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Question DetailsAsked on 1/1/2018

When showering the other day both of our bathroom vents began dripping cold water. Is this condensation?

It has been very cold here recently. We renewed our bathroom and 2 new vents installed. The vents were the best we could find for moving humidity out instead of having it stand on the mirrors. The cold water drip lasted a short time. We use both vents when showering. The bathroom was totally redone and finished in May, 2017.

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1 Answer


Assuming this has not happened before, almost certainly yes. Condensation can form in the exhaust duct both from normal household airflow up the vent condensing in the ducting (and the vent hood on the roof or wall) at the warm air hits the cold ducting and cools below the dew point (so basically causes fog in the duct which condenses in the ducting) and in cold weather (below about 10-15 degrees in attic commonly) can even frost up in there. In real cold weather (say attic below about 0-10 below) the moist bathroom air from showering can also condense in the cold ducting.

So there is really three mechanisms for moisture coming out of a bathroom (or kitchen) vent - condensation of normal minor household air flowing up through it condensing and running back down the inside of the ducting and dripping out at the fan area, active condensing and running down while especially humid air is blowing through it using the fan (early on in showering or after turning kitchen fan on) until the duct warms up enough to no longer condense moisture out of the air, and thawing out of frost in the duct when you first turn the fan on. Both the latter two usually result in dripping durin gthe first minute or few the fan is on, the first can cause dripping (and ceiling staining if it leaks out of the ducting).

A more unusual fourth mechanism which I have seen is a dut which is leaking cold air from outside because the damper is not fully closed, or just because it is bitterly cold around the duct, so moist air getting into the attic frosts up on outside of the colder metal ducting, then melts when the fan is run. This can also happen in the roof vent which the duct exits into, causing icing in the hood which then melts when the fan is run, running down the outside of the ducting and staining the ceiling around the fan housing. Commonly does not get past the insulation (which it wets), but can be more than that - I have also seen as much as a foot or more of frost buildup around the ducting because of leaking duct or blowback from the vent hood because the ducting was not sealed at the roofline, so there is a foot square or so hole in the roof sheathing under the vent which allows the moist air in the hood back in around the duct, building up frost and melted ice around the duct which can then run down to the ceiling when it melts - either from the fan running or from outside weather getting warmer. Ditto to frosting on the underside of the roof sheathing because of a vent ducting leak.

Attic frosting and icing has to be fixed by sealing the air leaks. The water dripping down from inside the fan unit when the fan is turned on can be solved in most cases by running the fan for at least 15 and more commonly 30 minutes at least daily to evaporate out the condensation/frost - best if it runs a few minutes BEFORE you start adding a lot of moist air (showering, boiling/frying food, etc) and then let it run long enough to remove the moisture from the room so high humidity does not work its way up the duct. Course, in REAL cold conditions, and especially if your furnace is not keeping up with the demand so it is running continuously or near continuously, venting that much inside air may not be desireable.

The frosting on the inside of the ducting can also be alleviated (though not totally eliminated) by insulating the heck out of the ducting - there are slip-on insulation sleeves which work OK, though in cold areas (below about 10 degrees in attic) I double up two sizes - say 3 or 4 inch size on the ducting itself, then 6 or 8 inch size over that for a double layer. Or just double wrap with R-13 batting. I use unfaced fiberglass batting so it can evaporate readily if it gets damp from a leaking duct joint or condensation in the vent hood.

If the problem is chronic and due to frosting/icing in the vent hood, then you need to change to a which has continuous ducting to the outside, not discharging into an open hood (which are typically totally uninsulated so exactly the wrong way to vent in cold climates) - but with small size ducting can be hard to find a damper arrangement which fits and does not freeze up.

Answered 2 years ago by LCD

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