condensation problems while taking a shower with newly installed bathroom exhaust fans

[youngi]

I've been trying to find an answer to this problem for over 2 weeks now.

Here is the situation:

2 newly installed panasonic 80 cfm bathroom exhaust fans(the kind that fit in a 2x4 joist space) in two different bathrooms. These fans have their own internal flapper.

Bathroom 1: 8 x 4 x 8 Bathroom 2: 8 x 5 x 8

I'm using 3" sheet meal ducting(the kind you have to "snap" together) going horizontally out the side walls instead of going straight out the roof. They duct out to exhaust hoods which also have a flapper on them. I also have put insulation on the whole duct run.(not tried putting insulation on the fan housing yet due to mold concerns.) The duct runs are about 4' each from the exhaust fan and slope downward from the start of exhaust fan to the wall, so you would think condensation would just drip out towards the wall. You can go outside while the fan is running and seems like its pushing out plently of air. The fans are mounted in between the shower and the vanity right above the toilet. The are mounted at a downward angle since the sheetrock ceiling has a downward shape that matches my open beam ceiling slope patter. My whole house is all open bean ceilings, there is no attic. The duct run is made above newly installed sheetrock and the open beam ceilings, with a 4" space between the sheetrock and the open beam ceiling. My open beam ceilings are vaulted going downward towards the side wall, so the ducting runs down the similar open beam downward slope. The ducting is not bent at all in the larger of the bathrooms, and has a slight bend in the smaller of the two bathrooms going out the side wall. When I take a shower in the smaller of the two bathrooms, after about

5-6 minutes, condensation starts leaking from the fan housing thru the grille. I thought it was due to it being pretty cold outside, but today was fairly warmer and the same results happended. In the larger of the two bathrooms, it takes slightly longer to condensate when taking a hot shower since its a bigger room, but it happens in that bathroom as well.

Can anyone shed some light as to how I should go about solving this issue, as I am out of ideas.

thanks, Isaac

[Joe]

Every output needs an input. You need an air makeup grill in the wall, door or wherever. The makeup air can come from any house area that is dryer than the bath. You will be amazed at how much more output your fans will have and I doubt you will see any more condensation. Reminds me of the many times I've seen techs fight a wiring problem in a car for hours only to find after all the struggle that it was a problem in the ground, not the positive side. Remember moving air goes in circuits, too. HTH

Joe

[youngi]

So you don't think its too cold in the space between open beam celing and sheetrock?

The one thing I did which made it not leak at all, was to leave a space heater on while taking a shower....the fan did operate much better not even letting the bathroom mirror to fog up. So in this case your theory proves correct except it wasn't just any air....it was WARM air I was adding into the bathroom. Any thoughts about this as well?

thanks so much for the prompt response

[Joe]

Pondering your duct sizing, could there be a mismatch? 3 x 14's are

42" sq,, 4 x 10's are 40"sq., 7" round 38.5" sq., Many bath fans use 4" round outlets (12.6 sq.") which prompted the question. Might be your moist exhaust air is meandering down the ductwork and being over- cooled on the trip.This suggests a) increasing the velocity with smaller ducting, b) using fiberglass or insulated ductwork or, c) all of the above and paying atterntion to makeup air, of course. HTH

Joe

[youngi]

Nope this fan uses a 3" duct and says so in the manual. its a panasonic model FV-08VS1,,,,and for some reason they don't have it on their website. Since it is a smaller fan(fits in a 2 x4 joist instead of a 2x6 joist), that is the reason for the smaller ducting. The makeup air does not seem to help, as leaving the bathroom door wide open still results in a good amount of condensation. The condensation is definitely coming from the fan housing itself as the duct slopes downward AWAY from fan and has fiberglass insulation wrapped around the ducting. Are you saying that by using even smaller ducting, that it will increase the overall velocity?

I should also mention that we had the same problem using a nutone model 695 which had 70cfm.

Perhaps we just need a stronger vent like 110 cfm?

[yourname]

It would appear that since the duct slopes away from the fan, the fan housing is cold enough to condense the moist air right away. If you cannot insulate around it you may just have to deal with it.

BTW, I noticed my bathroom fogs much less with the door closed than open. It is my thought that the easily available [dry] air from the bedroom gets to the fan before all the air in the bathroom. Closing the door makes it draw from the room more, perhaps.

[youngi]

I can try insulating around it, but am fearful that the insulation will just absorb moisture leading to mold. I also notice that the bathroom fogs a tad less with the door and window closed, but still fogs and the fan still condensates big time(a few tablespoons actually).

I am pulling my hair out trying to find an answer to this tough puzzle.

[Joe]

oversized duct for a small fan. The ratio, sizewise, is an amazing 6 to 1. Your output velocity therefore is so slow as to allow a backdraft of cold air to flow back to the fan unit and cause condensation there. Your options may be simple, like use a 3" PVC pipe for a vent to take advantage of less heat loss than a 3" metal duct would have. Theoretically you could add 5 more Panasonics, obviously not the practical thing to do. Vertical ducting may be a solution, since warm air rises the system would have some advantage from a natural draft. IMO this could be the best way to get a workable system, one that doesn't flout the laws of physics. Good luck.

Joe

[youngi]

quoted text -

do you mean my duct run is too long?....i'm not sure where you get the

14" measurement from.

The 3 " duct size fits the opening perfectly on the housing.

[trader4]

Hide quoted text -

I don;t see why you;'re reluctant to insulate around the fan housing. The exhaust pipe slopes down, so we know the water isn't condensing there and running back. It has to be condensing inside the fan. Putting insulation around the fan will keep the fan from getting cold. Any moisture should be reduced and it should be contained on the inside of the fan housing and will not get the insulation wet. I would check any electrical rating concerns related to how close insulation can be to the fan housing.

[youngi]

Hide quoted text -

Putting insulation around thefanwill keep thefanfrom

I am going to try both insulating the fan housing itself as well as insulation my floors of both bathrooms since I know that cool air is coming up through the walls from the crawlspace.

I'll let you guys know my results.

[Joe]

My error...your stated 3" duct size led me to believe it was 3" round ducting. It must therefore follow, that your outlet size is 3" x 14" rectangular. That being the case, keeping the troubled area warm by insulation and heat tape may solve the problem. The heat tape logically would be be powered by the fan circuit. So far as which to try fist, the heat tape would be much easier to install. If it does the job, quit there. If you judge that some insulation might help, go for it.

Je

[youngi]

what do you mean by outlet size??? the outlet of the fan that connects to the ducting?....that is also a 3" round.