Extractor duct condensation

[Selfmade]

An extractor fan I fitted has been shorting out due to water ingress. I insulated the duct and sealed the fan to the duct, as well as the cable entry, with putty. But the problem has not gone away. I have now removed the fan as I am concerned about the danger of water on the electrics, and my customer was obviously not happy about the lights tripping out all the time.

Now I have asked the customer to get the (possibly asbestos) duct removed so I can fit a moisture trap and new duct. I had planned to fit flexible duct as I am more confident about sealing this effectively to the fan, having used it before. But now I find you can't buy moisture traps for flexible duct.

So that leaves me with several questions: if I was to fit a short length of rigid duct between the fan and trap, and then another short length above the trap, taped to insulated, flexible duct, would this work? Or would the water drip from the folds of the flexi-duct right through the trap, missing the drain? How would I support the rigid duct? And how would I drain the trap, given that there is no access to soffits (dormers and coombed ceilings), and thick stone walls several floors up which cannot feasibly be drilled. The existing duct rises nearly vertically about 1.5m from the ceiling mounted fan to the roof vent.

Would it be a better idea to fit an inline fan close to the roof vent? How close can it be without risk of freezing due to outside temperature? Any other problems with this approach?

Any help would be so much appreciated. I've found a lot of threads, including some on here, about similar problems, but most relate to very long runs and flexible ducts with low points etc, none of which apply here. I'm not really sure why the condensation problem is so severe (fan swimming after only a day or two), with a short, now insulated duct.

Sorry for such a long post, thanks for reading, even if you can't help.

 

[Geordie Spark]

condensation trap - Google Search

Use standard overflow pipe for the condensate discharge and direct it into a convenient gutter or drain.

 

[Selfmade]

condensation trap - Google Search

Use standard overflow pipe for the condensate discharge and direct it into a convenient gutter or drain.

Thanks for that. I'll maybe have to go back and see what I can find in the way of gutters and drains. I'm terrified that I'll connect it into something wrong and they will end up with water from the central heating pouring out of the extractor fan or something... Tempted to refer them to a plumber.

 

[Geordie Spark]

Thanks for that. I'll maybe have to go back and see what I can find in the way of gutters and drains. I'm terrified that I'll connect it into something wrong and they will end up with water from the central heating pouring out of the extractor fan or something... Tempted to refer them to a plumber.

if you can, poke the end of the overflow pipe out under the eaves so it discharges into the gutter of the house - problem sorted!!

Otherwise, run 32mm plastic waste pipe from a convenient drain up the wall and discharge your overflow pipe into the end of it using an elbow on the waste pipe. do NOT seal the gap between the open end of the 32mm waste and the overflow elbow!!

 

[Marvo]

If there's as much condensation as you say there is then there's several things I'd recommend you do.

There are in-line condensate traps that will accept a flexible duct either side of them. The trap needs to be installed in the lowest point where the condensation naturally collects. If there's nowhere to run a drain pipe to them you might need a small plastic collection tank and a condensate pump which will pump it uphill when the level gets high enough. It's not a cheap option though and it will probably cost more for the pump system than you paid for the fan.

Relocate the fan far away from the lowest point of the system. The last thing you want is condensation to collect or sit in the fan. Install it at a convincing angle so the water runs out fast.

Insulate, insulate and insulate again. You only get condensation when the temperature of the air drops as it goes the the system. Insulate everything including all ducting, the fan itself and the suction and discharge vents. Make sure the system is watertight, if the insulation gets sodden it's useless.

Overrun. You want the fan to overrun after the bathroom has been vacated so the system drys out. I wouldn't be scared to set an overrun as long as 15 minutes on a system with condensation issues.

Make-up air. When the fan sucks air out of a bathroom new air migrates into the room to replace the air that's leaving. You want this make-up air to be as warm as possible. If the make-up air is coming predominately through an open window for example then it will be very cold and won't hold much moisture. If you put a louvre into the bathroom door so the make-up air is coming from the house then it should be warmer air which will help with the condensation.

 

[a1colly]

Rigid duct will always be better than flex, as already stated insulate and then add some more, personally I don't use the stuff off the shelf it's better to wrap yourself with a space blanket insulation if you hot major concerns. If it is going through a roof tile vent check the free air is sufficient for the fan as this may also cause a build up of condensation. Also check that it isn't the vent that is letting moisture in. Is the duct run straight or do you have turns in it think about your resistance curve.

 

[Geordie Spark]

 

[Selfmade]

Thanks so much all of you for the very helpful replies. I am going to do something like Geordie Spark's picture above, but with a flush-fit fan (as I already have one) and probably mostly rigid duct with just a short length of flexi to join this to the roof vent. I have still to work out the drainage, hoping to return tomorrow to do this. No access from loft to eves or soffits unfortunately, as it's effectively an attick flat with large flat-roofed dormer windows. So I'm hoping to connect it into a waste pipe, or an overflow from a water tank in the loft.

Looking on Google Street View, I think I may have discovered one of the causes of the problem: the existing duct appears to stick up a long way from the roof - maybe about 60cm. That of course is uninsulated and exposed to cold outside air. The customer's having a new vent fitted so that will fix that. I actually wonder whether the existing 'duct' was originally installed as a boiler flu, it seems quite inappropriate for fan!

 

[HandySparks]

The customer's having a new vent fitted so that will fix that. I actually wonder whether the existing 'duct' was originally installed as a boiler flu, it seems quite inappropriate for fan!

If the customer's getting a roofer in, it might be worth seeing if there's any scope for moving the roof vent to give the shortest possible run of ducting.

 

[Selfmade]

There are in-line condensate traps that will accept a flexible duct either side of them. The trap needs to be installed in the lowest point where the condensation naturally collects.

Been searching for one of these and had no luck, so I've bought a regular one and some rigid duct.

If there's nowhere to run a drain pipe to them you might need a small plastic collection tank and a condensate pump which will pump it uphill when the level gets high enough. It's not a cheap option though and it will probably cost more for the pump system than you paid for the fan.

Thanks for the suggestion, I hope it won't be necessary, but thanks anyway.

Relocate the fan far away from the lowest point of the system. The last thing you want is condensation to collect or sit in the fan. Install it at a convincing angle so the water runs out fast.

Presumably with a condensation trap, the fan should be below this. So with the fan fitted into the ceiling, and the trap just above the joists (to allow for a slope on the drain) this should be alright? I read elsewhere that there is little condensation close to the extract point (where the air leaves the room) as it's still warm at this point.

Insulate, insulate and insulate again. You only get condensation when the temperature of the air drops as it goes the the system. Insulate everything including all ducting, the fan itself and the suction and discharge vents. Make sure the system is watertight, if the insulation gets sodden it's useless.

Will certainly be doing this. Presumably the fan won't overheat from being insulated?

Overrun. You want the fan to overrun after the bathroom has been vacated so the system drys out. I wouldn't be scared to set an overrun as long as 15 minutes on a system with condensation issues.

Have suggested this to the customer. Would have to be extra though as there is currently only a switch live so wiring would need altering.

Make-up air. When the fan sucks air out of a bathroom new air migrates into the room to replace the air that's leaving. You want this make-up air to be as warm as possible. If the make-up air is coming predominately through an open window for example then it will be very cold and won't hold much moisture. If you put a louvre into the bathroom door so the make-up air is coming from the house then it should be warmer air which will help with the condensation.

The air is certainly not coming in through a window, as there is none - the bathroom is internal. But I will check the ventilation in/under the door when I next visit. (I probably did this when I first fitted the replacement fan, but this was a few months ago now so I forget.)

 

[Selfmade]

If the customer's getting a roofer in, it might be worth seeing if there's any scope for moving the roof vent to give the shortest possible run of ducting.

Yep thought of that, thanks. But the distance is not that great and probably couldn't be reduced that much. I thought of it to lower the angle so the water would run down more slowly and have more time to re-evaporate. But I think the cost is probably too high and the other measures will (I hope) do the job. Thanks anyway for the suggestion.

 

[Selfmade]

Rigid duct will always be better than flex, as already stated insulate and then add some more,

Looks like I will be I will be using rigid, and of course insulating a LOT.

personally I don't use the stuff off the shelf it's better to wrap yourself with a space blanket insulation if you hot major concerns.

I would dearly love to wrap myself in a space blanket and forget about this one...

If it is going through a roof tile vent check the free air is sufficient for the fan as this may also cause a build up of condensation. Also check that it isn't the vent that is letting moisture in.

New vent should hopefully fix all that.

Is the duct run straight or do you have turns in it think about your resistance curve.

No turns at present, little or none with new duct.