Blocked off Chimneys on Party Wall (Leave, Vent, or... ?)

16 Aug 2019
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United Kingdom
So, we have 3 chimney breasts that are blocked off and are on the party wall - they are all papered over and I presume plasterboard/wood or similar has been used to block the gap. These are not vented at the bottom, which I thought had to happen. (Please note that I'd prefer not to have a vent there!) They are open at the top.

Now the problem is we are going to be having these rooms re-plastered, so off goes the wallpaper and the chimney breast will be plastered over completely. Do I really need an interior vent? We have no visible problems with damp in any of the rooms, perhaps because the chimneys are all on the party wall. They would have been blocked off years and years ago, probably decades (hmm... perhaps the means of blocking the chimneys isn't air tight and enough air can pass through the wallpaper?).

To be honest, opening up the chimneys might unveil all manner of things, perhaps things that are better left alone, but if we need to add a vent then I'd need to have them opened up and swept.

Any advice would be appreciated. Perhaps you've encountered a similar issue? Thanks!
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Blocked off fireplaces should be fitted with 9 inch by 9 inch vents in the room, the chimney pot should be vented. The cold air in the stack condenses which can lead to damp, the room vent and pot vent allows warm air to be drawn up to stop condensation.
Thanks for your reply... I was kind of hoping we could get away with avoiding vents as they haven't been vented for decades!
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Thanks for posting - yes, I'd love to leave them, but the chimney breast does need plastering and I'm worried that there will be an issue with the plaster once on. Hard to be sure.

Has anyone else successfully gone with an entirely blocked up chimney (bottom that is)?
Been in thousands of properties where no vents have been installed with no issues.
There is no reason to think otherwise after getting it plastered.
Thanks for your reply. My worry is that maybe once plastered the chimney is air tight at the bottom, as at the moment it is boarded and papered so possibly not air tight... so, perhaps that could create more likelihood of mold? Not really sure if opening/sweeping would be a good idea, although it would allow me to stick a chimney sheep in for more insulation (assuming nothing is in there now).
Just had work done in the kitchen- opening up a fireplace that had been sealed closed for god knows how long. About the time the glowworm coal fired boiler was removed. Chimney uncapped at the top.
Zero damp or plastet damage. Original lime plaster.

I agree with the others - if it's been fine so far, you are ok
Thanks for your reply!

So, if I leave 'as is' and get them plastered over, if mold was present or became present would it only be an issue if it came through the plaster itself?

Probably a daft question, but I'm concerned that there could be (in the future) damp/mold fumes that we may be unaware of until it started to get through to the plaster. Again, hypothetical, but as we have young children I want it to be safe! Is mold only an issue if you can 'see' it on a wall, blocked chimney, etc?
If in the unlikely event a problem arises you can fit a flue vent retrospectively.

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