Chimney Lining – Acid Attack!

Part 5.  The internal consequences of acid attack.

This is a continuation of the article on chimney liners and their problems, stretching from the inglenook to the modern chimneys and the retrofit liners we have at our disposal today.

This part focusses on the internal damages from acid attack to the cement mortar.  Remember, if you think your chimney may be suffering from these problems, and you wish to discuss it with someone, your certified chimney sweep is the best person to advise you, as his entire business life revolves around chimneys, their problems and their cures.

Acid Attack on CementChimney Leaking Acid Attack

As explained earlier, the acid in the flue gases attacks the cement.  This causes the individual cement grains to puff up and expand, which disrupts the texture of the cement causing it to lose all strength and expand.  This causes the original mortar to expand and fall off, sometimes in quite large lumps exposing the mortar between the bricks which then crumbles and expands.  This can get so bad that breaches are made between two flues, allowing flue gases to flow through into unused flues where they can fall down into the living space.

Chimney Breast Staining and Damp Patches

Also, when the acids attack the cement they cause salts that penetrate deep into the structure of the stack.  Normally this is balanced by the air flowing up the chimney, but if this flue is blocked off at the bottom, then this will allow the water and salts to penetrate so far that they appear on the chimney breast in the bedroom, causing damp patches and staining.  This will not go away, and requires proper treatment by a builder to prevent it getting much worse.

Mid-feathers and Acid Attack

There’s more.  There is one piece of bad design in Victorian chimneys.  Where two flues join together, to save materials they only put a single layer of bricks between the flues.  Worse, the bricks were merely stacked there, and not keyed into the chimney proper.  These so called ‘feathers’  are attacked from both sides by the flue gases, so it comes as no surprise to learn that the mortar holding them in place is destroyed twice as fast.  This leaves a stack of bricks standing there unsupported, and when the chimney sweep cleans the chimney, the pressure of the brush is often enough to send these bricks cascading down the inside of the chimney.

Chimney Pot Reduction

At the very top of the chimney there is another problem.  The best chimney pots on Victorian chimneys are the square ones with internal dimensions of 9” x 9”, the same as the flue.  The huge problem is that so often such pots are replaced with smaller pots with an internal bore of 8” diameter.  These pots reduce the flow of gases out of the chimney which can cause enough back pressure to make the fireplace smoke badly.  Ignore people who tell you that the smaller pot makes the smoke go out faster.  That statement is true, but it ignores the fact that it also increases back pressure.

In addition, the sudden change from 9” sq to 8“ round causes turbulence which increases the soot deposited, and also the amount of acid condensate which attacks the cement that holds the pots in place.  This can mean that the passage of the sweep’s brush will actually lift and loosen the pot from the stack.

So, to recap

  1. Acid attack can damage the internal surfaces of the flues.
  2. It can even loosen and make chimney pots unsafe.
  3. Damp patches on the chimney breast mean that action needs to be taken.
  4. A certified chimney sweep will be able to advise you as to the steps that need to be taken.


Check out our articles in this Chimney Lining series:

  1. Chimney Lining – Inglenooks
  2. Chimney Lining – Georgian Chimneys
  3. Chimneys and their liners – the Victorian Era
  4. Chimney Lining – Victorian Chimneys
  5. Chimney Lining – Acid Attack
  6. Chimneys – Modern Design
  7. Chimney Liner – Lining a Chimney

For more advice, or to book a chimney sweeping appointment please call us on 01223 964305 or email us at

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VAT # 218 3459 04
Registered in the UK # 7725 203
Wight Initiatives Ltd,
23 King Street,
Cambridge, CB1 1AH
The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH
Registered in the UK # 7725 203
VAT # 218 3459 04