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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9 thoughts on “Flue Diameters”

  1. I am installing a open log fire in an inglenook fire place and I am not sure on the flue diameter the flue length will be 6.5m long

  2. Are there any benefits to a larger flue diameter? We have a 7” single wall coming from the wood burner, and this increases to 10” double wall (10” internal diameter) at ceiling height, through the first floor and then out through the roof. Would it be better running 7” all the way? Thanks

    1. There is absolutely no need to increase from a 7″ flue system (already particularly large and only required if your stove features a 7″ diameter outlet) to a 10″ Twin Wall system! 7″ internal diameter flue all the way will be sufficient.

  3. Hi ,is there any way I can connect a 5 inch flue liner to a 6 inch outlet as 5 inch is all that will fit down the chimney, I would be very grateful if you could get me out of this situation

    1. Unfortunately, as your stove features a 6” flue outlet, that is the minimum diameter flue system that can be used. The flue diameter cannot be reduced as this may have an adverse effect of the draw within the flue and therefore the efficiency of the fire. I would suggest perhaps purchasing a fire with a 5” outlet collar, or even discussing this with your HETAS installer who may be able to advise on an alternative route for your flue.

  4. That does depend on the brand/manufacturer. 5″ is used as an industry standard term for the internal diameter but this can measure 120mm, 125mm, 130mm as you have seen. Each brand will show you what their pipe measures precisely and ours are all 125mm.

  5. Is there a benefit to increasing a flue from single 5″ to 6″ twin wall before it goes through the roof? I’m asking because the installer that quoted for our wood burner is recommending this is done.

    1. The only reason it would be required to increase from 5″ single skin pipe to 6″ Twin Wall flue is if your stove is not DEFRA approved (and has a 5″ diameter outlet). In that case it is a building regulation requirement to attain a minimum 150mm internal diameter for the flue system. Otherwise, you are fine to stay in 5″ (125mm) flue the entire way (if DEFRA approved).

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