Soot fires inside chimneys are one of the most dangerous incidents linked to solid fuel boilers, stoves and fireplaces, starting suddenly and sometimes burning for hours once they take hold, according to the home improvement website BudujemyDom.
The outlet said the response in the first minutes is critical: homeowners should cut off the air supply to the firebox, call the fire service immediately and never attempt to douse a burning chimney with water.
Soot builds up on the inner walls of a chimney as fuel burns. When enough of it accumulates, and the deposits contain flammable tar and resin, it can ignite suddenly and burn intensely, according to BudujemyDom. The process is often accompanied by loud noise from the strong airflow and rapid combustion inside the flue.
Chimneys are not built to withstand prolonged exposure to open flame at such high temperatures, the site said, meaning the liner, insulation and other parts of the flue system can be damaged.
Why soot catches fire
Soot fires develop after deposits build up over a long period, usually because fuel was burned incorrectly or the chimney was not cleaned regularly, BudujemyDom said. Burning damp wood is one of the most common causes, since wet fuel burns less efficiently, produces more smoke and leaves behind more residue. Poor quality fuel creates a similar problem.
Restricting the air supply to the firebox too much also causes incomplete combustion, increasing the amount of soot and tar that builds up and gradually narrowing the flue, according to the site. Ignition can follow a sudden rise in flue gas temperature, for example during very intense stove lighting, and if a large amount of flammable deposit is present it can catch fire quickly.
How long a soot fire can last
How long the fire burns depends on how much residue has built up and on the draught conditions inside the chimney, BudujemyDom said, adding that it can last anywhere from a few minutes to several hours. Throughout that time the chimney is exposed to very high temperatures, and the longer the fire burns, the greater the risk of damage to a ceramic liner, deformation of steel components or destruction of insulation.
The site noted that danger does not end as soon as visible flames stop. The inside of the chimney can remain extremely hot for some time afterwards, so the end of visible burning should not be treated as a signal that the heating system can be switched straight back on.
Risk to the building
The most serious consequence of a soot fire is cracking that breaches the chimney, according to BudujemyDom. If hot flue gases or flames escape through a crack, they can reach structural parts of the building.
Areas where the chimney runs close to combustible material, such as wooden floor beams, are particularly at risk, the site said, because the high temperature at those points can ignite structural elements and let fire spread. A damaged chimney also creates a separate hazard: gaps can let flue gases, including carbon monoxide, leak into living spaces, which is why the chimney needs a thorough check before the heating appliance is used again.
What to do if soot catches fire
BudujemyDom said the priority in a soot fire is to limit the intensity of the burning and call for help. The air supply to the firebox should be cut off immediately, since oxygen feeds the flames, and the fire service should be called without delay.
Even if the fire appears to be confined to the inside of the chimney, only a thorough inspection can establish whether the flue has already been damaged or whether the high temperature threatens the building's structure, the site said. Special fire extinguishing powders designed for chimneys can be used to cut off oxygen to the burning deposits, but BudujemyDom stressed that they are not a substitute for the fire brigade.
Why water should never be used
A burning chimney should never be doused with water, which the site described as one of the most important safety rules. A hot chimney is already under enormous thermal stress, and water causes a sudden change in temperature that can crack the liner or other parts of the flue.
That can breach the chimney and let hot flue gases or flame escape into the building, BudujemyDom said, meaning that using water instead of controlling the fire properly can end up causing more serious damage to the system.
Relighting the stove after a fire
A heating appliance should not be relit until the chimney has been thoroughly checked, according to the site. High temperatures can cause damage that is not visible from outside, including cracks in the liner, damaged insulation or deformed steel components.
Relighting a device connected to a chimney that is no longer airtight can let flue gases into the room or trigger another fire, BudujemyDom warned. That is why an inspection by a chimney sweep is needed afterwards to assess the chimney's condition and decide whether it can keep being used or needs repair.
Inspecting and repairing the chimney
The assessment should be carried out by a qualified specialist, the site said. One of the most effective methods is a camera inspection of the interior along the whole accessible length of the chimney, which can reveal cracks and other defects that would not be visible from a standard external check. The results determine what repair, if any, is needed.
The scope of work depends on the type and extent of the damage, according to BudujemyDom. In some cases the chimney needs to be milled out to prepare it for restoration. Where it is damaged or no longer airtight, a new liner can be fitted, using materials that include acid-resistant steel liners and ceramic liners, among them elements made using isostatic pressing. In cases of very serious damage, the chimney may need to be partly or fully rebuilt, and the site said the extent of repair should always be based on a technical assessment rather than the chimney's outward appearance.
Preventing a soot fire
The most important preventive step is regular cleaning of the chimney to remove built-up deposits, which BudujemyDom said should be done several times a year for solid fuel appliances, depending on the type of system and fuel used. Using the right fuel matters just as much: appliances that burn wood should be fed dry timber, since damp fuel encourages smoke, soot and tar deposits.
Controlling the combustion process properly also helps, the site said, since restricting the air supply too much worsens combustion quality and increases the amount of residue entering the chimney. Fuel should be allowed to burn as completely as possible rather than being smothered for long periods. Soot-burning products and chimney draught attachments can also be used, but BudujemyDom said they do not replace regular cleaning and inspection.
Does chimney design affect the risk
The design of a chimney system can make it easier to keep in good condition, according to the site. Modern ceramic systems use glazed inner liner surfaces, among other features, which reduce how much dirt sticks to the walls.
Even so, BudujemyDom said this does not remove the need for correct use. Even a good quality chimney requires regular cleaning, the right fuel and proper control of the combustion process.
