A open vent boiler, also known as a regular, conventional or heat only boiler, is a system involving two tanks, a hot water cylinder and the boiler itself. Open vent boilers are typically found in larger properties as they can better meet the demand for hot water across multiple taps and shower heads.
What is a open vent boiler?
In this blog, we explain how open vent boilers work and how they differ from other heating systems.
How do open vent boilers work?
Open vent boilers work by heating water and distributing it throughout a home using a system with various controls, including motorised valves, pumps and tanks. A cold water tank stored in the loft will take water from the mains and feed it into a hot water cylinder.
The boiler will then heat the hot water cylinder, at which point the hot water is supplied to taps and showerheads when needed. A pump will also circulate heated water to radiators when the heating is turned on.
Homes with a open vent boiler will also have a feed and expansion tank to maintain a consistent level of water in the heating system. Water is sometimes lost from the system through leaks or evaporation, which is why the feed and expansion tank will replace water in the system to maintain a constant level of pressure.
What is the difference between a open vent and system boiler?
Unlike system boilers, open vent boilers maintain pressure using a feed and expansion tank, whereas system boilers use a pressurised heating circuit topped up by mains water. Heat only boilers utilise a feed and expansion tank to maintain water pressure whereas the expansion vessel is usually internal in a system boiler. System boilers require less space within the property than open vent boilers and require less piping due to the need for tanks and other elements of an open vented system.
However, both systems store hot water for later use, as opposed to heating it on demand like combi boilers.
What is the difference between a open vent and combi boiler?
As mentioned above, combi boilers heat water instantly on demand as opposed to storing it for later use. Combi boilers do not require a cold water tank, an external hot water cylinder or a feed and expansion tank, which means they take up much less space in a home. This makes a combi boiler a much better option in properties where storage space is limited.
The advantage of a open vent boiler is that it can supply water to multiple taps and showerheads more easily than a combi boiler. The hot water tank can store large quantities of hot water and deliver it instantly, making it the ideal choice for larger homes with a larger hot water requirement.
What size heat only boiler do I need?
The size of open vent boiler you will need will depend on the size of the property and its level of insulation. You should always speak to a boiler expert before purchasing a new boiler, but below is a general guideline of typical boiler sizes.
Number of bedrooms
Level of insulation
|More than 4
|More than 4
- A open vent boiler heats a home using a cold water tank, a hot water cylinder and a feed and expansion tank.
- Open vent boilers are typically found in larger homes because they are better able to supply hot water to multiple radiators, taps and showers at the same time.
- Regular boilers differ from system boilers in several ways, including the way that they maintain pressure using a feed and expansion tank.
- Heat only boilers are more suitable than combi boilers for larger homes with greater demands for hot water, while combi boilers are better suited for smaller homes.
- The size of open vent boiler you will need will depend on the size of your home and its level of insulation.