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What Is An Open Vent Boiler


Although some homeowners won’t be aware, the open vented boiler system may already be heating their home. Open vent boilers were once an incredibly common choice of system for home heating and hot water in the United Kingdom and are also regularly called conventional or Heat only boilers.

Learn about conventional open vented boilers, how they operate, as well as the benefits of heating a home using an open vent boiler with this helpful guide.

What is an open vent boiler?

Generally heating systems follow a similar design, though the main factor that sets an open vented boiler apart from a combi or system boiler is it does not operate on a sealed system, the system in this case is subject to external atmospheric pressure, hence the name “Open Vent”.

Open vent heating systems will typically have two cold water storage tanks; a large one for the hot water cylinder which refills the cylinder when the hot taps are used and a smaller tank which connects to the boiler and radiators. This smaller tank allows the water in the radiators to expand. This is commonly referred to as the “expansion tank” or “Feed tank”. It is also where the “open vent” is found. This vent is just a simple piece of pipe which allows water from the system to be ejected if it gets too hot. Any hot water ejected will fall safely back into the expansion tank.

It’s important to note that some hot water cylinders don’t have a tank in the loft and are fed directly from the mains (unvented cylinders). However, your boiler and radiator system can still be open vented with this type of cylinder. In this case there would be just one tank for the radiators.

The tanks are usually found in the loft or sometimes in the airing cupboard and do take up space. They also require mains water to be permanently connected to them so that the water level in the system can be maintained.

Some open vent boilers can be used with sealed heating systems. The “expansion tank” or “feed tank” is replaced by a device called an “expansion vessel” this allows for the water in the system to expand safely when it gets hot. This removes the need for tanks in the loft and a permanent connection to mains water.

What are the benefits of an open vent boiler?

Open vented boiler systems operate relatively similarly to other solutions but have certain advantages.

  • Open vent boilers are usually very small in comparison to a combi or system boiler. This smaller size allows them to be fitted in tight spaces like kitchen cupboards for example.
  • They don’t have a built-in pump like a system or combi boiler which means they have more flexibility when designing a large heating system.

Which properties are best suited to an open vent boiler?

Some properties are better suited for an open vent boiler system than others, mostly based on size, existing heating infrastructure and water demands. If your property fulfils any of the following criteria, it’s possible that you’d benefit from installing an open vent boiler.

Large homes with multiple occupants requiring multiple sources of hot water simultaneously or continuously

Properties that have older pipe systems that may not handle pressurised sealed systems

Properties that have an existing open vent boiler or storage tank system installed

Generally, we’d recommend against a fresh install of an open vent boiler for the majority of homes, as you’d need to install the necessary storage tanks and hot water cylinders in the respective locations - typically the attic and airing cupboard. If your property already has storage tanks installed or is a larger home that is in strict need of consistent hot water, then an open vented boiler could be the correct choice.

What are the drawbacks of an open vent boiler?

Open vent boilers are a perfectly fine choice for a home, though there are some considerations that could affect your choice of heating system.

The main drawback of an open vented system comes in the nature of the system itself - the exposure to the atmosphere. It’s more prevalent - but still not common - for air to get trapped in the system, leading to loss of performance and build-up of sludge within the system.

As the tanks are often in the loft space, freezing pipework is a risk if the home is left unheated for extended periods in very cold weather. The exposed pipework will need insulating to help prevent this.