Flue Liners

A 'flue liner' is the material used to line chimney stacks. Since 1965 houses with chimneys were usually built to include flue liners. They tended to consist of a clay liner which was designed to last the life time of the building. Prior to 1965 lining was not common place and chimneys were simply rendered with lime mortar.

There are three basic types of flue liners on the market today; cast in situ refractory lining, metal and sectional, solid liners.

Cast in situ refractory lining

A toughened rubber tube is inserted down the length of the flue and inflated. The structure of the flue is temporarily opened up at intervals of 2 meters and bends so that refractory blocks can be inserted to ensure the tube is in the correct position. A refractory concrete insulating mix containing pumice or vermiculite is then poured into the flue to surround the outside of the rubber tube. The tube is deflated once the mix has set, leaving behind a perfectly formed smooth cylindrical flue. Cast linings are the least expensive and amongst the simplest to install. However the success of the flue heavily depends on the skill of the specialist contractor. Water in the refractory mix can cause decay and migration and if the tube is not centralized properly, once installed it almost impossible to remove. The tubes are available in a range of sizes to accommodate different flues and fuel types.

The standard 9 x 9” flue creates a 185mm diameter cast in situ lining and in accordance with The Building Regulations is suitable for both solid fuel open fires and gas effect decorative fires. The ratio of the flue size against the size of the fireplace is an important factor to consider. Get this wrong and it could result in a weaker draught. In most cases the ratio of the cross sectional area of fireplace opening to the flue should be a maximum of 8:1 (6:1 for single storey properties). The average register grate is 12”-18” wide and 18-22” in height above the grate so most fireplaces will accommodate a flue as small as 5mm diameter.

The flue size of fireplaces built before the 19th Century will require additional calculation. Most old chimneys need to be relined before they become re-useable as even those built with a lining may have a flue that is too big or inefficient for modern stoves and fires. Installing a new liner with a correctly sized flue will ensure you have a new sealed, efficient and safe chimney. Both Cast and Metal liners are suitable for solid fuel and wood appliances.

Metal Liners

There are three variations on metal liners available; flexible double skin liners, rigid metal liners and gas flex.

Flexible double skin liners are made from stainless steel whilst the inner skin is smooth the outer side is corrugated. They are installed by pulling the liner either up or down the flue on a rope where the ends can then be fixed into position. This type of lining is suitable for solid fuel, wood or gas effect fires and available in a range of diameters.

Usually used to reline large flues in wood burning stoves rigid metal liners are made of stainless steel with a minimum thickness of 1mm. and are installed and held in place with special clips at the joints. Space around the liners is back-filled with perlite or vermiculite.

Gas flex is used for closed gas fires and boilers. It is a light single skin liner providing a flexible and economic lining for gas burning appliances.

Sectional, Solid Liners

Sectional, solid liners are more likely to be used in the case where the existing stack is large or straight or if the stack is re-built, and will then incorporate the liner in the stack as it is re-built. Made of refractory concrete or pumice concrete the fuel liner is highly durable and like metal liners, solid, sectional liners are reversible and therefore have the advantage of not involving the introduction of water.

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