Live Direct Debit rates for UK average since 1 October 2026.
The difference
- A combi boiler heats water instantly when you turn on a hot tap. There's no tank.
- A system or regular boiler heats water into a hot water cylinder, ready for when you need it.
The running cost of a hot water tank
A cylinder full of hot water slowly loses heat, even when nobody uses it. A modern, factory-insulated 150-litre cylinder, for example, is rated to lose about 1.24 kWh a day. Replacing that heat with a gas boiler costs about 12p a day, or £42.44 a year at today's prices.
Older cylinders with a loose jacket rather than foam insulation lose more. Adding a jacket or replacing a very old cylinder can pay for itself.
| Combi boiler | System boiler + cylinder | |
|---|---|---|
| Hot water | Instant, on demand | Stored, ready to use |
| Heat lost from a tank | None | about £42.44 a year (modern tank) |
| Several taps at once | Pressure can drop | Handles it well |
| Best for | Flats and smaller homes, 1–2 bathrooms | Bigger homes, 2+ bathrooms |
| Works with solar or a heat pump later | Less easily | Yes, the tank can be reused |
Which is cheaper overall?
For most small and medium homes, a combi costs a little less to run because there's no tank loss. For a busy home with two or more bathrooms, a system boiler can be more practical, and the extra running cost is modest.
If you might switch to a heat pump in future, a hot water cylinder is a plus, because heat pumps need one.
Tips for both
- Combi: turn the flow temperature down to around 60°C for heating. See how.
- System boiler: keep the cylinder at 60°C. That's hot enough to stop bacteria growing, without wasting gas.
- Both: shorter showers save more than any boiler setting. See what a shower costs.
Sources: Gledhill cylinder data via Heat and Plumb, Nesta, Government in-situ boiler study, Octopus Energy tariff data.