February 2018
Managing Risk: support for business Open chimney fires and stoves Managing Risk: support for business Managing Risk: support The risk of an open chimney fire in many pubs, hotels, farm estates, farm houses and family homes is often overlooked. With the rise in gas and electricity prices, businesses and homes are opening up their once boarded-over fireplaces, which is leading to an increase in chimney fires. It’s estimated there are more than 30,000 chimney fires in the UK each year, with 9,000 resulting in serious fire damage.
What causes chimney fires? Chimney fires can start due to: • Burning wet or unseasoned wood • Failing to sweep the chimney often enough • Chimney blockages, restricting air flow • Incorrect use of appliances, for example if a stove is too large for the room • Smouldering wood embers left unchecked for long periods of time
Chimney fires are extremely dangerous. Internal flue temperatures can reach more than 1,000 degrees Celsius, emitting intense radiant heat through chimney walls. A devastating fire can start quickly. Flames and sparks can leap from the chimney top or through cracks in the flue, igniting the roof or other parts of the building. The chimney’s bricks can become hot enough to combust nearby flammable materials, such as thatch and wooden beams and affect adjoining houses and nearby trees. Even with no apparent damage to the outside of the chimney breast or flue, Chimney fires burn hot enough to damage chimney liners and crack chimney walls and pots, and even damage factory-built metal chimneys. Sweeping the flue/chimney removes the build up of deposits of carbon- based fuels, such as coal, wood, oil and gas. It also makes sure there’s a clear and safe passage for combustible gases created in the burning process, so reducing the risk of a chimney catching fire. Sweeping also removes chimney obstructions such as nests, cobwebs and loose brickwork.
February 2018
How often do you need to sweep? Depending on which fuel you use, sweep your chimney as follows: • Smokeless fuels – swept at least once a year • Bituminous coal – at least twice a year • Wood – quarterly when in use • Oil – once a year Managing Risk: support for business Managing Risk: support • Gas – once a year Burning sap in ‘green’ wood is the main cause of tar deposits in a chimney. Seasoning wood properly reduces the sap and therefore tar substantially. Storing seasoned wood No matter how dry a shed or garage, storing wood inside will result in ‘sweating’. This results in sap condensing on the surface of the wood. Stacking wood outside exposed to the wind and sun properly seasons wood. Store wood under a shelter or cover the top layer to protect against excessive rain. Chimney liners You can use many sorts of chimney liners, for example pumice liners or vermiculite for woodburners. These liners insulate the chimney keeping flue gases as hot as possible, and therefore minimising tar deposits. Problem signs to look out for Old or poorly maintained chimneys can deteriorate allowing smoke and hot gases to leak into upper rooms, the roof space, or directly into any thatched roof. Telltale signs you need to take seriously if this is happening include staining of plasterwork or wallpaper around the chimney breast, or black or brown localised deposits on the chimney in the roof space. Soot on cobwebs in the loft is another sign something is wrong with the chimney. Chimneys built before 1960 are most likely a single brick thick and parged, not lined. The internal protective parging or roughcast plasterwork crumbles and disintegrates with age, leaving the chimney’s brickwork or stonework exposed to acidic condensates created by modern appliances. These chemical reactions can accelerate the attack on the mortar leading to further and rapid deterioration. In period and thatched buildings, chimney construction can be unconventional. Period buildings built before Building Regulations were introduced often have timber lintels over the fireplace and timber joists built into the chimney stacks. The disintegration of protective parging inside the chimney can expose these to scorching. Safety precautions for chimneys and flues • Ensure the chimney is swept regularly. If it’s in a period building inspect the mortar inside regularly and immediately after any chimney fire • Sweep chimneys according to the type of fuel used • Keep chimneys and flues clean and well maintained • Always use a fire guard to protect against flying sparks and hot embers • Make sure any fire is properly out before leaving a property • Use good quality fuel • Never block air vents or air bricks
• Don’t burn cardboard, paper, painted or unseasoned wood – even in wood burning stoves February 2018
Safety devices Tar removers You can apply chemical products to the embers of the hearth fire or firebox in a stove. These dry out tar deposits in the flue causing them to disintegrate. Repeat this treatment as recommended to prevent new deposits from forming. You’ll still need to sweep and scrape the flue regularly, especially where heavy tar deposits persist.
Managing Risk: support for business Managing Risk: support Stovepipe monitors Using stovepipe thermometers to monitor flue gas temperatures leaving the appliance give a good indication of how hot the upper part of the chimney is. You can manage the firebox to ensure the flue gas temperature does not exceed approximately 200ºC at the level of the thatch. Thatched roofs Although thatched roof fires are rare, more than 90% are due to a faulty flue or chimney. The thatch is designed to repel water, which makes it difficult to extinguish such fires. 90% It was believed in the past, that the major cause of thatch fires was attributed to sparks or burning brands issuing from the chimney and igniting the thatch. However, controlled tests have shown that it is difficult to ignite thatch with such a short-lived point of ignition as a spark. In fact, experiments show it’s the temperature of the flue gases inside the chimney heating the surrounding brickwork that heats the adjacent thatch around the chimney. If this reaches a critical temperature of about 200°C where the chimney and thatch join, charring and pyrolysis of the thatch occur and will eventually develop into due to a faulty a roof fire. This process develops over long periods of time. flue or chimney
Safety devices for thatched roofs Spark arrestors In the past, it was thought sparks coming from the chimney and igniting thatch were a major cause of fires in thatched properties. As a result, many owners of thatched properties installed spark arrestors on chimney pots. Although, this approach is no longer recommended, it’s important to still keep spark arrestors clean to allow it and the chimney to work efficiently. The National Association of Chimney Sweeps recommends sweeping chimneys with spark arrestors every three months including cleaning the spark arrestor itself.
Heat detectors Some systems use heat detectors in the thatch around the chimney. The system provides an audible warning when the brickwork and thatch approach a critical temperature. This early warning system allows you to take measures to reduce the temperature of the chimney – for example, extinguishing the fire in the grate and inspecting the chimney. Fireproof barrier If any major renovation work or reroofing is planned, consider building a fireproof barrier between the roof timbers and the thatch layer. So even if the thatch burned it would help stop the roof fire from getting into the structure of the building. Also insulating the chimney flue to reduce the risk of heat transfer is good practice.
February 2018
Key questions to ask There are several key questions to ask regarding open chimney fires and wood burning stoves: • Has there been a chimney fire previously? If so, was the chimney inspected and certificated by a member of a qualified chimney sweep body? In the event of a major thatched roof fire, was the roof lined with fireproof panels or boarding during any remedial or renovation works? • Are there joists or timber floors close to the chimney flue? The age of the building will provide
Managing Risk: support for business Managing Risk: support a clue as to whether there are joists or timber floors close to the chimney flue. If so, the mortar’s condition will be especially important as it will show evidence of a previous fire, otherwise unknown if a building is hundreds of years old • How often is the chimney swept based on the fuel used – and has it been certificated by the sweep? If burning wood, use only seasoned wood • Are there inspection hatches to help the chimney sweep and fire brigade in the event of fire? • If a stove is fitted, what type of liner has been installed in the flue?
Other useful information You can find more technical information on chimney safety, here: • The Guild of Master Sweeps – www.guild-of-master-sweeps.co.uk • The National Association of Chimney Sweeps – www.nacs.org.uk • The Association of Professional and Independent Chimney Sweeps – www.apics.org.uk
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