Chimney stack issue?

21 Mar 2021
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United Kingdom

In the last year we moved into a 1960s semi detached. The house has a chimney stack which is no longer in use but, from what I can see, has no vents.

Over winter, we had some issues with condensation, one of which was on the chimney stack itself (see pictures, darker discolouring due to a picture frame where the ink came through due to moisture).Weve looked in the attic and can see no signs of leaking around the chimney, and we've had a roofer repair some of the flashing around the chimney stack itself as a precautionary measure.

My thinking is this is a condensation issue caused by no ventilation in the chimney. As such, I was looking to install two internal vents (one in the bedroom and one in downstairs living room). To resolve this before redecorating. Could anyone tell me if they believe this is a correct course of action and, if so, tools needed to secure a vent? (if there's anything else going on here please feel free to tell me so we can look into it!)



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G_a94, good evening.

OK the "building science" is that an unused chimney flue should be vented.

Venting of a flue [chimney] are generally achieved by installing a vent in the old fireplace in rooms on first and ground floors, combined with the chimney pots having vented caps fitted, if both of these criteria are met then all is well [or should be?]

Suggest as you are suspecting, install two air vents in each room.

Suggest you remove a brick about 300 mm. above floor level [roughly] centrally in the old fire places and fit a "hit and miss" vent

What has probably happened is that in an un-vented flue [chimney] condensation will occur, the water then reacts with the soot, the soot contains Sulfates a direct result of burning [say] coal, logs. peat, Etc.

When water [condensation] interacts with soot, it produces a weak form of Sulfuric acid, this weak acid attacks mortar and produces dark, black marks, should smell of combusted materials / tar?

Hi Ken,

This is incredibly helpful, thank you so much. Just as an added point, beneath the plaster it is cinderblock rather than brick so we can't remove a single brick unfortunately. In this instance, is there a generally accepted workaround?

Thank you again!
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is there a generally accepted workaround?

Yes, find a vent that you are happy with.

Mark out the wall using the vent, BUT allow the vent to overlap these marks by about 15 / all round, using the inner marking as the line to which you cut out the block and plaster Etc.

As for cutting? if you have a drill + large masonry drill, then you are away, "stitch drill" around the inner line you have as above, keep the drill holes fairly close together, if you can drill right through the block, try to keep the holes as horizontal as possible.

once you have drilled all round, you can use the drill and go in at an angle from one hole across several other holes, then carefully use a hammer and cold chisel to remove the block.

The more holes you drill into the block in the central area of the block the easier it will be to remove the bit of block you are working on.

Hope this assists?


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