Gas Boiler Flue Position Regulations


One thing surveyors routinely check for is any potential danger with the siting of boiler flues. This shouldn’t normally be an issue because installation of new boilers must be carried out by qualified installers (GasSafe registered for gas or OFTEC for oil).   But mistakes sometimes occur and poorly executed subsequent alterations to buildings can also lead to flues causing risks to occupants.

But you don’t need to pay for a survey to check that your flue is sited safely.  The following rules for positioning flues are designed to ensure that waste gases from combustion don’t compromise safety, cause a nuisance or damage property.


Domestic boiler flue installations


Information kindly provided by for fanned draught boiler installations


*  The boiler flue terminal should be positioned away from doors, windows or other openings. There should be a space of at least 300mm above, below or to the side of any opening.

Harmful gases should be properly expelled from the property, so they cannot re-enter. Flue terminals should also typically be sited at least 600mm above flat roofs.



*  Boiler flues should be positioned at least 75mm below any material that could be damaged by heat, such as guttering. They should also be at least 150mm to the side of vertical drain or soil pipes. The waste gases expelled from the flue can be extremely hot, which could cause external damage.


* Waste gases can be dangerous if they are breathed in and the plume from condensing appliances can cause a nuisance if the terminal is not properly positioned, so avoid positioning the boiler flue where it would discharge onto public spaces like pavements wherever possible. Keep the flue at least 2m off the ground.


* If it isn’t possible to keep the flue over 2m off the ground, you must install a terminal guard. It should provide at least 50mm clearance from the flue terminal to ensure passersby won’t get burned accidentally.


* A boiler flue cannot be routed through another property. It can, however, be routed through a community area as long as access is always available.


* Condensing boilers produce a plume from the flue terminal — and it’s against regulation for it to cause a nuisance. To avoid an issue, ensure the plume doesn’t cross a frequently used area, public route or neighbouring property.

You should also make sure that the flue terminal and the plume produced won’t trigger infrared security lighting or obscure any security cameras.


* If your boiler flue is positioned horizontally, angle it slightly upwards. Dripping water vapour could be harmful so it’s important this drains back into the boiler rather than onto the exterior of the property or, worse, anyone who happens to be walking past.


*  Don’t conceal your flue! Boarding over or encasing a boiler flue, particularly vertical ones, might look a little better but you must always ensure you don’t cover up access points or joints in the flue.

A flue’s predominant job is to expel waste gases, so it should not be obstructed.


* Install inspection hatches if your flue is positioned so it cannot be physically accessed. An engineer should be able to visually inspect your boiler flue joints — and may classify it as ‘At Risk’ if they can’t.


* The Gas Safe technical bulletin TB008 provides guidance on meeting regulation when it comes to flues in voids. Boilers may run through ceiling, floor or wall voids if they are located away from an external wall.

Inspection hatches should be installed at 1.5m intervals along the flue run to allow for physical access and examination. These hatches should be 300mm by 300mm in size.


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