What a combi boiler actually is
A combi boiler does two jobs in one wall-mounted unit: it runs your radiators, and it makes hot water instantly, on demand, straight from the cold mains. There is no cylinder storing hot water and no tank in the loft. Turn on a tap and the boiler fires; turn it off and it stops. That on-demand design is why a combi is compact, efficient (no stored water cooling down and being reheated), and the cheapest boiler type to install. It is also the source of its one real limitation, which the advisor below checks for your home.
Is a combi the right boiler for your home?
Three quick questions about how your home uses hot water. We recommend combi, system or conventional, and explain why.
Recommended boiler type
A guide based on hot-water demand. A Gas Safe engineer confirms the final choice with a flow-rate and pressure check.
Recommendation logic based on simultaneous hot-water demand and cylinder space. Verified June 2026.
Combi vs system vs conventional
Every UK home runs one of three boiler types. The right one is decided almost entirely by how much hot water you need at the same time, and how much space you have to store it.
| Type | Hot water | Best for | Watch out |
|---|---|---|---|
| Combi boiler | On demand, no tank | Flats and small-to-medium homes with 1 bathroom | Struggles with two taps/showers at once; flow drops on low mains pressure |
| System boiler | Stored in a cylinder | Homes with 2+ bathrooms used at the same time | Needs cylinder space; hot water can run out until it reheats |
| Conventional (regular) | Cylinder + cold loft tank | Older/larger homes with existing tanks or low mains pressure | Bulky (cylinder + 1-2 tanks); most space of any system |
The belief that puts the wrong boiler on the wall
The common assumption is that a bigger or more powerful combi can do anything a system boiler can. It cannot. A combi heats water as it flows through, so its ceiling is flow rate, how many litres a minute it can warm, not storage. No matter how many kilowatts it has, split that flow across two showers and both go weak and lukewarm.
This is why the right question is never "which combi is biggest?" but "how many hot outlets do we run at once?" Answer that honestly and the boiler type chooses itself.
The insider view: flow rate, not kilowatts, is the number that matters
Installers quote combi boilers in kW, and buyers fixate on it. But for hot water, the figure that decides your shower experience is the flow rate in litres per minute at a given temperature rise, and it depends as much on your incoming mains pressure as on the boiler. A 35 kW combi in a home with weak mains pressure can deliver a worse shower than a 30 kW combi on strong mains.
The test the showroom skips Before anyone quotes you a combi, the installer should measure your mains flow rate and pressure at the kitchen tap. If it is low (under about 10 litres a minute), a combi may disappoint no matter its kW rating, and a system boiler with a stored cylinder is the safer choice. An installer who quotes a combi without measuring your flow is guessing. This single measurement, not the brand, is what predicts whether you will be happy with the shower.
What to actually do
Count your simultaneous hot taps
One bathroom, rarely two taps at once: a combi is ideal. Two showers together, often: look at a system boiler.
Ask for a flow-rate test
Insist the installer measures your mains flow and pressure before recommending a combi. It is the number that predicts your shower.
Right-size it, do not over-spec
Use the boiler size calculator for a sensible kW. Bigger costs more and short-cycles.
Compare against a heat pump
If your boiler is failing and the home is insulated, check the total-cost case for a heat pump before defaulting to a combi.
The bottom line
A combi boiler is the right choice for the majority of UK homes: compact, efficient, cheapest to fit and endless hot water for a single bathroom. It is the wrong choice when you regularly run two hot outlets at once, where a system boiler wins. Decide on simultaneous demand and mains flow, not on kilowatts or brand, and you will put the right boiler on the wall the first time.
Combi boiler FAQ
The Selectra expert answers your questions
A combi (combination) boiler is a single unit that provides both your central heating and your hot water on demand, heating water straight from the mains the moment you turn a tap. It needs no hot-water cylinder and no cold-water tank, which is why it is compact and the most common boiler in UK homes. The trade-off is that it heats a limited flow of water at a time, so it suits homes that rarely run two hot outlets at once.
For most flats and homes with one bathroom, yes: a combi is the efficient, low-cost, space-saving choice. With two bathrooms it can still work, but if two showers often run at once the flow drops and a system boiler is usually better. With three or more bathrooms, a system or conventional boiler with a stored cylinder is the right answer. Use the advisor above for a recommendation based on your home.
A new combi costs roughly £1,800 to £3,200 fitted in 2026 (budget to premium), which is cheaper than a system or conventional install because there is no cylinder to fit. See our new boiler cost guide for the full breakdown and the grants that can reduce it.
Most UK homes need a 24 to 35 kW combi, set mainly by the number of bathrooms and radiators, not floor area. Bigger is not better: an oversized combi costs more and short-cycles. Our boiler size calculator gives you a kW figure to check any quote against.
A combi is cheaper to fit and works in any home; a heat pump costs more upfront (even after the £7,500 grant) but can cost less to run on the right electricity tariff. If your boiler is failing and your home is well insulated, it is worth comparing both. See combi vs heat pump cost for the full total-cost comparison.