Sometimes it is impossible to fit the correct size of flexible liner (generally 150mm / 6") due to some restriction or tight bend within the existing chimney system and there is no alternative but to fit a liner with a smaller diameter (125mm / 5"). For some people fitting a smaller diameter liner can also be seen as a way to save money. However, it is important to understand the requirements of the stove manufacturer along with your legal requirement to comply with Building Regulations before the liner is specified.
Stove manufacturers determine the appropriate size of the connecting flue pipe to ensure that their stove will work safely and efficiently and therefore the flue liner should be no smaller than the size of the flue pipe aperture. In the case of 125mm flue outlets, generally on stoves with 5kW output and below, ideally the liner should be one size larger (150mm) as this diameter will make it suitable for burning high volatile fuels, such as coal, or replacing the original stove with a larger output stove at a later date.
Some manufacturers recommend a flue of the same size as the flue aperture throughout the system. The flue pipe or flexible liner used should never have a smaller diameter than that of the stove's flue aperture and that specified by the manufacturer.
In England and Wales, Building Regulations Document J (table 2) states that 125mm flues should only be used with smokeless burning appliances or those appliances exempted by DEFRA under the Clean Air Act and which allow you to burn wood in a UK Smoke Control Area. 150mm flues should be specified for appliances burning other fuels such as wood, peat and bituminous coal. It also states in paragraph 2.6 that for multi-fuel appliances, the flue should be sized to accommodate burning the fuel that requires the largest flue. This is because high volatiles fuels such as bituminous coal quickly attach deposits to the inside of the flue during combustion and over time could potentially reduce the usable cross sectional area of the flue.
So to sum up, if you fit a 125mm flexible liner and you want to burn wood, in order to comply with Building Regulations, you should only specify a DEFRA Approved Smoke Exempt stove and when burning multi-fuels in this stove only use clean burning approved smokeless fuels. If you specify a non-DEFRA Approved stove then you should not burn wood at all and you should only burn clean burning approved smokeless fuels.
If you need to use a 125mm liner and you wish to burn wood in a stove with a greater output than 5kW (which usually means a 150mm flue outlet) then you can only use a stove with the higher output that also features a 125mm flue outlet. This will severely limit your choice as there are only one or two DEFRA approved stoves with larger outputs that also feature 125mm flue outlets, for example the Broseley Serrano 7, being the most well-known. The same applies if you wish to use a higher output stove and burn multi-fuel – you should only burn approved smokeless fuels and specify a stove that unusually features a 125mm flue outlet.
Liner Dia: 125mm
Proposed Fuel Appliance: Wood
Appliance / Fuel Requirement: Defra Approved 5kW Stove with 125mm flue outlet burning wood with less than 20% moisture content (or approved clean burning smokeless fuels).
Liner Dia: 125mm
Proposed Fuel Appliance: Multi Fuel
Appliance / Fuel Requirement: Multi Fuel 5kW Stove with 125mm flue outlet only burning Approved Smokeless Fuels – but NOT wood.
Liner Dia: 150mm
Proposed Fuel Appliance: Wood / Multi Fuel
Appliance / Fuel Requirement: Any non-Defra* or Defra wood burner or multi fuel stove with a flue outlet size of 125mm to 150mm. *Subject UK Smoke Control Area regulations.
Please note: Without regular cleaning all flue liners could become constricted or blocked.