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We had our chimney capped a couple of months ago (the next-door semi were having theirs done, so we said "yes" to their contractor doing ours at the same time). The contractor has removed the chimney pots and all that is visible from the ground is sloped mortar on top of the stack. It's possible there is no ventilation at the top - there's certainly nothing protuding very high above the top of the stack and no air bricks are visible.

The fireplace is still open (just covered with a sheet of plywood with a hole in it) in the room below.

A couple of days ago a very strong dusty/sooty/musty smell started in the room with the fireplace. There is also a noticeable draught coming from the fireplace. Neither of these were present at any time before the chimney was capped. This has coincided with a period of strong winds (though I can't see any damage to the chimney stack).

Is it possible the smell is due to condensation from the now inadequately ventilated chimney, and the draught is just due to air pressure changes in the room (annoyingly it's worse with the windows open) sucking air in and out of the closed chimney? Or is it more likely the stack has been damaged nd leaking?

And am I right in thinking the correct action is to get the chimney properly ventilated at the top? Is it going to need cleaning as well?

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You are likely correct with your analysis.

All the properly capped chimneys I have seen have an air vent top and bottom to allow slower air movement compared to the original chimney.

When people have "hidden" the fireplace but not capped the chimney, they sometimes just put a mesh over the top end of the chimney to prevent birds getting in and an air grate as part of the fireplace cover.

Allowing the air to move is a crucial part of the chimney "removal" process. Either improve the ventilation or consider removing the chimney totally or at least taking the lower part away and leaving the part in the attic - make sure it has sufficient support.

  • Since the op said no bricks visible you could not just remove the bottom part without removing the upper, I have capped quite a few non serviceable fireplaces as long as the flu is not being used for anything else a solid cap is the best way to go. My family had a chimney sweep company and if the stack was not lined or unsafe we usually installed a stainless cap that was sealed. Mortar caps leak over time and stainless last decades. We suggested that the owners wait until the roof was replaced to remove the brick work so the roof could be repaired at the same time costing less in the long run – Ed Beal Aug 12 at 15:23
  • @EdBeal no, the op states “sloped mortar on top of the stack” so the stack is still visible and that is probably brick. It is difficult to follow your comment as it lacks some punctuation in places. – Solar Mike Aug 12 at 15:33
  • Sloped or not mortar caps tend to leak. A properly sealed stack needs no ventilation and if the stack height is reduced and a vent is there there will be down drafts. Not a good idea. – Ed Beal Aug 12 at 15:45

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