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Sealed chimney - what are the negative effects?

Postby blah » Tue Jun 20, 2023 2:24 pm


A few years back we had a chimney repointed by what was in hindsight a bit of a dodgy roofing firm. As part of the chimney repointing they also said they'd seal up the chimney with a lead "cap" and punch holes through it for ventilation. I'm (now) aware that they should have added a cowl instead, but I believe the chimney itself was in pretty bad nick and didn't have a chimney pot to work with.

Long story short but I had a separate company look at it recently and they say that it has been sealed up with concrete instead of having a lead cap added and it seems there's no ventilation from the top. The fireplace in the living room is still open so is ventilated from that direction i.e. it's not completely sealed. We don't have any noxious gases from heating system as it is all electric as is the cooker.

I'm wondering what are the downsides of having the chimney sealed on top like that and does it need correcting? I asked the current roofer but he says there's "theories" on the downsides and isn't quite spelling out all the downsides. I'm aware that damp could be encouraged by this kind of setup, is there anything else?

Hope you can help - thanks!
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Postby stoneyboy » Thu Jun 22, 2023 9:27 pm

Hi blah,
It is difficult to give you a definitive answer.
If you do not have problems with your sealed chimney stack leave well alone. If however you are getting brown damp stains internally towards the top of the chimney you could instal a cowl and seal up the downstairs fireplace leaving a small hole for ventilation.
Regards S
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