Glossary · Central heating, radiators and hot water
Open vented vs sealed heating system: what is the difference?
An open vented heating system has a feed and expansion tank, usually in the loft, that keeps the water topped up and takes up expansion. A sealed heating system is closed off from the air, is filled with mains water through a filling loop and uses an expansion vessel instead.
Key points
- An open vented system has a feed and expansion tank in the loft. A sealed system does not, and uses an expansion vessel and a filling loop.
- Heat only (regular) boilers are usually installed on open vented systems. System and combi boilers work on sealed, pressurised circuits.
- Ideal says a sealed system suffers less corrosion than an open vented one, which produces sludge that affects efficiency.
- Energy Saving Trust says heat only boiler header tanks must sit higher than the highest hot tap or radiator, so they rarely suit flats.
- Anyone working on gas appliances in a home must be Gas Safe registered, whichever system you have.
How the two systems work
Both systems heat water in a boiler and pump it round the radiators. They differ in how they cope with the water expanding when it heats up, and in how the water is topped up.
An open vented system has a feed and expansion tank, usually in the loft.[3]Baxi, Glossary of Heating Terms Baxi describes two tanks in the loft: a larger one that feeds the hot water cylinder and a smaller one that keeps the heating system at the right level.[3]Baxi, Glossary of Heating Terms Vaillant says the feed and expansion tank replaces water lost through leaks or evaporation, so the level stays constant.[1]Vaillant, What is a open vent boiler? Boilers that suit this layout are heat only boilers, also called regular or conventional boilers. Ideal says they are usually installed on an open vented system and do not have a filling loop.[2]Ideal Heating, Glossary of Terms
A sealed system has no feed and expansion tank in the loft. Baxi says it is filled with mains water through a filling loop, with an expansion vessel in the boiler to take up the expansion.[3]Baxi, Glossary of Heating Terms Ideal says the water is sealed from the atmosphere by the expansion vessel.[2]Ideal Heating, Glossary of Terms Vaillant adds that system boilers use a pressurised heating circuit topped up by mains water, with the expansion vessel usually inside the boiler.[1]Vaillant, What is a open vent boiler?
The differences side by side
| Open vented | Sealed | |
|---|---|---|
| Water top-up | Feed and expansion tank in the loft | Mains water through a filling loop |
| Expansion | Tank | Expansion vessel |
| Typical boiler | Regular (heat only) | System or combi |
| Loft tanks | Yes | No |
Energy Saving Trust describes the heat only layout this way. It needs extra space for two header tanks, and hot water pressure can end up a lot lower than cold water pressure. Those tanks must be higher than the highest hot tap or radiator, so the layout is unlikely to suit a flat.[5]Energy Saving Trust, Boiler types explained Worcester Bosch says open vent systems are typically common in older properties.[4]Worcester Bosch, Boilers Explained
Ideal says a sealed system offers benefits over an open vented one because less corrosion takes place, so the boiler and radiators stay efficient for longer. It says an open vented system produces sludge that affects efficiency, and that every system should be cleansed and treated with inhibitor on installation.[2]Ideal Heating, Glossary of Terms Our page on central heating sludge explains what that sludge is.
What you can check yourself
On a sealed system the boiler has a pressure gauge. Energy Saving Trust explains how to top it up on a combi boiler: if the gauge is in the red, open the two taps on the short filling pipe to let water in until the gauge is back in the green, then turn both taps off.[6]Energy Saving Trust, Eight quick DIY jobs to save energy Always follow the user guide for your own boiler.
Anything inside the boiler, its flue or the gas pipework is not a household job. Anyone employed to work on gas appliances in domestic premises must be Gas Safe registered and competent for that work.[7]Health and Safety Executive, Domestic gas: frequently asked questions If you are thinking of changing from one system to the other, ask a Gas Safe registered engineer for a survey. Our central heating team can arrange one.
Example
Harpreet in Keighley has a 1960s semi. In the loft there are two tanks, a big one and a small one, and the boiler has no filling loop. That matches Ideal’s description of a heat only boiler on an open vented system.[2]Ideal Heating, Glossary of Terms Her neighbour’s boiler has a pressure gauge and a pipe with two taps underneath, and nothing in the loft. That is a sealed system with a filling loop.[3]Baxi, Glossary of Heating Terms
What people get wrong
People hear “sealed” and think it means the heating can never lose pressure. A sealed system can still drop pressure and need topping up, and the boiler gauge is how you spot it.
The other common mix-up is between the heating circuit and the hot water cylinder. They are separate questions, and our page on vented vs unvented cylinders covers the cylinder side. Ideal says an unvented cylinder is one with no vent pipe, like a sealed system.[2]Ideal Heating, Glossary of Terms
The bottom line
An open vented system uses a feed and expansion tank in the loft. A sealed system is filled from the mains through a filling loop and uses an expansion vessel. The type you have follows from your boiler and your loft, and a Gas Safe registered engineer can confirm it. Ideal says sealed systems corrode less, while Vaillant says open vented suits larger homes with several taps running.[2]Ideal Heating, Glossary of Terms[1]Vaillant, What is a open vent boiler?
Common questions
How can I tell if my heating system is open vented or sealed?
Look in the loft. Ideal says a small expansion tank plus a larger cold water storage cistern means a heat only boiler, usually on an open vented system with no filling loop. A sealed system has a filling loop instead.
What does a sealed heating system mean?
The water in the system is sealed from the atmosphere by an expansion vessel and is filled from a filling loop, according to Ideal. It has no feed and expansion tank in the loft.
Is a sealed system better than an open vented one?
Ideal says a sealed system has less corrosion, so the boiler and radiators stay efficient for longer. Open vented suits some homes: Vaillant notes it can supply several taps at once.
Can I top up the pressure on a sealed system myself?
Energy Saving Trust describes it for a combi boiler: if the gauge is in the red, open the two taps on the filling loop until it is back in the green, then close them. Follow your boiler's user guide.
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 open vent boiler?. Checked 11 November 2026. ↩
- Ideal Heating. Glossary of Terms. Checked 11 November 2026. ↩
- Baxi. Glossary of Heating Terms. Checked 11 November 2026. ↩
- Worcester Bosch. Boilers Explained. Checked 11 November 2026. ↩
- Energy Saving Trust. Boiler types explained. Checked 11 November 2026. ↩
- Energy Saving Trust. Eight quick DIY jobs to save energy. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩