The short answer
A combi heats water on demand with no cylinder. A system boiler heats a stored cylinder of hot water and takes its water straight from the mains. A conventional boiler, also called a regular or heat-only boiler, heats a cylinder too but is fed by tanks in the loft. For most homes with one bathroom, a combi is the simplest and cheapest choice. For busy homes with two or more bathrooms, a system boiler usually wins. A conventional boiler is mainly the right call when your existing pipework or water pressure depends on loft tanks.
The best type is the one that matches how many people shower at the same time, how strong your mains water is, and where you have room for a cylinder.
How each one works
Combi boiler
Cold mains water passes through the boiler and comes out hot at the tap. The same box runs your radiators. There is no cylinder to heat up and no hot water to run out of, but the boiler can only heat water at a certain rate. That rate is measured in litres per minute and is set mostly by the boiler’s output in kW (kilowatts, how much heat it can produce at once) and your mains flow.
System boiler
The boiler heats your radiators and an unvented hot water cylinder, usually kept in an airing cupboard. The pump and expansion vessel are inside the boiler, so there are no loft tanks. Because the cylinder holds a store of hot water at mains pressure, two showers can run at once with good pressure.
Conventional boiler
The oldest layout. A cold water tank in the loft feeds a vented hot water cylinder, and a small feed and expansion tank tops up the heating. Water pressure at the taps comes from gravity, so showers on the top floor are often weak unless a pump is fitted. Many older homes still run this way.
Side by side
| Compare | Combi | System | Conventional |
|---|---|---|---|
| Hot water cylinder | No | Yes, unvented | Yes, usually vented |
| Loft tanks | No | No | Yes, two |
| Space needed | Wall cupboard | Boiler plus airing cupboard | Boiler, cylinder and loft space |
| Two showers at once | Weak | Good | Depends on gravity or a pump |
| Hot water runs out | No, but flow is limited | Yes, when the cylinder empties | Yes, when the cylinder empties |
| Needs good mains pressure | Yes | Yes | No |
| Typical like-for-like price | £2,200 to £3,400 | £2,800 to £4,000 | Usually close to a system swap |
Prices are typical fitted national ranges including VAT, checked October 2026. Your installer will give a fixed price after seeing the job.
Which homes suit which
A combi usually suits:
- flats and homes with one bathroom
- households where only one shower runs at a time
- homes with good mains pressure and flow
- anyone who wants the airing cupboard and loft space back
A system boiler usually suits:
- homes with two or more bathrooms, or an en suite used at the same time as the main bathroom
- larger families with a busy morning routine
- homes planning a heat pump later, because a heat pump also needs a cylinder
A conventional boiler usually suits:
- homes with weak mains pressure, where a combi or unvented cylinder would struggle
- older properties where the pipework was built around loft tanks and is not easy to change
- homes where you want to keep the existing setup and avoid a bigger job
Should you change type or stay the same?
Staying with the same type is almost always the cheapest job. The pipework, flue route and controls already suit it, and most like-for-like swaps are done in one or two days. See combi boiler replacement, system boiler replacement and conventional boiler replacement for what each swap involves.
Changing type costs more but can be the right move. The common switch is from a conventional or system boiler to a combi, which frees the airing cupboard and loft and removes a tank that can leak or freeze. A combi conversion typically costs £3,000 to £4,200 fitted and takes one to two days.
Before you agree to a conversion, ask the installer to measure your mains flow rate and pressure at the kitchen tap. If the flow is weak, a combi will give a disappointing shower and no boiler upgrade will fix that. A good installer will tell you to keep the cylinder rather than sell you a combi that cannot keep up.
If two people in your home regularly shower at the same time, think hard before swapping a cylinder for a combi. A combi shares its flow between taps, and no amount of output fully fixes that.
Questions to ask on the survey
- How many bathrooms and showers could run at the same time?
- What is the mains flow rate in litres per minute, and the pressure?
- Is the existing cylinder in good condition, and is it vented or unvented?
- Will the flue stay in the same place, or does the boiler need to move?
- What output in kW does the installer recommend, and why?
The output question matters for every type. A combi is sized for hot water flow, while system and conventional boilers are sized for the heat your home loses. Our guide on what size boiler you need covers this in more detail.
Thinking longer term
If you plan to fit a heat pump in the next few years, a system layout with a cylinder makes that switch easier, because a heat pump stores hot water in the same way. Ripping out a cylinder to fit a combi now, then putting one back later, costs you twice. If a heat pump is on your list, compare both options on our heat pump installation page before you decide.
For everyone else, match the boiler type to how your household uses hot water, keep the existing layout if it works, and only pay for a conversion when it solves a real problem. Our boiler replacement cost guide breaks down what the full job should cost.
Common questions
Which type of boiler is best?
None is best in every house. A combi suits most one-bathroom homes with decent mains pressure. A system boiler suits homes with two or more bathrooms used at once. A conventional boiler mainly makes sense where the pipework or water supply depends on tanks in the loft.
Can I switch from a conventional boiler to a combi?
Usually, yes. The cylinder and loft tanks come out, and some pipework is changed. A system or regular to combi conversion typically costs £3,000 to £4,200 fitted (checked October 2026), and it only makes sense if your mains pressure and flow can supply your taps directly.
Does a combi boiler give less hot water?
It gives endless hot water, but only at a set flow rate. Run two showers at once and both weaken. A system or conventional boiler stores a cylinder of hot water, so several taps can run together until the cylinder runs low.
Is a system boiler the same as a conventional boiler?
No. Both use a hot water cylinder, but a system boiler takes water directly from the mains and has the pump and expansion vessel built in. A conventional boiler also needs a cold water tank and a feed and expansion tank in the loft.