The 30-second answer
1 bathroom, 1–4 people, decent mains pressure → combi. Cheaper, smaller, no cylinder, hot water on demand.
2+ bathrooms with simultaneous use, low mains pressure, or a big family → system boiler + unvented cylinder. Costs more but delivers full flow to every outlet at once.
Side-by-side comparison
| Combi | System + cylinder | |
|---|---|---|
| Install cost (fitted) | £2,100–£3,400 | £2,700–£3,900 |
| Space needed | Boiler only (kitchen cupboard sized) | Boiler + hot water cylinder (airing cupboard) |
| Hot water at 1 outlet | Instant, unlimited | Instant until cylinder empties (~200L) |
| Hot water at 2+ outlets | Pressure drops noticeably | Full pressure at every tap |
| Mains pressure needed | 1.5 bar / 12 L/min minimum | Same, but works better on marginal supplies |
| Efficiency | 92–94% (no cylinder losses) | 89–92% (small cylinder losses) |
| Servicing complexity | One appliance | Two — cylinder G3 check yearly |
Rules of thumb from the tools
- • Test your mains flow first. Fill a bucket at the outside tap for 30 seconds. Below 6 litres = don't go combi.
- • Count simultaneous outlets. Two showers at once = system. Otherwise combi is fine.
- • 3-bed / 1-bathroom = combi 9 times out of 10. This is the most common UK setup and combi is almost always the right call.
- • Cylinder-in-a-cupboard? Keep it. If space and infrastructure exist, a system replacement is often cheaper than ripping it out for a combi.
- • Don't oversize a combi. A 30kW is plenty for most homes; a 40kW just cycles more and wears out faster.
Regular (heat-only) boilers — the third option
Also called "conventional" — the boiler feeds a hot water cylinder and a cold tank in the loft. Rarely the best choice for a new install now, but common in older homes and sometimes the sensible replacement when a full system rebuild would be disruptive.
Related reading
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