Why is a Flue necessary?

Why is a Flue necessary?

Flue damage

The primary function of the flue or chimney is to remove the unpleasant gasses produced by the combustion process away from the stove to be diluted by the atmosphere. However, the flue also performs two other important tasks.

1. It allows fresh supplies of air to be introduced into the stove. Because the fresh air should be supplied constantly and proportionally to the fire size, the flue should also exhaust the combustion gasses constantly and proportionally to the size of the fire. However, the flue is constantly being subjected to conditions that make this challenging to achieve.

2. Because some of the elements of the gasses produced by the combustion process are potentially hazardous to the fabric of the flue or property, serious consideration must be given to the correct installation of the flue system when installing a stove to avoid damage to the property or endangering the people within it. We strongly advise that an existing chimney is lined with a suitable stainless steel flue liner of the correct diameter.

For a stove to perform efficiently the flue system needs to be warmer than the surrounding air to ensure the gasses produced by the combustion process are removed.

All surplus heat is wasted heat, but insufficient heat will not only give a poor flue draught, it will allow the gasses from the combustion process to damage the flue.

The chemical changes which occur during the combustion process resulting in water being formed. As long as this water remains a vapour it causes no problems, water vapour weighs less than air and increases the flue’s efficiency.

However, if the vapour cools sufficiently to condense, it will mix with other gaseous products of combustion to form acids and tars which will be deposited on the flue wall. The acids eventually eat their way through the fabric of the flue and the tars will cause the flue to block, posing a fire risk, causing unpleasant smells and will potentially run down the flue to the stove and hearth.

The damage caused by insufficient flue temperatures can often be seen on the outer walls of older properties, where the flue is no more than a brick or stone duct.

As stoves are being designed to give ever improving efficiency, the temperatures of the gasses leaving the stove become progressively lower and the flue’s design and construction become increasingly important to avoid the risk of tar build up and the destructive effects of acids.

To reduce the risk the condensation within the flue the exit route of the gasses must be as simple and as smooth as possible and retain as much of the original gas temperature as possible. For this reason we recommend that a flue liner is used.

Note: All flue installations and modifications to existing chimneys and flues are governed by regulations and legal requirements and any such work should only be undertaken by a suitably qualified person.


An existing chimney may have only its masonry duct forming the flue way, which is completely unsuitable for a modern highly efficient stove for several reasons.

1. The masonry will be capable of absorbing moisture from the atmosphere which, unless the stove was used continuously, would never dry out. This moisture would absorb so much heat from the flue gasses that they would begin to condense which would deposit even more moisture into the masonry.

2. The walls of a masonry chimney represent such a large surface that even if the masonry was dry, the heat loss through the masonry would cool any flue gasses to an unacceptable temperature. For example, if the stove manufacturer recommends a 5” flue and the surface area in the chimney was increased even to 9”, the stove would need to heat an increased volume of 239%. The increase on demand for a masonry chimney would be significantly higher than this figure.

3. The large surface area of the walls, even if they were smooth, would deteriorate over time with increased voids, cracks and loose or missing masonry. The poor standard of wall surface will cause turbulence to the flow of gasses further slowing and allowing more heat to be extracted.

Many chimneys follow a torturous route of bends and twists to align the flues from several rooms to an orderly group of terminals at the top of a single chimney structure. For flues such as these, a flexible stainless steel liner designed specifically for wood and coal burning appliances can be fitted which reduces the abrupt changes in direction in such flues and gives a smoother wall. For additional insulation vermiculite or similar insulating materials can be poured between the liner and the chimney duct.


Whatever flue lining system is used it is important to use the correct diameter as specified by the stove manufacturer. An oversized diameter will lose heat unnecessarily through its increased surface area and the slower moving gasses will have more time in which to lose even more heat.


Always have the flue swept by a qualified sweep regularly and before significant deposits build up. The sweep will be able to advise you on the condition of your flue and if tar deposits are excessive. Attention should also be paid to the fuel being used and the way the stove is being operated.