System boiler
Checked against its sources: Updated
A system boiler heats the radiators and a separate hot water cylinder, and generally needs no tank in the loft to fill the system.1 The water stored in the cylinder is what lets several outlets run together.2
- Covers
- Sealed heating systems with a storage cylinder, UK homes.
- You
- Read the gauge, set the timer and controls, top up pressure as your own user manual allows.
- A professional
- The boiler internals, gas, flue and any unvented cylinder safety parts. A Gas Safe registered engineer for gas work.
1 What makes it a system boiler
The name is about where the parts live. Vaillant says the expansion vessel is often inside a system boiler, and that the heating circuit is pressurised and topped up by mains water3. So a house with this set-up has a gauge and a filling device near the boiler, a cylinder somewhere, and no feed and expansion tank in the loft1.
A coil in the cylinder carries hot water from the boiler, heating the stored water around it without the two mixing3. More on that vessel under hot water cylinder.
2 How it compares
| Combi | System | Regular | |
|---|---|---|---|
| Hot water | Made on demand | Stored in cylinder | Stored in cylinder |
| Cylinder | No | Yes | Yes |
| Tanks in the loft | No | No | Cold water tank and usually a feed and expansion tank |
| Pressure gauge to watch | Yes | Yes | Depends, see regular boiler |
The table is drawn from Worcester Bosch, Vaillant and Baxi descriptions123. Real homes vary, and which boiler have you got shows how to check yours.
3 Sizing and the wait to reheat
Vaillant gives a typical sizing guide of around 50 litres of stored hot water per person, and says a depleted cylinder can mean a wait of 30-40 minutes for the boiler to reheat it3. Those are one maker's general figures, not a standard, and the installer sizes the real thing.
Worcester Bosch notes that a system boiler usually costs a little more than an equivalent combi, largely because of the cylinder, but gives no figure2. It also lists solar water heating as compatible, and the freeing of loft space as a benefit2.
4 Pressure and upkeep
Like a combi, this system loses pressure if water escapes, and manuals differ on the figures. For a Worcester example, see the Greenstar i System Compact guide, which covers sealed systems and its external filling loop4. Start with boiler pressure for the table by model.
Sources
- Glossary of heating terms, Baxi read 3 November 2026.
- What is a system boiler, Worcester Bosch read 3 November 2026.
- Vaillant, what is a system boiler read 3 November 2026.
- Worcester Greenstar i System Compact 27i/30i user guide 6720807727, section 4.1, 2013/05, read 3 November 2026.