Chimneys

A chimney safely removes the products of combustion from the fireplace or appliance to outside without causing danger to the occupants of the house or setting the house on fire.

For any fire to work successfully and efficiently, it must be connected to a sound chimney and correctly sized flue.

A chimney works because hot air rises and always moves from high to low pressure. Factors such as running the appliance at a very slow rate, or cold air leaking into the flue, will cool the gasses and affect the performance of the chimney.

In houses built since 1965 (when Building Regulations were introduced) all flues must be built with liners during their construction. This is usually done with clay liners, which should last the life of the building. However, in houses built prior to 1965, lining was less common. Flues were usually “parged” (rendered) on the inside with lime mortar. This parging suffers attack from acids and tars produced during combustion, and gradually deteriorates. The flue is then in poor condition, often leaking fumes or tars into the walls or other parts of the building. Sadly, many houses built since 1965 suffer similar problems due to badly installed flue liners and need further attention.

• Chimney

  - A chimney is the structure surrounding one or more flues.

• Chimney Terminal

  - This is the pot, cowl or other method of finishing the top of the chimney.

• Flue

  - The flue is the void or passageway through which the products of combustion are removed from the fire to the outside.

• Flue Liner

  - The flue liner is the material used to form the flue or flues within a chimney.

• Flue Pipe

  - Is a metal pipe used to connect an appliance to the flue in a chimney.

There are a number of reasons why an old chimney may need lined

• The flue is leaking smoke and fumes into other rooms or parts of the building.
• Condensates or tar are seeping through the chimney walls causing staining, either inside or outside the building (a common problem with wood burning stoves)
• The flue is too large for the type of fire or appliance being used.
• The flue is too cold, particularly if on an outside wall, and is not drawing properly.
• If the chimney was built since 1965, but with the liners fitted the wrong way up, tar and condensate leakage may occur.
• The old flue surface is eroded and rough, causing frictional resistance to the flow of the gasses resulting in poor updraught.

Before any new lining is installed it is essential to have the chimney swept to remove all soot and tar deposits.

Several methods are available for inserting a lining into an existing flue but by far the most common and cost effective method we use is a flexible metal liner in a continuous length lowered down the chimney.

There are two distinct types of flexible metal liners available and it is very important to distinguish between them.

• Solid fuel flue liner

  This is a double-skinned liner made from an extremely high quality stainless steel and is smooth on the inside, corrugated on the outside and must be installed the correct way up. It is specifically designed for relining flues for solid fuel and wood.

• Gas flue liner

  A light gauge single skin liner commonly available in coils. It cannot be used for solid fuel or wood burning appliances or open solid fuel fires. Readily available sizes range from 4” - 8" sizes diameters. The minimum size permitted for closed (glass fronted) gas fires and stoves is 5” (125mm).

• There is soot in my chimney; how did that happen?

  If the temperature is too low you might experience tarry soot in your chimney.

• Smoke is coming out into my living room when I open the stove door. How do I avoid that?

  Do not open the door before the wood is burned all the way down to embers. When opening the door, a lot of cold air will get in contact with the flames which may cause smoke inconveniences. If you really have to open the door before the wood is burned to embers, do it carefully. Let the door stand on a 2cm gap for a moment before opening it completely. Another reason might be poor draft conditions. A chimney sweep is able to measure the draft.

• There is not enough draft in the chimney. What can I do?

  If the chimney is too low, not well insulated or has a leak, you may experience draft problems. Let the chimney sweep take a look at it and measure the draft. It is essential that the chimney is well heated during the lighting period (see our page on Lighting)

• What kind of chimney is the best?

  Steel chimney or a well-insulated brick chimney. They both have to be min. 4 meters to get an optimum draft in the chimney. As well as the height of the chimney you have to ensure the chimney is free from the roof ridge to avoid down-draft. The diameter of the chimney must be between 150-200mm. If a brick chimney is not insulated the diameter must be more than 200 mm which will also reduce the draft conditions. We therefore recommend an insulated chimney. Ask your chimney sweep for the best advice.

• How high should the chimney be?

  The chimney must be at least 4 meters from the pipe connection to get an optimum draft. However, it also depends on how much draft the individual stove demands. The chimney demands 3 - 5 Pascal’s per meter depending on wind conditions and the surroundings (trees, buildings etc).

• How should the diameter of the chimney be?

  The diameter should be at least 150mm and not more than 200mm as this will reduce the temperature of the smoke and thereby cause poor draft conditions.

• How long should the flue pipe from the stove to the chimney be?

  The flue pipe from the stove to the chimney must be as short as possible to avoid cooling down the smoke. If the smoke does not have the right temperature, it may cause poor draft conditions and smoke inconveniences. Only one elbow should be connected to the stove as the smoke will face too much resistance.

• Are the draft conditions different if the stove is fitted with a rear flue exit instead of a top rear flue exit?

  It’s not easy to measure any real difference in the draft conditions between a stove fitted with a top flue exit compared to a rear flue exit. We’ve found a top flue exit is an advantage if the draft conditions are not optimum as the smoke will face less resistance and draft and smoke inconveniences can therefore be avoided. Most stoves are fitted with both a rear and top flue exit.

• How often should a chimney sweep check the chimney?

  Once or several times a year depending on your requirements/stove use. It is recommended that you clean up the stove once or twice a year yourself. It is essential to remove soot and ashes on the baffle as it will make the baffle last longer.