The short version
- Flats and 1 to 2 bed homes usually suit a combi of around 24 to 28kW, and 3 bed homes around 28 to 32kW.
- For a combi, hot water demand and mains flow rate decide the size more than the number of radiators.
- Homes with 2 or more bathrooms in use at once usually need a system boiler and cylinder sized to heat loss.
- A boiler that is too big cycles on and off, wastes gas and wears faster.
- These are rough guides. The survey confirms the size by measuring flow and looking at the home.
The short answer
Most homes need a smaller boiler than people expect. As a rough guide for a combi: flats and 1 to 2 bed houses with one bathroom suit around 24 to 28kW, and 3 bed houses with one bathroom suit around 28 to 32kW. A 3 to 4 bed house with a bathroom and an en-suite may suit around 32 to 38kW, but only if the mains water flow can support it. Bigger homes where 2 or more bathrooms are used at the same time usually need a system boiler and cylinder, sized to the home’s heat loss.
| Home | Bathrooms | Boiler | Rough size |
|---|---|---|---|
| Flat or 1 to 2 bed house | 1 bathroom | Combi | Around 24 to 28kW |
| 3 bed house | 1 bathroom | Combi | Around 28 to 32kW |
| 3 to 4 bed house | 1 bathroom and en-suite | Larger combi | Around 32 to 38kW, if mains flow allows |
| 4 bed or more | 2 or more used at once | System boiler and cylinder | Sized to the home’s heat loss |
Treat these as a starting point. The survey confirms the size, and the rest of this guide explains what it looks at.
What the kW number actually means
A kilowatt rating tells you the most heat a boiler can put out. On a combi, the headline figure is usually its hot water output, because heating water instantly for a shower takes far more power than keeping radiators warm. The heating output on the same boiler is often lower, and it is listed separately in the specifications.
That is why combi sizes go up with bathrooms, not bedrooms. A small house with a powerful shower can need a bigger combi than a larger house with a modest one. For a system or regular boiler, there is no instant hot water to provide, so the size is based on the heat the home needs plus the job of reheating the cylinder.
Mains flow rate: the limit on any combi
A combi heats cold water from the mains as it passes through. It can only heat as much water as arrives, so the flow rate coming into your home, measured in litres per minute, sets a ceiling on your hot water. Pressure and flow are not the same thing: a home can have reasonable pressure with the taps shut and still deliver poor flow once a shower is running.
If the flow is low, a bigger combi will not give you a better shower. It will just heat the same small amount of water with more power in reserve. In that case the honest answer may be a smaller combi, or a system boiler or regular boiler with stored hot water. Measuring the flow on the survey is the only way to know.
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Start my quoteBathrooms and how you use them
The question that matters most is how many showers or baths run at the same time. One bathroom, even in a large house, is usually comfortable on a combi. A bathroom plus an en-suite is where it gets borderline: a larger combi can cope if the mains flow is good and you rarely use both together.
If 2 or more showers often run at once, a combi will struggle however big it is. A cylinder stores a quantity of hot water that several outlets can share, so a system boiler is usually the better answer there. See combi boiler installation for where combis do well, and our guide on combi water pressure and flow rate.
Heat loss: what your radiators really need
Every home loses heat through its walls, roof, floor and windows. The heating side of a boiler only needs to replace that loss on a cold day. A heat loss calculation works it out room by room, from the size of each room, how it is built, the windows and the insulation.
For most flats and ordinary houses, the heat loss is well within what any modern boiler can supply. It matters most for system and regular boilers in larger homes, and for homes that have changed: a loft conversion, an extension or new windows all shift the number. A Victorian terrace with solid walls loses heat differently from a well-insulated newer home of the same size.
Why bigger isn’t better
It is tempting to choose the biggest boiler to be safe. It usually backfires. Modern boilers turn their output down to match demand, but only so far. If the smallest output a boiler can run at is still more than your home needs, it heats up quickly, switches off, cools, and fires again. This is called cycling.
- Each start-up wastes gas before the boiler settles into efficient running.
- Frequent starts put wear on the fan, ignition and valves.
- A condensing boiler is most efficient when the water coming back from the radiators is cool. Short bursts of high output keep it running hotter than it needs to.
- A bigger boiler usually costs more to buy, for no benefit you will notice.
How the survey confirms the size
A proper survey looks at your hot water use, measures the mains flow, checks the gas supply pipe is big enough for the boiler, and considers the home’s heat loss. The gas pipe is easy to overlook: a larger combi can need more gas than the boiler it replaces, and an undersized pipe from the meter has to be upgraded.
You can start the process from home. Tell us your bedrooms, bathrooms and current boiler in the quote form, add a photo or two, and an engineer will suggest a size and explain the reasoning. If anything needs measuring, we check it before confirming the price. For what changes the total, see boiler installation cost.
If two quotes suggest very different sizes for the same home, ask each installer how they arrived at their number. A clear answer about your bathrooms, flow rate and heat loss is a good sign. A vague one is worth noticing.