Home / Guides / Combi or system boiler?
Combi or system boiler: which one suits your home?
For most homes with one bathroom, a combi. For homes where two showers or baths are used at the same time, a system boiler with a hot water cylinder. Here’s how to tell which you are.
- 1bathroom: a combi usually fits best
- 2+bathrooms in use at once: think cylinder
- 0tanks in the loft with either
System and cylinder
When you’re replacing a boiler, the first question isn’t which brand. It’s which type. Get that right and the rest follows.
The difference comes down to hot water. A combi heats it as you use it. A system boiler heats a tank of it, called a cylinder, ready for when you need it. Neither is better in every home. It depends on how many bathrooms you’ve got, how many of you there are, and how strong your water main is.
Combi boilers
Hot water straight from the mains, as you use it.
A combi heats your radiators and heats water for the taps on demand, straight from the mains. There’s no cylinder and no tanks in the loft.
Good for
- Flats
- Homes with one bathroom
- Freeing up a cupboard
- Hot water that never runs out
Watch out for
The hot water is shared. Run a shower and a hot tap at the same time and both drop. How much hot water you get also depends on your mains, so we measure it at the survey before recommending one.
Combi
System boilers
A boiler and a cylinder of hot water ready to go.
A system boiler heats your radiators and a hot water cylinder. Most new ones use an unvented cylinder, which runs at mains pressure, so the showers are strong.
Good for
- Two or more bathrooms
- Busy households
- Showers used at the same time
- Homes thinking about a heat pump later
Watch out for
It needs a cupboard for the cylinder. Stored hot water can run out after a lot of use and take a while to reheat. An unvented cylinder also needs its own service every year by an engineer with the unvented qualification.
And regular boilers?
Older homes often have a regular boiler with tanks in the loft. Most are replaced with a combi or a system boiler, but where the water main is weak, keeping a tank fed system can still be the right answer.
Hot water cylinder
Which one sounds like you?
- Flat or one bathroomCombi boiler
Simple, compact and no cylinder.
- Two bathrooms used at onceSystem and cylinder
Strong showers in both.
- Weak water mainAsk at the survey
We measure it before we recommend.
- Heat pump in a few yearsKeep the cylinder space
Heat pumps usually need one.
Side by side
| Combi | System | Regular | |
|---|---|---|---|
| Hot water | Heated as you use it | Stored in a cylinder | Stored in a cylinder |
| Cylinder cupboard | Not needed | Needed | Needed |
| Tanks in the loft | No | No | Yes |
| Two showers at once | Flow is shared | Yes | Yes, at lower pressure |
| If the boiler breaks down | No hot water | Usually an immersion backup | Usually an immersion backup |
| Usually suits | Flats, one bathroom | Two or more bathrooms | Weak mains, older homes |
Every home is different. We recommend a type only after measuring your water supply and looking at how you use hot water.
Common questions
Can I swap my system boiler for a combi?
Often, yes, and it frees up the cylinder cupboard. We’d check your water main and how many bathrooms you use at once first, because a combi that can’t keep up isn’t an upgrade.
Will a combi run two showers at once?
Most combis struggle, because the hot water is shared between them. A higher output combi on a strong mains supply does better. We measure your flow at the survey and tell you honestly.
Which is cheaper to run?
There isn’t much in it for most homes. A combi only heats what you use. A modern, well insulated cylinder loses very little heat. Getting the right size matters more than the type.
Does a system boiler need extra servicing?
If it has an unvented cylinder, yes. The cylinder needs its own check every year, and our engineers can do the cylinder and the boiler in the same visit.
Do I need a cylinder for a heat pump?
Almost always. Heat pumps heat water more slowly than a boiler, so they store it in a cylinder. If a heat pump is on the cards, keeping the cylinder space can save work later.
How much does a new boiler cost?
It depends on the type, the model and what your home needs. Our online quote takes about two minutes and gives you a guide price, and we confirm everything after a free survey.
Related pages
Not sure what you need? Give us a shout.
Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We’ll tell you what we think, and if the answer is to leave things as they are, we’ll say that too.
Worcester Bosch Accredited Installer
Guarantees of up to 12 years on the boilers we fit. Gas Safe registered. Fife Trusted Trader. 400+ customer reviews.