Faulty extractor fan installation

[KeepCalmAndCurryOn]

We have a problem in a newly refurbished bathroom: there is damp coming through the ceiling from above. However, there is no sign of a leak on the storey above, no mustiness, nothing.

My plumber is of the opinion therefore that the cause is the extractor fan in the bathroom, which seems to be installed in a chain with the fan in the bathroom next door (our ensuite) Both were refurbished last summer and the fans are new. And it is true that, if I turn the extractor fans off, there is less mustiness after a few days.

Have done a smoke test, and with both fans on, more smoke comes out of the bathroom vent than out of the external wall vent.

So I think the problem is indeed the fan(s). But what to do - the builder who installed them refuses to come round and see or do anything. Has anyone had a similar problem?

The fans should not be sharing a common ducting under any circumstances, each fan should be ducted seperately otherwise one fan will simply move the air into the second bathroom. are the fans wall ducted or are they ducted via a roof vent?. If they are ducted via a roof vent this vent needs to be a specific type with includes a water/condensation drain which has a small pipe connected for drainage. Your fans should also have a run on time of a few minutes after the lights are turned off.

Golly, Conculainey, you do know your stuff. I think the fans are connected, given the amount of smoke that blows back through fan 1 when I hold a gas match under fan 2.

They have little grilles in the ceiling, which when you lift them out have a motor (?) attached, then a long papery tube through to the outside vent (about 8 feet away for the problem bathroom). It is apparent that the papery tibe has holes in it - caused by the condensing water??

No sign of a drainage pipe thing.

If the external vent is in the wall, eves of facia board then a drain is not necessary. The flexible tubing should be made of a thin plastic material which is waterproof and will cause not odours, I would also check that the smell is not coming from the external soil pipe vent (sewer pipe from toilet) as the extractor vents may be close to the top of the soil pipe allowing fumes to enter the bathrooms. Are there little louvre doors on the extractor vents in the ceiling that open and close when the fan is turned on?

No louvre doors - just a grille / grid ifyswim. I have wondered whether he used an extractor meant for a loo rather than a bathroom, and it can't handle the water.

The smell is a harmless wet plywood sort of smell.

Perhaps the wet plywood smell is a leak under your bath or perhaps a seal around a shower tray and the water is getting onto the plywood floor or possibly the plywood backing behind the shower tiles if one is fitted. I would be more inclined to look for a leak than worry about the fan since you mentioned the plywood smell.

It is the ceiling around the fan, Con: gone all brown. But no sign of a leak in the attic above that could be trickling down. That's why i focussed on the fan.