Hello all

Can anybody advise about the best course of action? I have a combi boiler that has its flue exhaust going out through the wall, but it appears that over time, the steam from the flue has had a damping effect on the external wall over a stretch of a good few metres above the exhaust. The paint has started to flake away, but most importantly, it looks like some of the moisture collects under an exterior windowsill about 3-4m above the exhaust, and this has eventually become a problem at this location, in that the damp has crept through to the interior wall, giving us a damp patch under the window on the inside. Although it is only a small damp patch at the moment, I am thinking it would be best to try and get a new flue exhaust, or better still just a new terminal guard, that might include a solid upper surface that would act as some sort of directional 'hood', to push the steam exhaust further away from the wall before it can rise upwards.

I have a feeling this would go a long way to sorting out the problem - I don't think it is rising damp in the normal sense, the wall beneath the boiler seems to be fine, and there seems to already be an additional damp proofing membrane in the wall above the exhaust at a height of about 2m, which doesn't seem to be preventing the problem, which would be consistent with the theory that it is moisture settling on the wall from the steam. Maybe this is just another sort of membrane though, I don't know enough about damp proof coursing to know exactly what it is.

Does anybody know where I could get a new terminal guard with some sort of directional hood that would help with this problem, and also, if there is any useful anti damp product that can be applied from outside to try and reduce the existing damage?

Thanks very much for any suggestions.

Regards

If water is bridging the cavity, there must be a problem with the window or weathering.

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Hi there

Yes, that's exactly what I thought to begin with - but I've been up there many times, and I just can't see any problem with the seal at all. The window did give me some trouble a few years ago: after a really bad rainstorm (I mean really bad, it was during the bad floods that got on the news) I did get a wet patch in a similiar place, and so shortly afterwards I replaced the old frame sealant carefully, and put practically a whole tube around the window and that seemed to sort the original problem out. But this symptom seems to be related to the position of the window to the boiler exhaust. The window is wooden, perhaps the moisture is just soaking through the wood somehow? Or maybe there is some old water trapped in there from the original problem and it is just a coincidence?

Cheers

Whilst I would agree, condensing technology is heading for a lot of trouble in the future, any amount of water on the outer skin, shouldn't be able to penetrate the cavity.

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ive seen galvansied metal sheet fixed above flues before for what i assume is a similar problem but i dont know enough about plumbing to know if its currently acceptable....

Im going to guess too that its a solid 9" wall, as per scruffs pointing out about a cavity, if its got a cavity, it must be seriously bad to manifest on the internal... which definately would indicate a problem with the window installation / bridging etc... Ive seen window cills fitted so far back so as to tip backwards, causing water to drain into the house rather than off the end of the cill....

I also assume there must be condensation on the glass? and lots of it? seeing as that would be as cold as the wall itself, and more likely to encourage water to condense on it?

the 'membrane' you mention, is it external? as in 'on the outside skin' as opposed to on the inside? it could well be that water is getting in above it, but under the window, where its trapped and forced back into the building... just guessing mind...



futher edit...
probably the window and its installation...
is it a cavity construction, timber frame or solid 9" wall?

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