The flue is an integral part of your gas boiler system because it allows waste gases created by your boiler to be removed from your home. These gases usually include carbon dioxide, water vapour and nitrogen oxide, and it’s important that they can’t re-enter your home or potentially harm members of the public. It’s for these reasons that there are strict regulations around where a boiler flue can and can’t be situated. 

When your boiler is installed, the heating engineer will adhere to these regulations to ensure that your flue is safe, legal and won’t damage your property.

Boiler flue regulations

When it comes to the regulations around where a flue can and can’t be situated, there may be two different answers depending on whether your boiler system uses a fanned-draught flue or a natural-draught flue. The latter draws fresh air into the system naturally, using a draw that’s created from the differing temperatures of the cold air and the hot gases. The former makes use of a fan to draw in the air for the boiler.

The minimum distances recommended in the manufacturer's instructions may differ depending on the type of system you have. Therefore below, we’ve provided two distances - one for fanned-draught flues and another for natural-draught. Generally, a flue needs to be further away from a fanned-draught system because there’s a danger that the waste gases can be pulled back into the system, along with oxygen.

  • Below or above an opening

An opening is an openable element, such as a window or door, or a fixed element, such as an air vent. The regulations state that in fanned-draught systems, the flue should be at least 300mm away from either of these things.

However, for natural-draught systems, the distance can depend on the size of the boiler, as outlined below. Regulations for below an opening are as follows:

  • 0 to 7 kW appliance - 300mm
  • 7 to 14 kW appliance - 600mm
  • 14 to 32 kW appliance - 1,500mm
  • 32 kw or larger appliance - 2,000mm

Regulations for above an opening are as follows:

  • 0 to 32 kW appliance - 300mm
  • 32 kW or larger appliance - 600mm

The distances are smaller for flues located above an opening, as the waste gases are more likely to travel upwards than downwards.

  • Horizontally to an opening

There are further regulations for flues that are placed horizontally to an opening. For natural-draught systems, the regulations are as follows:

  • 0 to 7 kW appliance - 300mm
  • 7 to 14 kW appliance - 400mm
  • 14 kW or larger appliance - 600mm

In fanned-draught systems, this again remains at 300mm.

  • Below gutters or drain pipes

When the flue is located below guttering or drains, including drain and soil pipes, the minimum distance is 75mm in fanned-draught systems and 300mm in natural-draught systems. This is because the gases that come out of the flue are very hot and can melt plastic, the material these items are most often made from.

Where there are vertical drains and soil pipes, the distance for fanned-draught systems changes to 150mm in systems over 5 kW.

  • Next to public spaces

In order to keep members of the public safe, a flue must be placed at least 2,100mm off the ground when it opens onto a public space, such as a walkway that leads down the side of your property. This is also the case for flues that open next to a pavement.

  • Flues facing other flues

Where two flues on neighbouring properties face each other, they must be a minimum of 600mm apart in natural-draught systems and at least 1,200mm away in fanned-draught.

What distance should the flue be from a neighbour?

In some cases, a flue may need to be located relatively close to a neighbour’s property. When this is the case and they’re parallel to the boundary, there must be a minimum distance of 300mm. A flue that is pointing directly at your neighbour’s boundary must be 600mm away and at least 2,100mm away from their doors and windows.

What are the regulations around extending a flue?

In some cases, a flue might need to be extended so it can exit the property at an appropriate point, away from any neighbouring properties or flues and not near your own windows or doors.

Flues can be extended so they’re up to 20m in length, as long as the pipework has a minimum thickness of 80/125mm and the pipes are properly supported with brackets. These should be a last resort - shorter flues are more desirable as it takes less time for the waste gases to escape.

For more information on extending your flue, take a look at our guide on how long a flue can be, which could provide you with more details. All flue work must be carried out in accordance with manufacturers instructions by a gas safe registered engineer.

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