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Preventing chimney fires

Every winter, chimney fires take their toll on a number of nationally listed buildings. Eighty per cent of these fires result in major damage, destroying the entire roof structure.

In theory, the operation of a fireplace or stove is simple: combustible matter is burned inside and the energy that is released is used to heat the space, while combustion gases are discharged through the chimney flue.

Yet, in practice, there are all sorts of things that can go wrong when using fire for heating. Most problems occur in and around the chimney flue. With the recent increasing popularity of wood stoves, the number of chimney fires only seems to be growing. Thatched houses present a heightened risk, as a fire in the chimney can quickly spread to the roof thatching.

Preventing chimney fires

A few crucial tips

  • Use your fireplace or stove responsibly. Make sure you understand the combustion process, the materials used and the risks involved.
  • Periodic maintenance is essential: have the chimney swept at least once a year.
  • Also schedule a periodic inspection at least once a year.

What are the most common problems with chimney flues?

Leaks in the chimney flue

Causes

  • Cracks caused by overheating
  • Subsidence
  • Structural faults
  • Poorly fitted connections in the chimney pipes
  • Joist ends bricked into the chimney flue (in old buildings)¬†

Consequences

  • Formation of toxic gases, including carbon monoxide. These gases can be extremely dangerous to people and animals.
  • Leaking hot gas can ignite materials near the leak; the risk is especially great when the chimney flue runs through the floors, ceilings and roof.
  • If there are joist ends in the chimney flue, they can catch fire through cracks in the masonry: the cracks allow heat to reach the joist ends, causing them to ignite. This can happen even after years of a chimney flue working problem-free.

Recommendations

  • Regularly check the chimney flue for leaks during the season it is in use. You can identify the site of a leak by the smoke coiling out. A strong smell of smoke is another sign of problems.
  • Watch out for discolourations in the paint and wood around the chimney, which can be caused by leaks in the flue.
  • If you suspect there is a leak, have the chimney inspected by a building contractor or chimney specialist. They can do a chimney pressure test. This should also be done if you start using a chimney that has not been used for a long time.

Schoorsteenbrand monumentale boerderij Hei- en Boeicop, foto RCE
Aftermath of a chimney fire at a farm in Hei- en Boeicop, February 2013 (photo: RCE)

Dirty chimney flue

Causes

  • Substances produced during combustion cause residue to build up along the inside of the chimney flue, called creosote. How quickly this creosote accumulates depends on the type of fuel used and how it is burned. For example, wood that is excessively damp will often cause more buildup, as will burning paper, painted and impregnated wood and composite wood such as plywood and particle board. Burning at a high temperature with a sufficient oxygen supply will cause fewer residues to build up compared with a slow, smouldering fire.

Consequences

  • When the temperature in the chimney flue grows too hot, the creosote on the flue walls can ignite and start a smouldering fire inside the chimney.
  • Embers that escape from the fire present a particular danger in the case of buildings with a flammable roofing material such as thatch as they can leap from the chimney and fall onto the highly-flammable thatch.
  • Temperatures inside a burning brick chimney flue can grow so high that the masonry may crack and burst.

Recommendations

  • Make sure the burning conditions are optimal such that all the fuel is burned as fully as possible.
  • Do not burn damp, impregnated or painted wood, composite woods such as particle board or plywood, or paper.
  • Only use dry wood. Normally, wood is suitable for burning once it has been seasoned by lying in a dry, windy spot for at least one year.
  • Have your chimney swept at least once each season, preferably by a certified chimney sweep.
  • If the flue has not been used for a long time, have it cleaned first.
  • If you have a thatched roof, it is essential to cover the top of the chimney stack with a spark arrestor. Many insurers also stipulate this in their terms.

Improper chimney design

Causes

  • The diameter of the chimney flue plays a crucial role in the combustion process. A fire needs air, and for a fire to burn effectively, this air has to be discharged through the stove and out through the flue at a certain speed. This process only works properly if the diameter of the flue is the right size.
  • Chimney flues are always designed and built for a specific type of use, such as for a coal or oil-fired stove. If a flue designed for this purpose is subsequently connected to a wood-burning stove, the diameter of the opening may not be suitable.
  • Lots of bends in a chimney slow down the air current. Bends may never exceed an angle of 30 degrees.

Consequences

  • When the chimney flue is too narrow it increases the chance of excessively high temperatures.
  • A chimney flue that is too wide increases the likelihood of heavy residue buildup along the walls.

Recommendations

  • Consult with a building contractor or chimney specialist about the condition and use of an existing chimney.

Heat radiation

Causes

  • Single-walled steel chimney flues can grow extremely hot.

Consequences

  • The heat can cause flammable substances in the vicinity of the chimney flue to catch fire.

Recommendations

  • Ensure that single-walled steel chimney flues are placed at a good, safe distance from other structures.
  • Furthermore, as these pipes also feed through the floors, ceilings and roof, take account of the risks this entails and ensure a sufficient distance from structural elements.

Fighting chimney fires

  • Chimney fires are characterised¬† by a roaring or rumbling sound inside the chimney.
    Close the chimney damper and draught excluder immediately and contact the fire brigade.
  • Never use water to extinguish a chimney fire!
    Using water can result in a rapid temperature drop that may cause masonry chimneys to crack and burst. Moreover, when it is hot inside the flue, water used to put out the fire will change into water vapour and can lead to even more damage.
  • Put out the fire using sand, salt or a dry powder extinguisher
    All of these should be directed into the flue from the hearth.

The fire brigade normally uses a chimney chain to extinguish chimney fires, which consists of a chain with a ball and steel brush or smaller plated chains attached. The fire brigade will use this to remove smouldering particles from the flue and then cool it in controlled stages. Burning soot is collected and extinguished in the hearth.