As the hot weather arrives, so do the enquiries from customers worried that soot, dust and debris is starting to fall into their fireplace.
This is because it’s highly likely that the chimney is suffering from something called Reverse Stack Effect.
The reason is down to simple physics.
Chimneys work through differential pressure – warm air from inside a property travelling up the flue into cooler air outside.
For most of the year, this works fine.
But in summer, the heat and warmth is greater outside the house, rather than inside. So the equation reverses.
This change not only stops the chimney from naturally drawing, it can – depending a range of factors – start the process of pulling air down the chimney and into the fireplace.
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This can cause loose debris – which pretty much all masonry chimneys harbour – to be knocked off by the draught and fall down into the fireplace.
It tends to happen more on open fires – particularly gas fires.
These flues always seem to be more crumbly, and the owners aren’t used to dealing with the usual muck and dust of a traditional coal fire.
Short flues (usually found in bungalows) and straighter flues with virtually no or few bends can be particularly prone to it.
Flues which have also just been swept can also suffer from this due to the disturbance of debris within a flue system during the sweeping process.
It’s worse in spring, summer and during windy weather, but can happen at any time of year.
So what’s the best way of dealing with this?
One handy hint is to get your chimney swept regularly. The act of sweeping knocks off loose bits of debris from the flue and allows it to be removed in a controlled manner.
Of course, as all chimneys deteriorate with age. It won’t stop debris from falling entirely, but it will help alleviate it.
The main thing is not to panic. It’s usually nothing more than a bit of science at work in a part of your property which is usually the most neglected.
Contact your local, professional sweep for bespoke guidance on how to help solve your own particular issue.
Mr Soot is a qualified master chimney Sweeps based in the North West of England.