5 Reasons Why Soot, Dust & Mortar May Continue To Fall Down Even AFTER A Chimney Is Swept

You’d think having a chimney swept would stop any dirt and debris from falling down into the fireplace opening.

After all, isn’t that the purpose of having your chimney swept in the first place – to banish it from the flue system in its entirety?

Well, not quite. Here’s 5 reasons why…

#1 – Some Chimneys Will ALWAYS Harbour Dust & Debris

By their very nature, chimneys are in a state of constant deterioration.

Some were built over 100 years ago and in use for many decades, bellowing out thick, black coal smoke. The soot, caused by that smoke, sits in the chimney and attacks the brickwork and mortar, causing it to disintegrate. The ageing process adds to this.

All this means most of the mortar inside a chimney will have reverted back to a sand-like substance. So even if you swept the chimney every day for the next 20 years, it would still happen. It’s a perfectly natural sign of ageing.

#2 – How The Chimney Is Constructed Can Have An Impact

Some flues, by the way they are designed, can make them more prone to debris retention, and therefore more dramatic sootfalls.

For instance, if your house has one chimney stack right at the top of the roof, chances are it will some have long arches internally with narrow 45 degree bends (see photo above showing these arches in a loft space).

Over the years, dust and debris gathers on these sharp bends – in extreme cases blocking the chimney.

Some of these flues will have soot hatches in the loftspace, to enable these ledges can be cleared.

But many have been plastered over, or blocked off, so complete clearance of these parts of the flue system may be impossible without breaking into the flue.

Understandably, very few people want holes in their walls. But if you can’t access these difficult-to-reach places, it can cause dust to build up and sometimes tumble.

On the flip side, straight flues, those with very few bends, or shorter flues may have more debris falling, as there are fewer opportiniuties for it to be caught on a ledge.

#3 – The Choice of Sweeping Method Plays A Part

Most professional sweeps have many different ways to clean a chimney, depending on how the flue is being used in the future.

On an older, working chimney, there is a fine balance between cleaning it and exacerbating its demise. The more mortar you get out, the greater the chance of it leaking and becoming unusable.

Even on chimneys which aren’t in use, more aggressive sweeping methods may remove more mortar, but it can loosen brickwork inside the flues and, in certain cases, cause them to fail entirely.

Professional sweeps will assess each and every chimney individually, taking into account many variables, in order to ensure it remains fit for purpose, whatever that purpose may be.

#4 – The Sweeping Process Itself Will Loosen Mortar

Every type of appropriate sweeping technique will disturb whatever is in a chimney to some degree. The individual method, as described above, will determine to what degree and also what happens next.

On a standard sweep of a masonry chimney, a brush will disturb bits of mortar and, if weak enough, will enable them to be swept off and collected in a controlled way.

But it will also disturb previously stronger pieces until eventually they fall down.

That may be the following day, it may be in 10 years’ time. But the ageing and deterioration process will continue.

#5 – Warm Weather Conditions Can Cause Sootfall

Debris fall is usually worse in warmer weather, especially in spring and summer, but it can happen at any time of year.

The reason is to do with basic chimney physics. Chimneys work by drawing warm air up the chimney into the cooler air outside.

But in hotter weather, it’s usually cooler inside a proper than outside. So the draught of the chimney reverses.

Even windy days can cause debris to fall as well. And we’ve not even mentioned other facts like adjacent building which, even in reasonable weather, can cause chimney issues…

So What Is The Solution To Stop Dust Falling Down?

Generally there are two ways to halt or minimise it. If the chimney is in use, then the best way forward would be to get it lined. If you can’t do this, then some fireplace users use a temporary solution such as placing an old pillow or scrunched up newspaper in the throat of the fire.

If the chimney is not in use, then the easiest thing to do would be to block it off alongside ensuring there is suitable ventilation.

But the most important thing to remember is that dust and debris falling after a chimney is swept is perfectly normal.

The point of sweeping is, primarily, to ensure a safe passage for flue gases to terminate from inside a property to outside. It’s not to eradicate every last bit of dust, mortar etc.

Not only would that be impossible, it could also, taking into account all the points mentioned above, cause serious damage to a flue.

And that’s not something anyone wants, – whether the flue is being used again or not…

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