How the boiler size calculator works
The calculator does two things. First, it estimates your home's heating demand from floor area and insulation level, using typical heat loss rates of 50–100 watts per m². Second, it works out the power needed to heat hot water on demand for your shower flow, since this usually decides the size of a combi.
Typical boiler sizes
| Home | Combi | System / regular | Cylinder |
|---|---|---|---|
| Flat / 1–2 bed, 1 bathroom | 24–28 kW | 12–15 kW | 120–150 L |
| 3 bed, 1–2 bathrooms | 28–32 kW | 12–18 kW | 180–210 L |
| 4 bed, 2+ bathrooms | 32–40 kW* | 18–24 kW | 210–250 L |
| 5+ bed, 3+ bathrooms | Not recommended | 24–30+ kW | 250–300 L+ |
*Only if showers aren't used at the same time and the mains water pressure and flow are good.
Before you buy
- A combi can only deliver as much hot water as your mains flow rate allows. Ask your installer to measure it.
- Larger boilers may need a bigger gas pipe from the meter. Your Gas Safe engineer will check.
- Thinking about a heat pump in future? A system boiler with a cylinder makes the switch easier.
- This is an estimate. A proper room-by-room heat loss calculation gives the most accurate answer.
Worked example: a 90m² three-bed semi
With average insulation (cavity walls filled, loft insulated), 90m² × 75W per m² gives a heating demand of about 6.8kW on a cold day. A good shower at 11 litres a minute, heated by 35°C, needs about 26.8kW. As in most homes, hot water, not heating, sets the size of a combi. The calculator suggests a 28kW combi.
If two showers are often used at once, a combi can't keep up, so the advice switches to a 12kW system boiler with a cylinder of about 180 litres. The boiler is much smaller because the cylinder stores hot water ready for use, so the boiler only has to heat the house and top up the cylinder.
Why heat loss matters more than the old boiler's size
Many UK homes have boilers far bigger than their heating needs, often because the old one was oversized and simply replaced like for like. Modern boilers turn down (modulate) to match demand, but every boiler has a minimum output. If that's well above what the house needs in mild weather, it cycles on and off, which wastes gas and wears parts. Insulating first shrinks the heating demand. Start with the loft insulation calculator if your loft is below 270mm.
Running a boiler efficiently
- Lower the flow temperature. Condensing boilers are most efficient when the water returning to them is cool. Many homes stay comfortable with the radiator flow set lower than the factory default, if the radiators are big enough. The radiator BTU calculator shows the sizes needed at lower temperatures.
- Use controls well. A programmable thermostat and TRVs on radiators stop you heating empty rooms.
- Service it yearly to keep it safe and efficient, and keep the system water clean with a filter and inhibitor.
Common mistakes
- Choosing the biggest combi "to be safe". More kW won't give more hot water than your mains supply can deliver.
- A combi in a busy household. Two bathrooms used together in the morning is the classic case for stored hot water.
- Ruling out a heat pump without checking. Government grants may help with the cost of switching. Check GOV.UK for the current scheme, and ask installers for a heat loss survey either way.
More home calculators are in heating, insulation and measuring. Our loft insulation guide shows one of the cheapest ways to cut heating demand.