New extractor fan installed - condensation

justbigboned · 16 May 2020

Hello chaps,

Our old bathroom fan of 72m3/hr died a few days ago so I've replaced it with a Vent-Axia Silhouette 100T, which has an extraction rate marginally higher at 108m3/hr. The bathroom is in the middle of the house with no windows and no external walls.

Unfortunately, the builders used uninsulated flexible ducting which exists through a vent on the outside of the building (about a 5m run) that is installed slightly higher than the fan itself.

The ducting is completely unsupported and runs off at a tight right angle from the fan, before (I assume) turning a further right angle for a run of 4m to the external vent. Due to the way the pipe runs, I don't think the previous extractor fan ever worked correctly. I always thought it was poor, but since everything else in the house is too, I never looked too closely. Apparently, it was because the back draft shutters would have been unable to open. The new one was the same, so I've had to remove them for the moment. It does extract now, but, we have condensation dripping from the fan, which I guess I expected.

Without cutting a huge hole in the bathroom ceiling or removing a glued down chipboard floor in the bedroom above (which is on ijoists so would be very invasive) I can't access the pipework at all to attempt to install any supports or ideally rigid pipework. There is no space in the void above the plasterboard ceiling to fit a condensate trap and nowhere to drain one anyway.

I've never noticed condensation before, but if the back draft shutters were unable to open as I suspect, then I guess it wouldn't occur with the old fan since warm air isn't meeting cold air?

External vent is straight through and not a louvred self closing type.

Given the above, is there anything at all I can do to help the situation that doesn't involve cutting large holes anywhere? I'm trying to minimise that kind of thing because I've found electrical runs all over the place in the en-suite upstairs and I've no reason to think the rest of the house would be any better.

I'd be most grateful for any pointers.

Thank you.

JohnD · 16 May 2020

you can run the fan continuously for a while, and it may eventually dry out the duct. An indoor duct won't be as cold and prone to condensation as one in a loft.

as you've alluded to, you may need to replace the old duct with a straight one, rising from the bathroom ceiling, then sloping gently downwards.

Maybe you could remove the external vent and poke a tube through? (edit: no, because of the bends)

justbigboned · 16 May 2020

JohnD said:
you can run the fan continuously for a while, and it may eventually dry out the duct. An indoor duct won't be as cold and prone to condensation as one in a loft.

as you've alluded to, you may need to replace the old duct with a straight one, rising from the bathroom ceiling, then sloping gently downwards.

Thanks John. I've got it set to 20 minute overrun and it does dry out it seems, so there is that at least. Theres only about 5cm of space above the top of the extractor fan exhaust so very little to play with.

The option of replacing the ducting I think would be extremely invasive as the external vent is just above a stone window lintel leaving no option to move downwards. To create a proper rigid run by moving it over about 1m, I think I'd have to remove (and therefore replace) most of the upstairs bedroom floor, take down a couple of partition walls to allow access and cut holes through ijoists for a new route out, then reinforce the ibeams near the previous holes and make good. Ouch.

JohnD · 16 May 2020

If it's a chipboard floor, you can do it once it cracks, creaks and squeaks unbearably. Might not be long.

delmel · 17 May 2020

justbigboned said:
Hello chaps,

Our old bathroom fan of 72m3/hr died a few days ago so I've replaced it with a Vent-Axia Silhouette 100T, which has an extraction rate marginally higher at 108m3/hr. The bathroom is in the middle of the house with no windows and no external walls.

Unfortunately, the builders used uninsulated flexible ducting which exists through a vent on the outside of the building (about a 5m run) that is installed slightly higher than the fan itself.

The ducting is completely unsupported and runs off at a tight right angle from the fan, before (I assume) turning a further right angle for a run of 4m to the external vent. Due to the way the pipe runs, I don't think the previous extractor fan ever worked correctly. I always thought it was poor, but since everything else in the house is too, I never looked too closely. Apparently, it was because the back draft shutters would have been unable to open. The new one was the same, so I've had to remove them for the moment. It does extract now, but, we have condensation dripping from the fan, which I guess I expected.

Without cutting a huge hole in the bathroom ceiling or removing a glued down chipboard floor in the bedroom above (which is on ijoists so would be very invasive) I can't access the pipework at all to attempt to install any supports or ideally rigid pipework. There is no space in the void above the plasterboard ceiling to fit a condensate trap and nowhere to drain one anyway.

I've never noticed condensation before, but if the back draft shutters were unable to open as I suspect, then I guess it wouldn't occur with the old fan since warm air isn't meeting cold air?

External vent is straight through and not a louvred self closing type.

Given the above, is there anything at all I can do to help the situation that doesn't involve cutting large holes anywhere? I'm trying to minimise that kind of thing because I've found electrical runs all over the place in the en-suite upstairs and I've no reason to think the rest of the house would be any better.

I'd be most grateful for any pointers.

Thank you.

What was your old fan, it may have been a centrifugal fan which is far superior at moving air through longer awkward duct runs, the Vent axia replacement is a nice little fan and a decent brand but it isn't a centrifugal fan. Worth checking what you had before. You may be able to fit a condensation trap pending access.

justbigboned · 17 May 2020

delmel said:
What was your old fan, it may have been a centrifugal fan which is far superior at moving air through longer awkward duct runs, the Vent axia replacement is a nice little fan and a decent brand but it isn't a centrifugal fan. Worth checking what you had before. You may be able to fit a condensation trap pending access.

It replaced a Greenwood AXS100TR, which I think was just a bog standard unit.

justbigboned · 17 May 2020

Just an update to this. As the extraction ducting is all internal and sandwiched between two of our upper floors rather than in the loft, I decided to try and restrict presence of cold air by fitting a shuttered cowling externally. It seems to have done the trick when combined with allowing more air flow into the bathroom whilst the shower is running. Also, I'd previously noticed that the fan speed was being varied slightly, presumably due to wind entering the ducting and hitting the fan directly as I'd removed the back draught shutters. This has also been solved by the external cowl it seems.

I will continue to monitor and it might be that a more invasive solution is required, but I thought I'd post this for anyone else living in a modern 3 storey new build who might find themselves in a similar situation.

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New extractor fan installed - condensation

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