What is a balanced flue?

You may have noticed a pipe that comes out of your house’s wall or roof and wondered what it is there for. Or you might have seen a white vapour or mist blow past your window and questioned where it came from.

The vapour is the waste gases that have been produced by the boiler. They are expelled via the flue, which is the pipe on your wall or roof. Boilers require a flue pipe to ensure that any dangerous byproducts of combustion, such as carbon monoxide, do not enter your home but are taken outside instead.

In most cases, the flue may have a rounded head, unless your boiler is over 15 years old - in which case it may have a square head. The flue can sometimes have a cage covering it. This is usually present when a flue is located quite low down on a wall.

There are lots of regulations that determine where a flue can and can’t be placed on your home, and a balanced flue is different to a conventional flue. Here, we reveal the difference and explain how a balanced flue works

Flues aren’t just a requirement if you have a gas boiler. You’ll also need a flue if you have a gas fireplace. In each case, a balanced flue works in the same way.

A balanced flue normally exits a building horizontally, through a wall, or vertically, through your roof. Either setup is correct, but most heating engineers choose to have the flue exit through a wall as it’s usually easier to install this way.

When a boiler is installed for the first time, the flue will be installed too. Once it is in, it usually doesn’t need to be moved again, even if you have a new boiler installed. The position of the flue is important and needs to follow certain regulations to keep you and your family safe. 

The flue should be positioned a certain distance from any openable doors or windows, or air vents that might be positioned in the brick. This distance is typically between 30 and 60 cm, but check your manufacturer’s installation guide to confirm this figure.

It should also be a particular distance from any drains, gutters, soil pipes or balconies. This is because the gases are a byproduct of burning fuel, which means that they could be hot. This heat can melt plastic pipes, such as drain pipes. The minimum distance is usually the same as above (between 30 and 60 cm).

Flues that open onto public spaces can be a danger to anyone passing by. In this case, they should be situated at least 2.1 m high if the flue is positioned on a wall.

It’s likely that you already have a boiler flue somewhere in your home, so next time you go outside, take the time to look for it. The only system that does not require a flue is a back boiler heating system. This is because a back boiler is usually located behind a chimney breast and can use the chimney to expel any fumes and waste gases.

If you’re planning on getting a new boiler and you want to move the flue, you should check that it adheres to the above regulations. Otherwise, you may have to apply for planning permission to get the new location accepted.

Unlike a conventional flue, a balanced flue allows air to enter from outside to supply the appliance with oxygen. In the case of a gas condensing boiler, the balanced flue contains two tubes: one tube lets oxygen in, the other tube lets waste gases out. In this situation, the boiler doesn’t pull oxygen from your house but from outside.

A conventional flue only contains one tube that allows the gases to escape. It doesn’t contain a second tube to draw air from outside. Instead, a conventional flue system will take air from the room where it’s positioned.

The conventional flue can either be a chimney pot (usually found in older houses) or a prefabricated flue that is made out of metal.