Glossary · Boilers and boiler types
What is a system boiler?
A system boiler is a central heating boiler that heats a separate hot water cylinder and has the main system parts, including the pump and expansion vessel, built into its casing. It needs no loft tanks. It stores hot water ready to use, so several taps and showers can run at once.
Key points
- A system boiler has the pump and expansion vessel inside the boiler casing, so there are fewer separate parts to fit.
- It heats a hot water cylinder, usually in the airing cupboard, so you need space for the cylinder.
- It suits homes with more than one bathroom because stored hot water can feed several taps at the same time.
- It does not need water tanks in the loft, which removes the risk of frozen loft tanks and tank leaks.
- By law, anyone employed to work on gas appliances in a home must be a Gas Safe registered engineer.
How a system boiler works
A system boiler has the major parts of a heating and hot water system built in, including the pump and the expansion vessel.[2]Baxi, System boilers The boiler heats the water that runs around your radiators. It also heats a separate hot water cylinder, which is usually in the airing cupboard.[2]Baxi, System boilers
Inside the cylinder, a coil carries the hot central heating water through the stored water and heats it. The heated water then goes to the taps and showers.[1]Vaillant, What is a system boiler? Because the hot water is already stored, it can feed several outlets at the same time.[3]Worcester Bosch, System boilers
The heating circuit is sealed and under pressure. It is topped up from the mains rather than from a header tank in the loft.[1]Vaillant, What is a system boiler? That is why a system boiler needs no loft tanks.[2]Baxi, System boilers If you want to know how the sealed circuit differs from an older open one, see open-vented vs sealed heating systems.
What a system boiler includes
| Part | Where it is | What it does |
|---|---|---|
| Pump | Inside the boiler | Pushes heated water round the radiators and through the cylinder coil |
| Expansion vessel | Usually inside the boiler | Absorbs the extra volume as the water heats up (see expansion vessel) |
| Hot water cylinder | Separate, usually the airing cupboard | Stores the hot water for taps and showers |
| Loft tanks | Not needed | A system boiler works without them |
Worcester Bosch lists the benefits as a constant supply of hot water to any number of taps at once, no loft tanks and compatibility with solar water heating.[3]Worcester Bosch, System boilers Baxi adds that the downsides are the space the cylinder takes up and the wait while the water in it heats up.[6]Baxi, What are the different types of boilers?
Cylinder choice matters. Baxi says its system boilers suit unvented hot water cylinders.[2]Baxi, System boilers You can read about them on our hot water cylinders page and in the glossary entry on the unvented cylinder.
Boiler installation is gas work, so it must be done by a Gas Safe registered engineer.[5]Health and Safety Executive, Domestic gas: frequently asked questions Our boiler installation team covers system boilers in Bradford, Leeds and West Yorkshire.
Example
Priya lives in a four-bedroom house in Bradford with two bathrooms. Two people shower at 7am while the dishwasher fills. A combi boiler heats water as it is used and would struggle with that demand. A system boiler feeds all those outlets from the cylinder.
Vaillant says a cylinder is typically sized for around 50 litres of stored hot water per person.[1]Vaillant, What is a system boiler? For Priya’s family of four, that rule of thumb gives about 200 litres. If a house full of guests uses the whole cylinder, Vaillant says to expect a wait of 30 to 40 minutes while the boiler reheats it.[1]Vaillant, What is a system boiler? The right cylinder size depends on the household, so it is worked out at survey.
What people get wrong
A system boiler is not a combi boiler with a tank added, and it is not the same as a regular boiler. It has no hot water heat exchanger of its own, so it relies on the cylinder to supply hot water.
Ideal Heating says a system boiler does not have the hot water heat exchanger, which is why it is designed to work with a cylinder.[4]Ideal Heating, What are differences between a Combi, System and Heat boiler? The second mistake is thinking it has no stored water to manage. It does, and the cylinder takes space. Vaillant notes that a system boiler needs more room than a combi because of the cylinder, but less than a regular boiler with its tanks.[1]Vaillant, What is a system boiler? For the other side of that comparison, see system boiler vs regular boiler.
The bottom line
A system boiler is a boiler with the pump and expansion vessel built in, working with a hot water cylinder. It removes the loft tanks and gives plenty of hot water to several taps at once. It needs cylinder space and a Gas Safe registered engineer to fit it. It tends to suit homes with more than one bathroom.
Common questions
Does a system boiler need a hot water cylinder?
Yes. A system boiler heats water in a separate cylinder and does not heat taps directly. Ideal Heating says it has no hot water heat exchanger of its own, so it is designed to work with a cylinder.
Does a system boiler need tanks in the loft?
No. Baxi and Worcester Bosch both say a system boiler needs no water tanks in the loft. Baxi says the pump and expansion vessel are built into the boiler instead.
Is a system boiler the same as a regular boiler?
No. Both store hot water in a cylinder, but a regular boiler keeps the heating water topped up from a header tank in the loft. A system boiler uses a sealed circuit filled from the mains.
Can I run out of hot water with a system boiler?
Yes, if demand is high. Vaillant says a drained cylinder can take 30 to 40 minutes to reheat. A cylinder sized to the household makes this rare.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. What is a system boiler?. Checked 11 November 2026. ↩
- Baxi. System boilers. Checked 11 November 2026. ↩
- Worcester Bosch. System boilers. Checked 11 November 2026. ↩
- Ideal Heating. What are differences between a Combi, System and Heat boiler?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- Baxi. What are the different types of boilers?. Checked 11 November 2026. ↩