Fireplaces, chimneys and flues

6.8.21All – flue liners

Flue liners shall be unaffected by flue gases and suitable for their purpose.

To produce a suitable flue path, appropriate components should be selected to keep cutting and joints to a minimum. At changes in direction, including bends, offsets and tees, purpose-made components should be used.

Flue liners should be:

  • clay or purpose-made concrete, as specified in the design
  • handled carefully to prevent chipping or cracking
  • installed in accordance with the manufacturer’s instructions and the design
  • sealed at their joint with the starter block or throat unit (no cavity should be formed between the linings and the starter elements)
  • placed with the sockets or rebate ends facing up.

Liners suitable for solid fuel appliances, and generally suitable for other fuels, include liners whose performance is at least equal to the designation T450 N2 S D 3, as described in BS EN 1443, such as:

  • clay flue liners with rebates or sockets for jointing meeting the requirements for class A1 N2 or class A1 N1 as described in BS EN 1457
  • concrete flue liners meeting the requirements for the classification type A1, type A2, type B1 or type B2 as described in prEN 1857(e18) January 2001, or
  • other products that are independently assessed in accordance with Technical Requirement R3.

Alternatively, imperforate clay pipes with sockets for jointing as described in BS 65:1991 are acceptable.

Joints should be made in accordance with the manufacturer’s recommendations, generally using:

  • fire cement, or
  • refractory mortar

Joints should be fully filled, and surplus material cleared from the inside of each joint as the flue is built.

Spaces between the lining and the surrounding masonry should be:

  • filled with weak insulating concrete, or
  • in accordance with the manufacturer’s recommendations, with the specified material providing adequate protection.

Ordinary concrete should not be used to fill the space between the lining and the surrounding masonry.

Suitable mixtures for weak insulating concrete include:

  • one part ordinary Portland cement to 20 parts suitable lightweight expanded clay aggregate, minimally wetted
  • one part ordinary Portland cement to six parts vermiculite, or
  • one part ordinary Portland cement to 10 parts perlite.