Chimney fires

Guidance to reduce the likelihood of your chimney catching fire

Ensure your chimney is swept regularly

Keep chimneys and flues clean and well-maintained. Find a chimney sweep on the NACS website (opens new window) or HETAS website. Chimneys should be swept according to the type of fuel used:

• Smokeless fuels - at least once a year
• Bitumous coal - at least twice a year
• Wood - quarterly when in use
• Oil - once a year
• Gas - once a year. Any work on gas appliances requires a gas safe registered installer or engineer

Use 'good' wood

All wood burned must have a moisture content of no more than 25 percent. Moisture meters are available to provide consumers with information about moisture content.

Get the correct size appliance - for example, a wood burner

It is important to purchase the correct size appliance for your room. An appliance that is too large will never be used enough to burn all of the fuel within the wood. Unburned fuel will pass up the chimney as smoke and condense within the flue as extremely flammable creosote.

Read further guidance on the use of wood burning stoves.

You should also:

• Be careful when using open fires to keep warm. Make sure you always use a fire guard to protect against flying sparks from hot embers
• Ensure the fire is extinguished before going to bed or leaving the house
• Never interrupt the air supply by blocking air vents or air bricks

Visit the NACS website for further technical information on chimney safety. 

What causes chimney fires?

Chimney fires occur when the deposits of combustion are left within the flueways. A chimney fire is the burning of soot or creosote within the appliance, outlet or flue system. This can result in improper function of the appliance, damage to the flue, house or surrounding structures and it can even start a house fire.

Chimney fires are usually started when high temperatures or flames from a very hot fire extend into the appliance outlet or flue and the combustible deposits catch light.

Signs of a chimney fire:

• A loud roaring noise occurs as air is sucked through the appliance or fireplace opening. This oxidises the combustible fuels within the system
• Sparks and flames can be seen shooting from the chimney top, which can be firework-like in appearance
• A glowing or shimmering appliance outlet or connector
• Vibrating appliance, outlet or connector
• Flames visible through any tiny cracks in the outlet or connector
• Smoke and odours noticeable in adjoining rooms or the loft space
• The heating up of the chimney breast or flue pipe, in the same room as the appliance and also other rooms that the flue passes through

Important! It should be noted that it is possible to experience a chimney fire without noticing any of the above characteristics. All chimney fires are different. The signs listed above are intended only as a useful guide.

All chimney fires are extremely dangerous even though their intensity and duration may vary. During a chimney fire, internal flue temperatures may reach a staggering 1,100 degrees Celsius. As a result, massive radiant heat is emitted through the chimney walls. With the addition of possible thatched or wooden roofs, a devastating house fire can start quickly. Flames and sparks can leap from the chimney top or through cracks in the flue and ignite the roof and other parts of the house. The bricks of a chimney can become hot enough to combust nearby flammable materials such as thatch and wooden beams.  Adjoining houses and nearby trees can also be affected.

If no apparent damage is visible on the exterior of the chimney breast or flue, it is still highly probable that damage may have occurred within the lining of the chimney. Chimney fires burn hot enough to damage liners, crack chimney walls and pots, and damage factory-built metal chimneys.

Tar and creosote build-up

Unfortunately, with all methods of sweeping a chimney, you cannot remove tar and creosote that builds up internally within the flue if wet wood or some forms of coal are burnt in them. The tar and creosote that this creates can ingress into the fabric of the chimney. When the chimney is hot, this tar can melt and vaporise meaning it can combust if a spark rises into the vapour - resulting in a chimney fire.

Tar and creosote can only be removed by chemical means. However, if you burn smokeless fuel in the appliance, the sulphur that this produces can reverse the tar and creosote build-up. However, remember smokeless fuel must only be burnt in appliances and flues designed for that purpose. Sweeping of the flue should be done by a chimney sweep until all deposits are removed and then should be monitored on a regular basis.

To minimize creosote production in a wood stove these steps can be followed:

• Once the fuel load has been ignited and the flue has been heated to its operating temperature, the stove's air supply should be adjusted to limit the amount of air. This is to avoid over firing and excessive heat loss up the chimney. However, there should be enough air to maintain moderate flaming combustion in the fire box. The flames should fill the entire window or fire box without being sucked up the chimney.
• To determine if this is maintained the condition of the fire should be checked through any glass panels. The density of the smoke as it exits the flue at the top should also be checked.
• An internal probe-type thermometer located within the flue can be used to ascertain if flue temperatures are of sufficient temperature 
• It is important when using a multi-fuel stove that you control the burning of the appliance by not using any dampers in the air inlets provided for this purpose. These could obstruct the safe passage of exhaust from being able to exit the appliance

Unsuitable terminals for live fires

These types of terminals are designed to give ventilation to unused chimneys only. They should not be fitted to chimneys that have working gas, oil or solid fuel fires or boilers, as they slow the escape of the fumes from the chimney and could cause smoke and carbon monoxide to enter occupied rooms. If you have one of these fitted to your chimney ensure it is removed if you wish to use the chimney.

After a chimney fire

After a chimney fire has occurred and been extinguished, the chimney must be inspected as soon as possible. A certified sweep should perform a thorough inspection before the chimney is used again to ascertain if the rapid and dramatic changes of temperature, which would have occurred within the chimney, have caused any damage and also to determine the need for any remedial measures. It is imperative that the chimney is not used prior to inspection.

If any damage is present and there is another chimney fire then it is quite likely the fire will spread to other parts of the building. If cracks are present within the flue then poisonous fumes can either filter through these cracks or damage. This may effect and dilute an effective warm air up draught and cause the fire to smoke. After a chimney fire, it is extremely important to sweep the burned "expanded" creosote residue from the flue, as this will cause obstructions and blockages within the flue. It is likely that, after a chimney fire, some damage will have occurred and remedial work will be necessary.

It is essential that a smoke test is carried out in accordance with the current Document J Standards.