central heating flue exiting a property horizontally

Installing a gas boiler is a lot more complicated than wiring a new plug socket or fitting a new radiator. There are lots of rules and regulations to follow, and this is why it must be done by a Gas Safe registered engineer who knows exactly what they’re doing. But before the boiler is installed, there may be some decisions that you need to make first. For instance, is the boiler staying where it is or will be the new one be installed elsewhere in the home? If this is the case, you may need to discuss available options with an engineer, who can advise you on the best place as well as whether the move is possible. When you wish to move a boiler, one of the most important considerations is where the new flue will go.

A flue is an essential part of a heating system. Not only does it draw in fresh air so that your boiler can safely burn natural gas, it also expels some of the excess gases that are produced during this process, such as CO2 and water vapour. The flue will either exit your home vertically (through the roof) or horizontally (through a wall), and there are certain regulations around where the flue should be positioned as well as how long it can be.

How long can a flue be?

The recommended flue length will depend on the type of boiler you have and its manufacturer. Different manufacturers often have contrasting guidance and regulations, so you should check what they recommend.

As a general rule, a flue should work effectively when it’s up to 10 metres in length. This is usually based on a standard thickness of 60/100mm. For flues that need to be longer than this, it’s a good idea to increase the thickness to 80/125mm to ensure that the gases are able to escape easily. This increased thickness should be fine for flues of up to 20 metres subject to the age of boiler, model make and maximum run lengths. You can check against your installation manual for your selected manufacturer model.

When flues are exceptionally long (i.e. more than a couple of meters), there are some further requirements that should be considered. The flue should be properly supported along its entire length, with brackets located every metre or so to hold it up sufficiently in line with building regulations BS-5440. It should also be continuous throughout its length, with any joins assembled correctly and fully sealed. Where this is the case, the Gas Safe engineer should perform a smoke test to ensure that no joins are leaking or allowing gases to escape.

Ideally, flues should be kept as short as possible and an ideal route should be discussed with a suitably qualified heating engineer (if needed) before any work is carried out. They shouldn’t pass through any other dwellings, as this can cause access issues when a boiler service is carried out.

There are a few issues that can arise from having very long boiler flues.

First, there is an increased risk of exhaust fumes leaking out before they reach the end of the flue to be expelled outside. A flue that is longer than it should be may not work effectively, and could potentially endanger household members.

Second, room sealed flues draw fresh air in from outside to assist the combustion process and ensure that the gas is burning correctly. A flue that is too long, therefore, could also cause problems with ignition and may affect combustion. When a boiler isn’t burning fuel correctly, there is a risk of carbon monoxide production.

So long as the flue is the correct thickness for the additional length, you shouldn’t run into problems like these.

Do all boilers need a flue?

Any boiler, no matter what type or the fuel it burns, needs a flue. When any fuel, including gas, wood or oil, is burned, it produces fumes, including smoke, soot, water vapour, CO2 and even carbon monoxide. These gases need to be expelled somehow, and a flue is the best way to do this.

The only type of boiler that doesn’t have a flue is a back boiler. This is because they’re fitted behind a fireplace and expel any waste gases via the chimney instead. These boilers are rare, as most old models have been removed, and can no longer be installed from new. 

How many bends can you have in a flue?

Ideally, a flue should be installed in such a way that gases can easily escape with little resistance. This generally means that it should be kept straight, with as little changes of direction as possible, and short. However, this isn’t always possible.

Where bends are required, they should be configured in such a way so that the flue has as little bends as possible and must not be so acute that they cannot freely allow the gases to escape. For this reason, bends should be angled at 45 degrees as a minimum. Each 90 degree elbow would mean an additional 1 metre flue length must be taken into consideration, however the maximum total length for the boiler must not be exceeded. Heating engineers will calculate the flue configuration based on building regulations.

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