help! solving condensation in ongoing build

4 Dec 2010
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United Kingdom
sorry for the long explanation...
We are having a small side extension built, currently halfway through the build and it has come to an impasse. Our boiler (to be replaced and resited) currently vents into the new room. The boiler vapour is causing the room to be saturated with water, with condensation gathering on the vapour membrane on the ceiling and dripping on the new blockwork and floor area.

Our builder has two solutions - 1, get the gas fitter to come and fit a flue extension to get the vapour outside (cost of flue plus gas fitter time plus chance it may go wrong!). 2, turn off our boiler to allow the room to dry out (2 days drying time plus 2 days plastering plus 2 days for the new boiler to be fitted in new location).

We are tending towards turning the boiler off and using fan heaters, which is not ideal given the weather. However we're concerned that there's too many ways for the temporary flue solution to go wrong and cost us a bomb (even allowing for the cost of heating the house with fan heaters!).

Any other ideas would be much appreciated! This can't be the first time a build has encountered this problem....

On another note - our builder has still not tiled the roof (don't ask me why!), and insists that by fitting a waterproof membrane over the top with some overhang he has made it weather tight. But it seems to me that any rain water will drip straight off the membrane onto the blockwork and soak through - making our efforts to dry the room ready for plastering completely redundant. Am I right or should I trust my reputable builder of 30+ years experience?!

Also any clues on how best to dry out the room would be great - are we better to shut the door and warm it with an IP rated heater, or keep it more ventilated, or heat it with the door open? Or all those things in sequence?

thanks in advance for any advice you can give!
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Vents into the New room? ? ?

You have to be kidding us? ?

The vapour that is emanating is [ amongst other dodgy noxious gas ] Carbon Monoxide, IT COULD KILL YOU!

Any builder who is reputable would have refused point blank to proceed along with this idea of allowing the vent from the C/Heating system to stay inside a confined space. it is positively suicidal.

turn it OFF NOW, or leave the external door to the new extension open but do NOT OPEN the door the the rest of the house.

under no conditions at all whatsoever allow the boiler to keek firing all night, the fumes could [ with a correct wind direction ] be blown back into the house, this stuff will kill you!

On the matter of the damp proof membrane on the roof, at some point in the future this ,material will [or should be terminated in a gutter, as such the water will be disposed of via the rain water system, as will the run off from the roof tiles.

Drying out the room? ?

Depends on dimensions, materials used to construct the room, length of time the mortar has had to dry out, and importantly has it been [ the mortar that is ] been exposed to any of the severe frosts we have had recently?
Installing "Dry Heat" namely electrical, under no circumstances use Calor Gas blowers, and some fans then the dry out may not take too long.
Have a look at the cost of hiring such things as air movers, and Infra red panel rads, these are dry heat sources and will assist.

CAUTION ! Depending on when the mortar was laid, the conditions it has been subjected to and the recent frosts the entire gambit is fraught with potential problems. remember in Scotland even the citys went down to minus 10 + minus 14 degrees.

Lets see what other contributors have to say on this post?
Yes you are right kengmac cant belive that post,as you say shut boiler down, get flue soughted quick.
Just for avoidance of doubt - the new room isn't in use; it isn't finished. The boiler is being replaced and put in a new position (venting outside of course!). Trouble is we can't get to that stage until the vapour problem is dealt with long enough for the blockwork etc to dry out- so the plastering can be done and the new boiler installed.

In the meantime, there is no door to the extension so it's not fully enclosed and the carbon monoxide doesn't accumulate in there (?- not sure this is adequate to reduce the risk?). The builders are working on the outside rather than inside the extension (the boiler can be off while the plastering etc takes place anyhow). We have a mox detector by the door leading to the unfinished extension to warn us in case of any fumes drawn back into the house somehow. Having said all that, we do appreciate the safety concern- sooner the room can dry out so new boiler can go in the better.

I'm just hoping there's a third way instead of having the boiler off to allow it to dry or doing the temporary flue extension but it may be these are the only options. Can't be the first time someone had to coordinate new boiler and new extension...
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Sorry, didn't explain properly - the new boiler is going in the new (under construction) room - which is currently wet and unplastered. Can't install either the boiler (or electrics for the spur obviously!) until it's dry and then plastered. can't get it dry and plastered and boiler fitted, without either: turning off the central heating and relying on fan heaters for most of a week; or extending the existing flue outside which I think could easily turn out a bigger job than expected. So I'm wondering if there is a creative (and safe) 3rd way that hasn't occurred to us.

KenGMac- on the subject of the roof membrane, what I meant was just that I'm not sure whether it makes sense to try and dry out the room before we have the tiles and guttering - as I think rain could run off the membrane onto the exterior wall and soak through. I realise water will divert to the drain when gutters are in place, but they aren't fitted yet. As I said, the builder assures us it's fine and the room is sufficiently weatherproof for us to go about drying it out, but doesn't make sense to me.
The reason you can't find anyone in a similar situation could be because it's a non issue! Just towel down any surface condensation, make structure weatherproof (plastic sheet over doorway?), and get the boiler moved.

Cables will clip to a damp wall as well as a dry one, and it wont cause any harm in the short term.

Only downside is that the boiler would have to be plastered round, not behind. You won't notice if the boiler is boxed in...
For one thing the blockwork does not need to be completely dry to be able to plaster.
In fact we wet ours down prior to plastering.

:idea: Can you not plaster just the area where the boiler is to be fixed then move said boiler? :idea:
Whilst I understand your frustration of not having any heating, temporarily reflueing the boiler isn't really a practical way to go.Do you have an immersion for hot water, if so just cut the boiler off and use some alternative method of heating.
Either way turn the boiler off get the structure weather tight and get the new boiler fitted, you can always run it off an extension lead for the time being.Meanwhile get your first fix electrics done and get on with your plastering.
Just because your builder has 30 years experience doesn't mean a thing, many builders have been doing it wrong for 30 years
thanks for all the comments, gave us food for thought. I'd still be grateful for any more comments on the safety aspect as per the first reply - given the room's not yet in use, should we still be worried?

anyhow the plan is now: Monday -turn the boiler off and wipe down, stick a IP rated tubular heater to help dry it. Tuesday - get the builder to put up the insulating plasterboard and skim the corner where the boiler needs to go (he says the plaster is needed for fireproofing). In the meantime have the gas fitter doing powerflush in preparation for the new boiler. Wednesday - gas fitter puts in new boiler running off extension lead (through the catflap into the socket in the adjacent kitchen!).
So by Wednesday pm hopefully we will be warm again and no more condensation going into the room - rest of the plastering can go ahead and electrics installed so the boiler can be connected to its spur....
Day 1

Plaster a small bit of the wall where the boiler will go, or just fix some plasterboard and scim it

Day 2

Fix the new boiler, either fix a permanent or temporary spur, run the pipes, connect to the main system

Problem solved

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