Tar in chimneys: How it’s formed and why it is a fire/carbon monoxide risk

Prevention is better than cure, and when it comes to tar in your chimney it definitely pays to get clued up. Despite what some may think, tar is not ‘just part and parcel’ of having a real fireplace. It’s usually causes by either operator error, or poor installation – sometimes both.

Some of the reasons why tar and creosote forms in a chimney include:

  • Burning wet fuel
  • ‘Slumbering’ your stove (i.e. – turning the air controls right down)
  • Having a stove which is too large for your room
  • Position of the flue in the house (gable end walls or exposed locations)

The main thing to remember is that tar is all to do with temperature – either fuel burnt at too low a temperature or the temperature of the flue being too low.

Burning fuel at the correct temperature will, in 99% of case, ensure that no significant tar deposits are formed in the flue.

The way it works is simple. A piece of fuel is burnt at a low temperature, and because of that, the energy released from the fuel doesn’t come out as heat, but as smoke. Smoke then travels up the chimney and then condenses on  cold surface (i.e. the flue) – a bit like when you’re in the shower and steam condenses on the window glass.

So as you can see, the lower the flue temperature, the greater the risk of tar or creosote forming. Once it’s formed, it needs to come out as it could present a major fire/carbon monoxide risk. Tar in a flue can catch fire quickly as it is highly flammable. It can also smoulder and cause Carbon Monoxide poisoning (aka the ‘silent killer).


Bag of tar from a chimney.jpg

In serious cases, a chemical treatment needs to be applied to get rid of the tar. Even if you are no longer using the chimney, it is strongly advised that you call in a professional to get rid of the tar as, quite apart from it smelling strongly, it can also seep through brick and plasterwork, causing black staining on inside and outside walls.

Preventing such a scenario isn’t difficult. Simple burn your fuel at the correct temperature, by using seasoned wood or dry coal, and never ‘slumber’ your appliance.

I advise customers to invest in a stove thermometer, which will help guide you on whether you are burning at the correct temperature. If you source your own wood or season wet logs, a moisture meter is an invaluable bit of kit. For more hints and tips on usage, visit www.burnright.co.uk which I helped set up with my trade association, the Guild of Master Chimney Sweeps.

And finally, remember to get your chimney swept regularly. Your local sweep will be able to tell you whether you’ve got  problem with tar, and will be able to offer bespoke advice.

Mr Soot Chimney Sweep is a HETAS Approved Sweep, and member of the Guild of Master Chimney Sweeps, offering a professional and friendly chimney sweep and stove service in Wigan, Stockport, West Lancashire, Chorley and surrounding areas. Book Mr Soot online or call 0800 0541154

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