Glossary · Boilers and boiler types
System boiler vs regular boiler: what is the difference?
The main difference is how each keeps the heating water under pressure. A regular boiler uses a feed and expansion tank in the loft. A system boiler uses a sealed circuit filled from the mains, with the pump and expansion vessel inside the boiler. Both heat a separate hot water cylinder and store hot water for later.
Key points
- Both types store hot water in a cylinder, so both can supply several taps at once, unlike a combi boiler.
- A regular boiler needs a feed and expansion tank and a cold water tank in the loft. A system boiler needs neither.
- A system boiler has the pump and usually the expansion vessel built in. A regular boiler has them as separate parts.
- A regular boiler suits low mains pressure and old systems you want to keep. A system boiler needs reasonable mains pressure.
- System boilers take less space than regular systems, but both need room for a hot water cylinder.
How the two types differ
Both boilers heat a hot water cylinder and store hot water for later, unlike a combi boiler that heats water as you use it.[2]Vaillant, What is an open vent boiler? The split is in how each keeps the heating system under pressure.
A regular boiler uses a feed and expansion tank in the loft to maintain the water level and pressure.[2]Vaillant, What is an open vent boiler? A system boiler uses a pressurised heating circuit topped up with mains water, and its expansion vessel is often inside the boiler.[1]Vaillant, What is a system boiler? Ideal Heating gives the same split. A system boiler contains an expansion vessel and pump, which lets it run on a sealed system. A heat only boiler is the simplest type and can run on an open vented system with a tank in the loft.[5]Ideal Heating, What are differences between a Combi, System and Heat boiler?
Our glossary entries on system boilers and regular boilers cover each in full.
Side by side
| System boiler | Regular boiler | |
|---|---|---|
| Other names | None in common use | Heat only, conventional, traditional, open vent |
| Heating water pressure | Sealed circuit topped up from the mains | Feed and expansion tank in the loft |
| Pump and expansion vessel | Built into the boiler | Separate parts |
| Loft tanks | None | Cold water tank and feed and expansion tank |
| Hot water | Stored in a cylinder | Stored in a cylinder |
| Low mains pressure | Needs reasonable mains pressure | Suits low mains pressure |
| Space | Less than a regular system, more than a combi | Most space of the three |
The sources behind each row are consistent. Baxi names the alternative terms for a regular boiler and says it needs a feed and expansion tank, a cold water tank, a hot water storage cylinder and a separate pump.[3]Baxi, What are the different types of boilers? Vaillant says regular boilers use more space than a system boiler and often need more complex piping.[1]Vaillant, What is a system boiler? Baxi says a regular boiler is a good option where mains pressure is low.[4]Baxi, Heat only boilers Ideal Heating notes that system boilers are sealed units and need decent mains pressure to work well.[8]Ideal Heating, What size boiler do I need? Try our power calculator
Example
Meera owns a large 1950s semi in Bradford with a loft tank that freezes most winters. She has two bathrooms. Both boiler types give her stored hot water for both showers. The system boiler would remove the loft tanks and so the frozen tank risk, because Baxi says no loft tanks reduces the risk of frozen pipes and leaks.[3]Baxi, What are the different types of boilers?
But if her old radiators and pipework are tired, Baxi cautions that an older system may not cope with the higher pressure of a system boiler.[4]Baxi, Heat only boilers Worcester Bosch makes the opposite case for a regular boiler: it can use the existing pipework and keep classic radiators.[6]Worcester Bosch, Regular boilers A survey of the house settles it, not the boiler label.
What people get wrong
People often assume a system boiler is a small regular boiler, or that regular means standard or older. In fact both store hot water in a cylinder. The differences are the loft tanks, the pressure in the heating circuit and where the pump and expansion vessel sit.
Vaillant sets out the same differences in pressure, tanks and the position of the pump and expansion vessel.[1]Vaillant, What is a system boiler? A second mistake is to think that moving between the two is a simple like-for-like job. It involves the cylinder, the pipework and sometimes the tanks. Gas work must be done by a Gas Safe registered engineer.[7]Health and Safety Executive, Domestic gas: frequently asked questions Our boiler installation page explains how we survey for it.
The bottom line
A system boiler and a regular boiler both give stored hot water for busy homes. A system boiler is sealed, has the pump and expansion vessel built in and needs no loft tanks. A regular boiler relies on loft tanks and suits low mains pressure and older systems. Your pressure, space and existing pipework decide between them.
Common questions
Which is better, a system boiler or a regular boiler?
Neither is better for every home. A system boiler removes the loft tanks and takes less space. A regular boiler copes with low mains pressure and is often the easier swap in an older home. A survey decides.
Do both need a hot water cylinder?
Yes. Vaillant says both system and regular boilers store hot water for later use, as opposed to a combi boiler, which heats it on demand. Both therefore need space for a cylinder.
Which needs loft tanks?
A regular boiler does. Baxi says it needs a feed and expansion tank and a cold water tank in the loft. A system boiler does not need water tanks in the loft.
Can I swap a regular boiler for a system boiler?
Often, but not always. Baxi warns that an older system may not cope with the higher water pressure a system boiler delivers. A Gas Safe registered engineer should survey the pipework and radiators first.
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. ↩
- Vaillant. What is an open vent boiler?. Checked 11 November 2026. ↩
- Baxi. What are the different types of boilers?. Checked 11 November 2026. ↩
- Baxi. Heat only boilers. Checked 11 November 2026. ↩
- Ideal Heating. What are differences between a Combi, System and Heat boiler?. Checked 11 November 2026. ↩
- Worcester Bosch. Regular boilers. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- Ideal Heating. What size boiler do I need? Try our power calculator. Checked 11 November 2026. ↩