Chimney Types

Class I flues

Most houses built before the 1960s will have a class I flue, which is has an internal diameter of 7 inches or greater.

Class II flues

Smaller Class II flues are often found in modern houses or houses that have had the flues relined. Class II gas fires are more efficient but some larger fireplaces and baskets cannot use them as they cannot cope with the volume of fumes created.

Precast Flues

Precast flues, or gas flue blocks are incorporated within a building block in more modern houses. Usually there is no chimney breast and a Ridge Vent on the roof instead of a chimney pot. Only fires labeled as suitable for precast flues can be used with them as they can only move a limited volume of flue gases.

No Flue: No Problem

If you would like a gas fire but your house has no chimney you can have a Powerflue gas fires designed to be fitted on outside wall.

More detailed Information on Class I Flues, Class II Flues and Precast Flues can be found here

The Flue or Chimney

The flue is the technical term for the usually vertical shaft that draws the smoke and fumes from the fireplace to the terminal. The terminal is the exit point for the smoke, usually a chimney pot.

Before you buy a new fireplace and definitely before installing a fireplace I strongly advised that you check that the flue is sound. It must not leak since.any leakage could be life threatening. If the smoke is going anywhere other than up the chimney associated with it then you must have it checked by an expert.

Chimney Liners

An existing metal liner within a brick flue may indicate previous problems which have necessitated the flue being lined. A single wall 5 inch diameter liner will work with class II gas fires only. A single wall 7 inch liner is for class I or class II gas fires. An 8 inch twin walled stainless steel liner are tested for solid fuel appliances and will also accomodate gas fires. The liners must be adequateley sealed within the flue at both the bottom of the chimney terminal and should generally be in one single length.

Smoke Tests

The usual method for testing a flue is the smoke test. Although you can do this yourself, I would always advise that an experienced person carry this out. Fireplace fitters, builders and especially chimney sweeps may be your best bet. A local chimney sweep from your yellow pages will clean your chimney, which should always be done when fitting a new fire, and should also be able to do a smoke test to check the flue for soundness.

Smoke tests are very simple. A smoke pellet is burned in the fire opening. Smoke should emerge from the correct terminal only (usually one chimney pot). This is a good indication that your flue is sound. However, since many houses have had their flues messed around with at some stage I feel that there is no substitute for seeing the bristles of a chimney-sweeps brush emerging from the top of your chimney pot!

While the smoke pellets (which are deliberately strong smelling) are burning, you should check all of the rooms that the flue passes through, including the loft space. There should be no leakage into any of the rooms above or below where the fire is to be fitted.

Roughly speaking, houses built before the first World War (1914) are likely to have leaky flues and must always be thoroughly checked. In my experience, solid fuel fires will damage the flue much more than a gas fire. Many houses built after the second World War tended to have gas fires fitted and so the flue will probably still be sound. However, since house builders were a mixed bag of variously qualified people, all flues should be checked prior to the installation of any fireplace

If there is any leakage, chimney sweeps can be very helpful. Good builders should also know how to repair the damage and there are also specialist flue fitting firms who do nothing else but fix leaky flues.

If you find yourself confronted by a solid brick wall where you think the fireplace used to be, remove the vent or knock a hole in the wall to enable a smoke check to be done on the flue.

Capped Flues

If the smoke stubbornly refuses to rise up the flue it may be because it is capped off at the top. Capped flues must be uncapped to do a flue smoke flow test.

However, it may just be that the flue is filled with moist, cold air. This air will be heavier than that in the room and prevent the smoke from rising. Warming the flue with a blowtorch or burning some sheets of newspaper inside it will often do the trick of getting the smoke moving up the chimney, enabling it to be checked

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