Venting Old chimney Flues

26 Oct 2014
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United Kingdom
I have a 1900 3 story building.
The chimney stack, serving 3 flues, was lowered 23 years ago as it was disused and leaning. This was done on the advice of a surveyor.
See photo attached. The chimney breast concerned is behind the single story extension and shows the chimney stack still in position. The whole stack was removed in September 2013.

The 2 of the 3 fireplaces served were blocked up over 20 years ago. The remaining one has been left open.

No air bricks were put in the flues at this point or subsequently.

The remainder of the chimney stack was taken down below roof level in September 2013. No one thought of putting air-bricks in!
The flues now open into the loft space and on the inside the roof.

I have had an increasing problem with damp in the top bedroom at ceiling level over these 3 chimney flues dating back 20 years. Up to September 2013 the flues were vented out through the chimney stacks though 2 of the 3 flues had no ventilation coming from the old fireplaces as these had been blocked up.

Many roofers and qualified people have looked at the damp over the years, and no one has suggested that it might be the chimney flues causing the problem. The problem was thought to be a leak in the roof, possibly the lead flashing around the remainder of the chimney breast.
I have had extensive work done on the roof in September 2013 and there is no damp coming in from the roof now.

It is possible that damp had been coming in through the roof, perhaps caused by faulty lead flashing around the chimney or defective pointing and that I am now left with legacy of poorly vented chimney flues. Nothing definitive was found at the time to explain the damp.

I have bought a damp meter and have recorded very high levels on the both sides of the interior chimney breasts (Top of the flues and the room side in the bedroom). I conclude that the damp comes now from condensation at the top of the chimney flues.
I plan to put vents internally in all the disused fire places. I understand that the flues will need air-bricks inserted on the exterior wall.
How many of these should there be,
what size and
where should they be located in the flues,
do I need more than 1 in each flue?
Any other suggestions - I am getting desperate.


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Why not post pics of the situation as it is now?

Are you saying that both stacks have been dropped below the roof line, and that the roof has been made over the dropped stacks?

Each fire opening has its own flue, this flue is not shared with any other flue or fire opening. Each fire opening is required to be open, or, if bricked up, to be opened up, and the flue swept. After sweeping, the opening can be bricked up again, and a vent inserted.

Each fire opening requires to be open, or vented, at the bottom of the flue, and left open, or vented, at the top of the flue. How many fire openings do you have in each chimney breast?

Do not insert intermediate vents in a flue between the top and bottom vents - its dangerous.

The external chimney breast, on the left, might have poor pointing - has it been closely inspected? Be cautious of standing on the extension roof if you choose to work from outside. Most sweeps would refuse to work from a pitched roof.

Go in the loft and inspect the brickwork of the chimney breasts for signs of water penetration.
Thanks for those comments.

The pointing was made good in Sept 2013.
I realise now that I have 2 single story extensions showing in the picture. The chimney stack concerned is the one on the left of the picture and above the extension that has a white door and pitched roof. All 3 flues to this chimney stack now terminate in the loft. The middle flue is "clean" and has an open fire place. The flue supplying the old fireplace to the bedroom concerned can be viewed from the loft in its entirety and is essentially clean. The third flue is inaccessible but does have an internal vent that can I hope be reopened.

I have now added a photo of the "exposed flues" taken during the renovation process and another one showing the roof after the stack removal.
The three flues exposed in the pic are all three sooty and need sweeping, sweeping is essential for redundant or active flues - i dont understand what you mean by "clean"?

Those flues will only require venting or leaving open at their fire openings ie. at the bottom not the top.

Unswept flues can cause sulpherisation which can appear on plaster or decorated surfaces as damp, or a crystallised kind of damp.
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Can you please post the pictures here ? It would be easy for us to answer your question. Well, it could be dangerous, if you do it yourself. Hire professionals, they know it better. Also,you could check here about the installation process.
Just to clarify; you say the flues terminate in the loft. You also say the middle flue has an open fireplace. I assume this isn't in use?

Your dampness is a combination of condensation and nitrate/chloride contamination. You could have a small piece of plaster tested to confirm but if the plaster is old and/or gypsum type undercoat plaster then there's no point because it needs to come off anyway. The wall should be re-plastered with a suitable salt resistant type or dry lined with a suitable system.

The flues do need to be vented. Standard 215mm open vents at the bottom of each flues should be sufficient. You could fit these externally but I wouldn't advise it with the flues vented internally at the top.

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