Twin wall flue systems for biomass boilers

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There are a number of considerations for designing the flue system for a biomass boiler installation. Our project management team will manage this for Treco customers, but some important considerations are listed below. We fit Twin Wall, Insulated stainless steel multi-fuel chimney systems for Treco installs, which are eminently suitable for modern biomass boiler applications.

However, please contact us on 01884 250 790 or email us at [email protected] to discuss the flue requirements for your biomass boiler project.


All twin wall flue systems for biomass boilers (including log boilers, wood chip boilers, wood pellet boilers) must be installed to comply with Document J of the Building Regulations in England and Wales and part F of the Building Regulations in Scotland, as well as to the flue manufacturer’s installation instructions.

Flue design

The biomass boiler twin wall flue system, from the point it leaves the building, should remain as straight as possible and be high enough to ensure sufficient draught to clear the products of combustion. The height necessary for this will depend upon the type of the appliance, the height of the building and the number of offsets within the system. However, a minimum flue height of 4.5 metres should is recommended in the Building Regulations.

Where it is not possible to construct a vertical flue for your biomass boiler system, Building Regulations dictate that no part of a chimney should form an angle greater than 45° from the vertical, except where it may be necessary to use a very short horizontal section, not exceeding 150mm, of flue to connect to a back outlet appliance. The run between offsets should not exceed 20% of the overall height of the chimney. In both instances, a maximum of two offsets in any one flue run should be used. Cleaning access must be provided in chimney systems with two offsets. It is also recommended that a vertical rise of 600mm should be allowed, immediately above the appliance before any change of direction.

Where the flue passes through a combustible floor or ceiling, an air gap clearance as per the manufacturer’s installation instructions is required. In multi-fuel applications and where the flue gas temperature is greater than 250°C, the ventilated fire stop and ventilated support components must be used at the bottom and top of the shaft. No joints can be made within the thickness of any floor or ceiling joists. The twin wall flue system must project below the ceiling in accordance to the diameter of the flue pipe.

Use of an adjustable length within the flue pipe connection or twin wall flue system immediately above the appliance enables removal of the appliance later without dismantling the full system.

In the room where the biomass boiler appliance is installed, provision should be made for an air vent. This is required to provide adequate air supply to allow the appliance and flue to operate efficiently. These requirements are specified in the Building Regulations. Where a flue passes through an external wall, a wall sleeve must be used.

Flue size

The size of the chimney system should be as recommended by the biomass boiler appliance manufacturer’s installation instruction and in most cases should not be smaller than the appliance outlet. Confirmation from the manufacturer will be required if you wish to reduce the flue size. Flue sizing for commercial biomass boiler applications will depend on individual site details and must be designed and sized in accordance with manufacturer’s criteria. For open fireplaces and inglenooks with openings larger than 500mm x 550mm, a flue size with a cross sectional area equal to 15% of the total face area of the fireplace opening is required. On multi-fuel or wood burning closed biomass boiler appliances of up to 30kw rated output, the minimum flue size must be a 150mm internal diameter.

Support components

Load bearing supports eg. wall supports and ceiling/joist supports are required to take the vertical weight of a twin wall flue system.  Non load bearing supports e.g., wall bands and roof/rafter supports give lateral support throughout the chimney system. Wall bands should be fitted every three metres and where the flue offsets to ensure the system is adequately supported. Where the flue is free standing above the roof line a height of up to three metres can be achieved unsupported with the use of a reinforced or extended locking band at the joint immediately below and every joint above the roof level.

Provision for sweeping, cleaning and maintenance

Provision should be made for inspecting and cleaning the flue. This is particularly important on solid fuel applications. It is recommended that flues serving solid fuel appliances be swept and maintained as frequently as necessary but at least once a year.

Please contact us on 01884 250 790 or email us at [email protected] to discuss the flue requirements for your biomass boiler project.

Do you have a biomass or low carbon project?

Speak to our biomass team today

01884 250790

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