Many chimney fires occur because the flue interior has been neglected by not being cleaned regularly enough depending on the fuel being burnt. Chimneys can become choked and partially blocked with an accumulation of soot deposit, left from burning coal or wood. Soot (Carbon) rises up in the smoke and is deposited on the surfaces of the flue.
Anatomy of a fire
For a fire to "live and survive" it must have three separate elements.
Take any one element away and the fire "dies". Sounds easy doesn't it; however, once a fire gets hold, its always searching for survival.
Carbon: The Black Stuff
Heated wood releases hydrocarbon gases. When these get hot enough, they mix with air and catch fire.
When open or wood stove fires smolder, unburnt gases condense in the cooler air of the flue/stack and deposit on the stove pipes or brickwork as runny acids and liquid tars that harden into creosote. A cool flue and steam from green or wet wood when first starting up a fire, create moisture, known as condensation. This condensation contains the hydrocarbon gases and when the flue/stack starts to heat up the condensation (water molecules) dry out and leave the residue compound Carbon, the base for creosote buid up.
Creosote can appear as any of the following;
a sooty powder
a sticky mess
a hard glaze
a deposit that looks like burnt expanding foam.
A creosote fire can burn with such blast-furnace intensity that it sets off this frightening chain of events:
"Crumbling and cracking mortar; Balls of flaming creosote shoot out of the chimney top onto the roof; Clay-tile flue liners crack open; Stainless steel liners warp, buckle and separate at the seams; Masonry in the chimney expands with such force that sections of the chimney can blow out; flames can spread to the structure or roof of the house even explode into the room".
Creosote: The Monster in your Chimney!
The buildup of creosote in your fireplace, wood stove and chimney is unavoidable, A natural byproduct of the burning of fossil fuels, resulting in various forms of creosote, black powdery, flaky or glazed coating on the inside of your chimney. Creosote is a potential fire hazard: it's the primary fuel in most chimney fires! During a chimney fire, the outside surface of the chimney can become hot enough to ignite surrounding walls, floor joists, rafters, insulation or roofing materials. Suddenly, you have a structure fire, which can raise your home to the ground.
This type of creosote makes for the hottest burning fuel for a flue fire. The thicker the layer of creosote, the hotter the fire. The heat generated by this infernal can raise to ignition point, the temperature of wood structures on the other side of a chimney, so that it also starts to burn threatening the entire house.
Wood doesn't necessarily need contact with fire in order to ignite. It just needs oxygen and enough heat.
A house may survive the first chimney fire, but the intense heat has started pyrolyzing* nearby combustibles, thus lowering their ignition temperature.
This makes the structure very vulnerable to a subsequent chimney fire. A damaged flue liner can no longer protect either the chimney or the house. And instead of being all burned out, creosote may instead be all puffed up to the point of partially or completely blocking the flue.
What is Pyrolysis.
Pyrolysis is a chemical decomposition caused by heat. Severely pyrolyzed wood can ignite as low as 100 degrees Celsius, while it would normally have a catch-fire temperature of about 260 degrees Celsius before it had any exposure to intense heat.
Carbon Monoxide: The Silent Killer
This odourless, colourless and tasteless gas can be fatal.
If your chimney is blocked or the stack/flue is not airtight, Carbon Monoxide may seep into your home unnoticed. Symptoms of carbon monoxide poisoning are similar to those of the flu: headaches, fatigue and nausea.
Leaves, bird nesting material, spiders webbing or fallen masonry debris can block your chimney and restrict a positive uplift in air flow. A crack or break in the flue or broken feathers (separating brickwork between two or more chimney flues) can interfere with the chimney’s ability to vent properly.
Your wood stove/fireplace may be blocked if smoke is blowing back into your home. Sometimes, it may be simple flow reversal -- negative pressures in the house, making the smoke blow the wrong way. Also a cold flue can cause blow back of smoke, a newly lit fire does not have the heat energy to force the cold air back up the flue, hence smoke flow reversal. Carbon Monoxide may be present in this smoke and Carbon Monoxide from gas appliances is not so noticeable because there is no smoke as an indicator of a blocked or limited positive air flow.
Carbon Monoxide: The Signs
Steps to safety
Fit a Carbon Monoxide detector in every room where there is a fire
Fit a stove thermometer and burn fuel between 150-250 Celsius
Store combustable material away from heat source i.e. wood bin, coal bucket, fire lighters
Ensure good positive ventilation i.e. air bricks - window air vents
Avoided burning paper, cardboard, green wood & food waste in your appliance
Regularly have your flue inspected and swept by a professional