Flue Diameters

The diameter of flue pipe required for your system will initially be dictated by the size of the flue outlet on your particular stove. From this point onwards the Building Regulations are very clear on the size you must be in for the remainder of your flue installation.

The most common regulation that the majority of flue systems in the UK must adhere to is:

“Closed appliances of up to 30kW rated output burning any fuel 150mm diameter or square flues having the same cross sectional area”

This clearly states that you are required to install a minimum flue diameter of 6” (150mm) and this is the case whether you are using Twin Wall insulated flue pipes or Flexible Flue Liner. But what if the outlet on your stove is smaller than this? Not to worry, this is quite often the case and you are simply required to use increasing adaptors to attain the 6” diameter by the time you convert to your Twin Wall or Flexible Liner.


The ONLY exception to the rule above concerns DEFRA Approved stoves most commonly installed in designated Smokeless Zones. The regulations for these models are as follows:

”Closed appliances of up to 20kW output which: a) burns smokeless fuels or b) is an appliance which meets the requirements of the Clean Air Act 125mm diameter or square flues having the same cross sectional area”

This permits a minimum flue diameter for a DEFRA stove of 5” (125mm) and does not require the increase to 6”. Please click here for our full range of DEFRA Approved stoves.

Below are a few basic rules for your flue pipes:

    • The diameter of your stove outlet is the minimum diameter your flue system can be
    • In no circumstance should you ever reduce the diameter of your flue system
    • The diameters quoted in the Building Regulations are minimum diameters only
    • If your stove outlet is greater than 6” then your flue system is going to be greater than 6”
    • DEFRA stoves burn cleaner and / or burn smokeless fuels which leave less soot and tar deposits hence the reduced diameter flue required
    • Some registered installers won’t even attempt to fit anything less than a 6” diameter flue


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