Fireplaces, chimneys and flues

6.8.11Gas – chimneys

Chimneys shall incorporate flues capable of safely conducting products of combustion from an appliance to the external air. The structure shall be capable of supporting the flue and providing adequate protection to adjacent materials. Issues to be taken into account include:

  1. flues and flue liners
  2. chimneys.

Flues and flue liners

Flue blocks for use with gas appliances should comply with BS EN 1858 (Concrete) or BS EN 1806 (Clay).

Table 4: Gas flue sizes

Rigid flue liners should comply with BS EN 1856 or be as described in Clause 6.8.6. Flexible flue liners are not acceptable in a new build.


Chimneys for gas appliances must not incorporate an adjustable draught control.

Masonry chimneys

Flues within masonry chimneys should be in accordance with the requirements relevant to flues for solid fuel appliances (see Clause 6.8.6b).

Brickwork or blockwork chimneys for gas appliances should, at minimum, have the same level of fire resistance as each compartment wall or floor which it forms part of, or passes through. The compartment wall may form the chimney wall where it is a masonry material.

Flue block chimneys

Terminals to masonry chimneys should:
■ where proprietary, comply with BS EN 1856, BS EN 1858 and the appliance manufacturer’s recommendations
■ where proprietary products are not used, have a free opening area a minimum of 2x the area of the flue; there should be openings (6-25mm in diameter) distributed uniformly around the terminal or on two opposite faces.

Flue block chimneys
Flue block chimneys can only be used for certain types of gas appliances and should be:
■ compliant with BS EN 1858 or BS EN 1806 with a minimum performance class of FB4 N2
■ constructed using units suitable for the appliance
■ constructed, jointed and weatherproofed in accordance with the design and the manufacturer’s instructions
■ correctly bonded to the flanking masonry
■ clean and sealed
■ checked for suitability, before connecting any appliance.

Connections between flue blocks and ridge terminals should be made:
■ in accordance with the design
■ using the correct fittings and supports as specified by the manufacturers of the flue blocks, flue pipe and ridge terminal.

Gas flue blocks are at least 140mm wide. Where this is wider than the wall leaf:

  • the extra thickness should be incorporated by increasing the overall width of the cavity
  • the flue block should be installed flush with the inside of the cavity and project into the room as a false chimney breast, or
  • where the cavity is reduced, the flue block should be protected by a vertical DPM supported by a layer of non-combustible insulation, in accordance with the manufacturer’s instructions.

Flue blocks should not be:

  • built into separating walls unless it can be shown that the wall has adequate sound resistance
  • plastered; a plasterboard lining with an air space or non-combustible insulation behind it should be provided (insulated dry lining may be unsuitable in this situation unless separated from the flue block).

Factory-made insulated chimneys

Factory-made insulated chimneys should:

  • be assembled, erected, anchored and protected in accordance with the manufacturer’s instructions
  • comply with BS EN 1856 and be installed in accordance with BS 6461, BS EN 15287-1 or BS 5440.