It may be ‘multi-fuel’… but mixing wood, coal and slumbering can damage your chimney liner

If there was one term I would ban in the world of stoves and fireplaces, it would be ‘multi-fuel’. The term implies that an appliance can take multiple fuels at the same time. And while strictly speaking it can, it’s actually not such a great idea.

Firstly, coal (both smokeless and ordinary household coal) have very different properties, so the way they burn is different. Wood requires air from above the fuel to combust, whereas coals need air from below. That’s why ‘multi fuel’ stoves have grates (which allows the ash to fall down into a pan) and woodburners just have a solid base.

Putting the two together on a grate means that there are two sets of fuels competing for the same amount of air.

More importantly though is the very real problem of sulphurs from the coals mixing with the moisture from the wood to create a highly acidic deposit which can and does corrode stainless steel liners.

It’s worth bearing in mind that even well seasoned wood gives off some moisture (about 20% of its mass) and that, when mixed with the sulphurs from coal, can create the liner-eating deposit.

The issue is made worse from ‘slumbering’ – turning the air controls right down on your appliance in order to prolong the burn. This causes both fuels to burn at too low a temperature (remember – the greater the air supply, the greater the heat) and that produces excess moisture to be given off. This is the case even if you just burn one fuel, as it can cause tars and creosotes in your flue which in turn can provoke a chimney fire.

Tar in a chimney liner

Tar in a stainless steel liner caused by burning wood at too low a temperature

My advice is that, if you have a ‘multi fuel’ stove attached to a stainless steel liner, then just stick to one fuel. If you are in in a smokeless area, with a non-DEFRA exempt stove, then by law you should only burn smokeless fuel. If you are not in a smokeless area or have a DEFRA-exempt stove, you have the choice of either.

Whichever you choose, burn them hot (investing in a stove thermometer can help) and don’t forget to get your flue swept regularly as this will help prolong the life for your liner.

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, Southport, Ormskirk, Chorley and surrounding areas. Book Mr Soot online or call 0800 0541154.



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