Flues or Chimneys

Your flue or chimney carries smoke or waste gases away from your fireplace outside thereby preventing coughing, choking or death. There are different kinds of flue which will affect the kind of heating apliance that you can use.

Class I flues

A class I flue has an internal diameter of 7 inches or more. Most houses built before the 1960s will have a class I flue.

9 inch square brick built flues are most common in houses of this age but these brick flues degrade with age and with use.

The flue should be checked before use. Local chimney sweeps will tell you if the flue is leaking by means of a smoke test. Having the chimney swept prior to installing a gas fire is also a ggod idea.

If a flue leaks then a gas fire cannot be used. and the flue must be relined. It will typically cost about £300 to reline a flue with a 5 inch liner. This turns the flue into a class II flue and only class II appliances can be used with it.

Class II flues

A class II flue has an internal diameter of less than 7 inchs. They are usually circular, metal and flexible and around 5 inches in diameter, like an elepha-nts trunk

They can be single or twin-walled.


Class II flues are often found in houses built after 1960 or in older houses that have had the flues relined. The gas fires that go with this type of flue will be generally more efficient than with a larger diameter flue.

Some larger fireplaces and baskets cannot use this type of flue with a gas fire, as the flue will not cope properly with the quantity of fumes created.

Precast Flues

Precast flues are incorporated within a Building Block. Usually there will be no chimney breast in the room and a ridge vent instead of a chimney pot on the roof.

Precast flues, or gas flue blocks were installed in houses from the 1960 onwards. They are the poorest in terms of the volume of flue gases that they can move. Only fires labeled as suitable for precast flues can be used with them.

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  • Greg Street
  • Reddish
  • SK5 7BS
  • UK


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