Boiler flues: What are they and how do they work?

Discover how boiler flues work, what regulations you have to follow and whether you'll have to pay for one to be installed.


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A boiler flue is the pipe that leads from your boiler to the outside of your home, preventing the waste gases from becoming potentially dangerous.

The flue is an essential part of the boiler typically located around the back. Waste gases disappear through this pipe and disperse safely into the air outside the house. Essentially, it’s an exhaust pipe for your home.

Your boiler almost definitely has a flue, regardless of whether it’s a regular boiler, combi boiler or a system boiler. The only exception is a back boiler, which is situated behind a fireplace and can therefore allow waste gases to disappear through your chimney.

Without somewhere for these gases to disappear, your boiler is extremely dangerous. There are several safety regulations that the flue must comply with. When a new boiler is being installed, you may have to pay extra to ensure these regulations are met.

How does a boiler flue work?

When gas is burned in a boiler’s combustion chamber to create heat, waste fumes are also created. The flue begins inside the combustion chamber, so the waste gas can safely make its way outside the house through it. The fumes are often guided towards the flue by an electric fan.

In newer condensing boilers, this waste gas will pass through an extra heat exchanger, which soaks up the heat from these fumes and passes it onto the cold water returning back into the boiler. This is why condensing boilers are so much more efficient.

Where is my boiler flue?

In most cases, your boiler’s flue extends from the back of your boiler through the nearest wall, allowing gases to exit from the side of your house. This is known as a horizontal flue.

However, it’s possible that your boiler has a vertical flue, which extends from the top of your boiler, allowing gases to disperse through the roof of your home.

There are several regulations regarding where in your home the flue can be placed. For example, it needs to sit at least 60 centimetres from the nearest door, window or air brick. This is to ensure that no harmful gases re-enter the home. It must also appear at least 60 centimetres away from the corner of a building and the same distance above flat roofs or balconies.

A flue must be completely sealed on both sides, at an angle which allows the gas to easily escape.

How much does a boiler flue cost?

If your boiler has a horizontal flue, the cost of this will be included. However, if you’re forced to install a vertical flue, this could set you back up to £600. This decision will be based entirely on where in the house your boiler can be stored.

You may have to pay extra for extensions or brackets to ensure that waste gas leaves your house safely. You can expect to pay between £50 and £100 for these.

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