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.

Building Regulations states:

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

This means you are required to install a minimum flue diameter of 6” (150mm). This is the case whether you are using Twin Wall Insulated pipes or Flexible Flue liner.

If your stove outlet is smaller than this you simply need an adapter that increases to a minimum of 6” diameter by the time you convert from stove pipe to 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”. 

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

Carbon monoxide alarm

You must have a carbon monoxide alarm fitted in the same room as any solid fuel burning stove. This is now a legal requirement and more importantly, vital for your safety. Carbon monoxide can escape for a number of reasons, such as a cracked chimney. The alarm will go off before reaching harmful levels.


These are just some of the regulations to be aware of for full details you can download the guidance here https://www.gov.uk/government/publications/combustion-appliances-and-fuel-storage-systems-approved-document-j


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