A combi heats water on demand with no store, limited to one outlet at a time. A system boiler works with a cylinder, storing hot water for simultaneous use. A regular boiler is the traditional arrangement with a loft tank. Simultaneous hot water demand is the deciding factor.
What each one is
Combi
Heats water instantaneously as it flows through, with no storage at all. Open a hot tap and the boiler fires, heating the water as it passes. Compact, no cylinder, no loft tank, and hot water available indefinitely – but only as fast as the boiler can heat it.
System
Heats a hot water cylinder, with the pump and expansion vessel built into the boiler. The system is sealed, so no loft tank. Hot water is stored and available at whatever rate the cylinder and its feed can deliver.
Regular (heat only)
The traditional arrangement. Boiler plus a separate cylinder, with a cold water storage cistern in the loft and usually a second smaller tank feeding the heating. The pump and controls are external to the boiler.
| Combi | System | Regular | |
|---|---|---|---|
| Hot water | On demand, no store | Stored in a cylinder | Stored in a cylinder |
| Cylinder | None | Required | Required |
| Loft tanks | None | None | One or two |
| Simultaneous outlets | Poor – one at a time realistically | Good | Good, at gravity pressure unless unvented |
| Space used | Least | Cylinder only | Cylinder plus tanks |
| Components in the boiler | Most | Many | Fewest |
| Hot water pressure | Mains | Mains if unvented, gravity if vented | Gravity, unless unvented |
| Complexity to repair | Highest – more in one box | Moderate | Lowest – components accessible |
The question that actually decides it
How many people want hot water at the same time?
A combi heats water as it passes, so its hot water output is limited by how much heat it can transfer instantaneously. Two showers at once means splitting that output two ways, and neither is satisfactory. That is physics, not a quality issue – no combi solves it.
A cylinder stores heat. Two showers at once draws from a store, which handles it comfortably provided the cylinder is sized for it and the feed delivers.
- One bathroom, one or two people, sequential use – a combi is frequently ideal, and you gain the cylinder cupboard.
- Two or more bathrooms used simultaneously – a cylinder, without much doubt.
- A family household with a morning rush – a cylinder.
- A large house with long pipe runs – a cylinder, because a combi’s delay before hot water arrives gets worse with distance.
- A holiday let or a second home – a combi has advantages, with nothing stored and less to go wrong when unoccupied.
The most common regret we hear is from households that moved to a combi for the cupboard space and then discovered that two people cannot shower at once. It is worth thinking carefully about the busiest twenty minutes of a weekday rather than about average use.
Combis: the honest picture
What they do well
- No cylinder, no tanks – the space back, and no loft pipework to freeze.
- Mains pressure hot water at every outlet.
- No stored water losing heat continuously.
- Hot water never runs out – it just arrives at a fixed rate.
- Simpler installation, generally lower cost.
- No legionella consideration from stored water.
What they do less well
- One outlet at a time, realistically.
- Flow rate is the limit, and it is worth understanding that a combi’s hot water performance is quoted as a flow rate at a given temperature rise – and that rise is greater in winter when the incoming water is colder. Combis genuinely perform less well in January.
- Delay before hot water arrives, which gets worse with distance from the boiler.
- Scale in the plate heat exchanger, which in a hard water area is the defining long-term fault – it reduces both temperature and flow, gradually, over years. See what limescale actually does.
- More components in one box, so a fault takes the whole appliance out.
- No backup. With a cylinder and an immersion heater you have hot water when the boiler fails; with a combi you have none.
- Marginal mains supply hurts them, because everything comes off the main.
System and regular: which of the two
Both use a cylinder. The difference is where the components live and whether there is a loft tank.
- System boilers have the pump and expansion vessel inside the appliance and run a sealed system. Tidier, fewer components in the loft, and they pair naturally with an unvented cylinder.
- Regular boilers keep the pump and expansion arrangements external, with the traditional loft tanks. More components outside the boiler – which sounds like a disadvantage and is actually useful, because those components are individually cheap and accessible.
- Regular suits an existing gravity installation being replaced like for like, particularly where the loft tanks are sound and the household is happy with the system.
- System suits most new installations with a cylinder, and is the natural partner for an unvented cylinder.
The bigger decision alongside this is vented or unvented cylinder, which is a separate question driven by the incoming mains – see unvented or vented.
Things that should not decide it
- What was there before. The household may have changed since.
- Fashion. Combis became the default because they suit small modern homes, not because they suit everything.
- A desire for the airing cupboard back, if the household genuinely needs stored hot water. It is a real benefit and it is not worth a decade of staggered showers.
- Installation cost alone. The cheaper installation is not cheaper if it does not meet the need.
- House size. A large house with one occupant may suit a combi; a small house with four teenagers may not.
What should decide it is peak simultaneous hot water demand, the measured incoming mains flow, and whether there is space for a cylinder.
Not sure which suits you? Describe your busiest twenty minutes on a weekday morning. That answers it more reliably than anything else.
Questions we get asked about this
Should I switch from a cylinder to a combi?
If you have one bathroom and modest sequential hot water use, it is frequently a good move and you gain the cupboard. If two people shower at the same time, or you have two bathrooms in use together, you will regret it.
Can a combi run two showers at once?
Not satisfactorily. A combi heats water as it passes, so its output is split between simultaneous outlets. This is physics rather than a quality difference between models.
Why is my combi’s hot water worse in winter?
Because the incoming cold water is colder, so the boiler has to raise it further to reach the same outlet temperature – and at a fixed heat output that means less flow. It is normal rather than a fault.
Is a system boiler better than a regular boiler?
Different rather than better. System boilers have the pump and expansion inside and run sealed, which is tidier. Regular boilers keep those components external, which makes them individually cheaper to replace. Both work with a cylinder.
Do I need a loft tank with a system boiler?
No. A system boiler runs a sealed heating circuit, and with an unvented cylinder there is no loft tank at all. With a vented cylinder you would still have the cold water cistern.
What size combi do I need?
It is determined by hot water demand rather than by the heating load, because hot water requires far more instantaneous output than radiators do. The flow rate you want at the shower is the design figure.
Regulations and standards referenced on this page
- Approved Document L: conservation of fuel and power (the Boiler Plus measures apply in England) — Ministry of Housing, Communities and Local Government
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