When coal soot deposits or wood tar deposits build up to a sufficient level there is the risk of chimney fire. The heat from the fire warms the deposits releasing the combustible volatiles until they ignite. The fire then migrates up the chimney as the burning deposits heat the chimney above.
How to tell if you have a chimney fire - You will often hear a roaring noise in the chimney, especially with an open fire. Outside masses of smoke will be pouring out of the chimney.
When you start to see flames coming out of the chimney you know that the chimney fire is near the it's end.
We had this page checked over by Avon Fire Brigade - our thanks!
Coal soot chimney fires can create temperatures up to 1000 degrees centrigrade inside the chimney.
Wood tar chimney fires can create temperatures up to 1200 degrees centrigrade inside the chimney.
Clay liners - a chimney fire will often cause clay liners to crack and therefore the chimney will probably need to be relined (you may be able to claim for this under your household fire insurance). Contact us for advice.
As the chimney heats up during the chimney fire it expands - this causes plasterwork to crack and even blow off and can cause structural damage to the chimney.
On one chimney fire job I looked at the headboard of the bed that was against the chimney breast was scorched and the plaster looked like a London Underground map. On another job there had been a wood tar fire to the stone-built chimney. The stones inside the chimney had a glassy sheen as they had been vitrified by the high temperature inside the chimney!
Chimney deposits are intumenscent and expand when heated. In the worst case scenario they can expand to the extent that they block the chimney. The chimney fire will then seek oxygen from the nearest available source - usually the stove or fireplace - which means that the fire can come out of the bottom of the chimney.
In extreme cases where the integrity of the top of the chimney is already compromised the top courses of brick can blow off due to the pressures inside the blocked chimney.
Heat from the chimney fire can transfer into joists and weaken them through smoldering and cause them to catch fire. Sparks and debris flying out of the chimney can also set fire to the roof if there are tiles missing.