Damp wall due to combi boiler exhaust

2 Jul 2007
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United Kingdom
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.

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You can fit directional exhausts on many flues but I'm not sure it would solve your problem. To get significant condensation forming on your wall the exhaust gases would have to be getting blown back onto the wall fairly consistently and no matter what direction you send them that would still happen. Is the prevailing wind blowing the gases against the wall? Or in other words (for most people) is the wall facing west?

This isn't a common problem so you may be seeing something else. Sounds like a painted wood wall? Could it be that the flue gases have stripped the paint and that the damp is simply a consequence of the exposed wood?
It maybe possible to change the direction of your flue outlet, or have a plume kit fitted. It's really ascertaining the cause. Wind. Is it in a wind protected corner, with wind blowing above, causing the pluming to get drawn in a certain direction?

Thanks for the interest. I'm wondering where I might get to look at some sort of the plume kit you mention, to get an idea of whether it is worth a try? It is a north facing brick wall, but what makes me think it is the condensation from the boiler is that the damp on the wall seems to exactly follow the route that the exhaust steam generally takes on days where there is not much of a breeze, i.e., its default. Is it because the wall being north facing does not get enough sun to dry up the condensation quickly enough? That's why I was thinking some sort of waterproofing coat might also be worth a try, to slow down the damp creeping through the mortar long enough to let the condensation dry off as it should do.

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Also I should have mentioned that it is an original wood window frame. The sealant around the window looks fine, I replaced it myself a few years ago, but perhaps the condensation has been soaking through the window frame itself somehow?
Did you solve this problem. I have exactly the same problem here - the front wall of my house above the vent is soaking wet!

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