Flue size update

doc_jIn October 2010 our Technical and Training Manager Andy Mathews issued an update on flue sizes, the issues highlighted by Andy remain prominent and can be read below. Andy writes: Through the ongoing HETAS inspections process we are realising that there are a number of installations that are failing in respect of the flue installation. One of the more common areas of non compliance has been recognised by our inspectors and these concern flue sizes and those associated flue connection arrangements. It has been observed that 125mm flues have been fitted instead of the required 150 mm flues. There is clear evidence in failure to observe the appropriate Building Regulations in force controlling the installation of flue liners.

All appliances up to 30kW

The Building Regulations 2010 state that appliances burning any fuel and rated up to 30kw would require a minimum 150mm diameter flue liner. The Building Regulations are more stringent than manufacturers’ instructions; therefore, the Building Regulations must be abided by unless the manufacturers’ installation instructions state a larger diameter. The quoted flue liner size should be as per manufacturer’s instructions but in all cases should be at least 150 mm diameter or larger where appropriate. The following extract confirms this: The Building Regulations (2010) Approved Document J: page 30, 2.6, that “For multi-fuelled appliances, the flue should be sized to accommodate burning the fuel that requires the largest flue.”

Smokeless Fuelled Appliances: and Defra exempted Appliances up to 20kW rated output

A DEFRA exempted appliance is one that meets the standard for burning non-authorised fuels in accordance to the Clean Air Act. The Building Regulations, Approved Document J, Table 2, suggest you may line the chimney with a 125 mm diameter liner for burning only authorised smokeless fuels or the fitting of an exempted appliance burning one of the fuels listed in the exemption record for that appliance. This assumes that the appliance manufacturer does not specify a
larger diameter flue is required.

Note: This is not to be confused with connecting flue pipes, where the connecting flue pipe should have the same cross sectional area as that of the appliance flue outlet, and should not be smaller than the size recommended by the appliance manufacturer. Where possible the Connecting flue pipe should be kept to the shortest practical length.

Registered installers should also refer to Chapter 11 of the HETAS Technical Handbook.

Andy Mathews
Technical and Training Manager

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