Cause of Chimney Fires
Ok, so let’s start with the cause. In very simplistic terms, chimney fires are caused when your chimney gets dirty and is then allowed to overheat. There is a handy expression for what is needed to start a fire, the ignition triangle, referring to the three things needed to start any fire.
- Enough air.
- An ignition source.
In chimney fires, the fuel source is the soot and tar deposited in the flue/chimney. This contains carbon and unburnt hydrocarbons in varying proportions, depending on what you burn and how. If you burn cheap or smokey fuels like house coal, or pine wood, there is a risk that they will give off higher levels of soot, and it is important to get your chimney swept regularly enough to keep those levels down to the point where they are unlikely to catch fire under normal use.
Always, it is essential to burn your fuel with a proper supply of oxygen/air. Soot and tars are the products of incomplete combustion, and if you add more air to the burning mix, then a previously sooty appliance will stop furring the chimney up so swiftly.
Open fires are not at such a risk from tars, (although it is possible) because open fires have a torrent of air passing through the fire bed, so usually hydrocarbons are burnt off in all but the most reckless situations. The downside to this of course is that open fires blast a LOT of heat into the chimney and out into the Great Outdoors, which is why we always recommend getting wood burners installed. To a certain extent, while charming, an open fire is a bit like throwing banknotes up the chimney.
Multi fuel stoves
Wood burners do require sensible useage. They need to be run at sensible rates, so that wood burns hot enough to burn off the volatiles quickly, leaving a nice bed on embers, so running the fire hot while the logs get going, and then turning it down is the way to go. This is especially important when it comes to keeping the fire in overnight, or ‘slumbering’ as it is sometimes known.
It is absolutely vital that the stove slumbers with a full belly of embers. In other words, towards bedtime, you fill the stove, then run it hot till the wood has burnt off the volatiles, then shut it down tight. What you must never do is let the fire burn down, fill it with wood, and then shut it down. This effectively turns the stove into a destructive distillation apparatus.
The heat is there to break down the wood, but there isn’t enough oxygen to strip the hydrocarbons down to their constituent atoms and oxidise/burn them all. So instead of the long chains of carbon atoms with hydrogen atoms all along the sides of the chain being completely burnt, only partial combustion takes place, and short chain molecules like creosotes and tars get blown up the chimney.
A golden rule is, if you get cheap fuel, and you suspect that it is going to be sootier, then you should get the sweep out an extra time that year, just to be on the safe side.
If your chimney has been the victim of serious tarring, the last thing you want is a chimney fire. A chimney fire in a tarry chimney can get very exciting indeed, and will cause the fire brigade quite a hard time putting it out. Also, it won’t get all the tar out in most cases.
So that’s where the fuel comes from. Now let’s look at the air.
Fairly obvious. A chimney in use draws air up it, and a chimney on fire even more so. This means that the air arriving at a chimney fire is preheated and ready to go. This is why, if you have a chimney fire, reducing the flow of air up the chimney is a good idea.
Chimney fires start when a portion of the soot or tar in the chimney is heated to the point where it ignites. Sometimes this is because the fire is burning very hot, the chimney is very dirty and a big hot spark/ember is sucked up into the chimney. Should it rest at a suitable point, you will soon get a chimney fire. One cause in big old chimneys is where soot has been allowed to build up on the register plate for ages. An ember dropping onto that will ignite it, causing what we call a ‘plate fire’ . I have visited establishments where you can see the layers where they have had one plate fire after another, with each fire leaving a distinct layer, separate from the others. However, since they didn’t ignite the chimney proper, the householder was blissfully unaware of the danger.
If your wood burner has got tar building up in the flue, then that can ignite, causing a tube fire. One cause is where someone lights the fire, then leaves the door open to get it going, and is then distracted. Someone did that once while I was in the house, and the first clue I had that anything was wrong was a distinct roaring sound that grew steadily louder.
On entering the room, the tube that went into the masonry chimney was glowing white hot, so that it looked like a six inch diameter fluorescent tube. NOT a good thing at all.
Burning unsuitable items
Then there’s the fire bomb. This is where the householder does something silly. This particular gentleman threw his fish and chip wrappers on the fire, and the draw was so intense that the blazing wrappers went thru the chimney like Drake’s fire ships through the Spanish armada with similar results. Much screaming and a spectacular fire.
Crisp bags can have a similar effect. Just please think before you throw things on the fire. ESPECIALLY Christmas decorations. 12th night is a traditional date for chimney fires. The holly wreath is taken down and put on the fire. This is done by people who forget that evergreen plants like holly and Christmas trees are full of waxes etc, and after two weeks in the warm living room, they are bone dry and will burn like petrol.
“It’s only a chimney fire, so what?”
So a lot, if I am honest. Chimney fires burn very, very hot. Your living room fire burns at 300 to 500 degrees C. It is lined with fire bricks. These are specially made to resist heating and cooling, and even so, they start to crack and break, and need to be replaced after a while.
Chimney fires can hit 1,000 C with ease, and your chimney, if original will be built from brick. HOUSE bricks, which are built for structural strength and not heat resistance, so they crack and break very quickly, especially if the firemen start squirting water down there. Then, they shatter. If the damage is severe enough, the integrity of the chimney is compromised, and in houses where there are more than one flue, the fumes and smoke from one flue can leak through into the other with possibly fatal results in extreme cases.
It has been known for a chimney fire to split the stack.
Why do chimney fires burn so hot?
Bear in mind that a chimney fire is like an upside down rocket. It is a tube of preheated fuel, being fed preheated air, and as such it burns at one heck of a rate. Were we able to make them burn the other way up, and we could interest the space agency, a chimney on the moon could be a very real possibility.
Chimney fires invalidate the liner warranty
So, what if the chimney fire is in a chimney that is already relined. Here’s a clue. If you have a chimney fire in a metal liner, your guarantee is null and void. The reason for that is this. Such liners are made of stainless steel, and alloy of iron mixed with vanadium, chrome, or both. Heat can upset this alloy.
Imagine that you have a large cardboard box, and you cover the bottom with a layer of oranges so they are all in straight lines, with a gap between each 4 oranges. This is called Face Centre Cube (FCC) packing. Then place a golf ball on each of the gaps, and another layer of oranges on that and repeat. That is crudely what the alloy looks like.
When you heat something, the atoms vibrate. The hotter it gets the more they vibrate. To emulate that imagine that you shook the box violently. As with the muesli jar, when you shake it all the nuts go to the top, and it is the same with the box, as the oranges congregate at the top.
In the alloy, what happens is that the vanadium and chromium migrate away from the heat leaving the metal there more like pure steel/iron. Under the effects of the soot and fumes containing acid chemicals that area will rot away rapidly leaving a hole in the liner, necessitating the liner’s removal and replacement.
Chimney fires and the damage they cause
Then there are other consequences. Blazing tar balls falling down the chimney can set the carpet on fire, and if you aren’t there to stop it, the house.
The chimney pot is terracotta usually and the intense heat and cooling can crack it badly. That’s expensive to get someone up there to replace it, but it’s worse if the crack is where you can’t see it. Along comes the sweep, and when his brush goes through the pot, it splits, and the pieces crash onto and through your roof, followed by your car, conservatory, or God forbid, someone’s head.
Anything on top of the pot is also history. Cowls won’t last long sat on the top of what amounts to a 30 foot high blowlamp, and I have seen an expensive extractor fan turned into an exploded chrysanthemum by the chimney fire that cored it out in record time. Yet another reason not to stick them on pots. Trust me when I say that is NOT a good idea to put extractor fans on chimneys.
Old properties have their own unique problems in the event of chimney fires. Thatched roofs make me sweat. I have read accounts of such roofs igniting due to a chimney fire, and the owners were lucky to get out with their lives it went up so quickly.
In addition, in old properties like that, the support beams are embedded in the chimney, with their ends protected by a bit of liner. When that falls out, the ends on the beams start to char, and then are a terrible fire risk. Wood will, if heated in the presence of air, ignite at 232 degrees C. (You remember that by Ray Bradbury’s book, ‘Farenheit 451’ The ignition temperature of books (and wood) 451, subtract 32 and divide by 1.8 gives you 232C).
Now this is where it gets interesting. If you set fire to wood and then put it out after a while, the charred end has become pyrolised. Pyrolised wood has an ignition temperature half that of normal wood. So, a charred beam will ignite spontaneously at 116C. If that doesn’t impress, remember that water boils at 100C, and pyrolised wood ignites at just 16 degrees higher.
Have your chimney swept after a chimney fire
Don’t, whatever you do, assume that a chimney that has had a chimney fire is clean. It’s true that a fire CAN burn a chimney clean, but that’s quite rare. I have swept a chimney after it had a fire and taken out a HUGE quantity of soot.
So what should you do if you DO have a chimney fire.
- call the fire brigade IMMEDIATELY.
- Do not throw water or salt on the fire.
- A bucket of dry sand is the best thing, but it you don’t have that to hand, a shovel of soil from the garden will work quite well.
- Once it is smothered, carefully pouring small amounts of water on will send steam up the chimney and that will damp the fire.
- Get a heavy blanket, wet it with water (not so much that it is running, you don’t want to make a mess) and hold that over the fireplace opening to reduce airflow up the flue. BE CAREFUL NOT TO BURN YOURSELF.
- have your chimney swept after the fire to ensure all tar/creosote is removed
Basically, since you can’t affect the flue, you want to minimise the amount of heat and air going up the chimney.