit all count. A typical air source heat pump costs £11,000 to £14,500 before any grant, and £3,500 to £7,000 after the £7,500 Boiler Upgrade Scheme grant in England and Wales (typical, checked October 2026). See our cost guide for the full picture.
Checking the work
You are entitled to see the heat loss calculation, and a good installer will share it. It should list each room, its heat loss in watts, the target temperature and the radiator chosen to cover it. If you are not sure what you are looking at, our guide to how a heat pump survey works explains what the installer measures and why.
City Boiler is not an installer. We connect you with independent MCS-certified installers who carry out a full heat loss calculation before they quote, so the size is based on your house.
Common questions
What size heat pump does a 3 bed house need?
There is no single answer, which is the point. A well-insulated three bed semi might need a much smaller unit than a draughty three bed Victorian house. Many three bed homes end up somewhere in the 5 to 10kW range, but only a heat loss calculation tells you where yours sits.
Can I use my boiler's size to choose a heat pump?
No. Most gas boilers are oversized for the heating they actually do, often because a combi is sized for hot water flow. A 30kW combi does not mean the house needs a 30kW heat pump, and it rarely does.
What happens if a heat pump is too big?
An oversized heat pump costs more to buy, can switch on and off too often, and may run less efficiently. It also needs more space and can be louder. Bigger is not safer with heat pumps.
What happens if a heat pump is too small?
On the coldest days it may not keep the house at the target temperature, and it may lean on a backup electric heater, which is expensive to run. Proper sizing to MCS standards is designed to avoid this.