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Boiler Flue Regulations Guide


Gas boilers are handy systems that help to heat rooms and water in a home. With all the benefits they have, it’s scary to learn that they expel carbon monoxide and other toxic gases that could kill you in your sleep if the installation isn’t done right. Luckily, Gas Safe engineers follow a strict set of rules and regulations during installations to help keep you safe and ensure your boiler is working efficiently.

Boiler flue regulations are designed to help keep you and your neighbours safe from the toxic gases. Having these regulations at your fingertips helps in the smooth planning of new boiler installation.

What is a boiler flue?

Gas boilers burn gases to produce heat that provides hot water and heating to your home. A boiler flue is a pipe that is attached to the boiler. It allows the fumes produced from burning of the gases to escape from the building.

For safety reasons including avoiding carbon monoxide poisoning, the boiler flue should be installed according to the boiler flue regulations. The boiler flue positioning is inspected annually during the gas safety checks. Given the health concerns associated with the expelled gases, boiler flue regulations are getting stricter by the years.

Where is the boiler flue?

Boiler flues can be seen exiting from the side of the building – usually through the wall that is closest to the boiler. You might have noticed steam coming from the side of your house or your neighbour’s house, especially during cold weather. These are often called horizontal flues.

In other buildings depending on the shape and type, the flue might be vertical and exits through the building’s roof. Modern boilers have a rounded flue top while older boilers have a square flue.

What are the boiler flue regulations?

  1. Condensing boiler flue siting regulations

Condensing boilers emit gas with a similar composition to conventional boiler gases. The gas is mainly water vapour and carbon monoxide. But the difference is that condensing boilers are cooler and therefore the water vapour turns to steam easily even when it’s warm. Also, since they are cooler, the gases do not rise and disperse in the air. Instead, they float at low levels.

Because of this, condensing boiler flues should be positioned 2.5 m from the adjacent building, wall, fence, or boundary.

  1. Distance from an open window

The purpose of a boiler flue is to transport toxic gases away from the house. According to the boiler flue regulations, the flue needs to be positioned a safe distance from any openings in the house, including doors and windows. The boiler size determines the distance. However, the size is usually between 30cm and 60cm to the side, above or below the window or door.

This regulation helps in preventing harmful gases from re-entering your home.

  1. Distance from the neighbour’s house

With harmful gases coming from your home, you don’t want to direct them into your neighbour’s home. The minimum boiler flue installation distance requirement is 600 mm from an opposite wall, building, or fence. If the flue is installed facing the boundary, but on the back garden, then the minimum recommended distance is 300 mm. This reduces the possibility of causing ‘Statutory Nuisance.’

  1. Height of flue kit

If your house is located close to public property and the horizontal flue protrudes too close to the public space, you need to call a boiler engineer for a reinstallation. According to the flue regulation, the flue should be fitted at least 2.1m from the ground to allow the toxic gases enough room to escape.

Other boiler flue regulations

Aside from the above-mentioned regulations, boiler installation engineers are required to follow the below boiler flue installations regulations.

  • The flue should be sealed on both sides – a gas safe engineer will seal the space through the wall with caulk, sand cement, or other recommended industry sealants. Sealing ensures that no gas is escaping or re-entering a home.
  • The angle needs to be right – this is especially important if you are shifting from a non-condensing boiler to a condensing boiler. If the angle is wrong, then some drip might escape.
  • Use screws to secure joints – all brackets, pipework, and extensions need to be fixed properly. If not, the joints become loose with time and become a safety hazard.
  • Ensure parties involved understand the length of flue used – when adding bends and extensions to a flue, you will need to know how long it will be. For instance, a 45-degree bend adds an extra meter to the final flue length.

Also, note that when too many extensions or bends are added, the flue will be too long for the gases to escape safely. The boiler will, therefore, experience an efficiency problem. The maximum flue lengths vary depending on the boiler.

For instance, the Viessmann Vitodens 050-W 29kw has its maximum flue length at 15m while the 35kw variant has a maximum of 20m. Though it boils down to health and safety, different manufacturers have different criteria that they use to determine the maximum length.

Should I move my boiler?

If your current flue does not adhere to the above boiler flue regulations, say it is installed below the 2.1m recommended height facing a public space, then yes, you will have to install a new boiler. However, most of the boiler flue regulations are only for new boiler installations. So before coughing up large sums of money, you should consult with a gas safe boiler engineer first.

In line with the UK condenser boiler regulations, all new boiler installations should be condensing boilers. This is because condensing boilers are more efficient. They feature two internal heat exchangers, which means that they lose less heat through flue pipes. When you replace your non-condensing boiler with a condensing boiler, you will have to move the flue’s location to meet the recommended condensing boiler flue regulations.

Moving the boiler

If you are considering moving your boiler to a different room for aesthetic reasons, or to free up space in the current room, you might have to move the flue as well. Moving the flue along with the boiler will ensure you meet the flue regulations. On average the cost of moving a boiler is between £300 and £800. However, extra costs come into play when the flue has to be extended or moved.

Do I need planning permission to fit a new boiler flue?

If the boiler flue you install meets the current boiler flue regulations, you will not need planning permission. However, if the boiler flue will be without the set regulations, you will need planning persimmon. To ensure your boiler flue is installed safely outside of the boiler regulations, you should consult with a gas safe engineer.

Do I need to involve an engineer?

Yes. You shouldn’t attempt to make any changes to flue positioning and general boiler setup without a registered and gas safe engineer. The results could be dangerous and cost you a pretty penny if anything goes wrong during the modifications.

Our gas safe engineers are qualified and trained to handle flue re-positioning and related tasks on different boiler brands, models, and types.

Be aware

You must be conversant with boiler flue regulations. If you are getting a boiler re-positioned or serviced, you should be wary of the ‘professional advice.’ Be cautious when a professional says a given rule is no longer listed in the regulations. If possible, look it up. Such phrases are used to get more money out of you.

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