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AllGoNoShow 10-30-2006 11:17 AM

Bathroom Exhaust Fan Questions
I have an older 1930 house that someone added a bathroom exhaust fan in directly over the shower (the bathroom is prolly 50 sq. ft.). I went up into the attic and realized there is just cheap non-insulated flexible plastic ducting that is 10 feet long that attaches the exhaust fan to the roof vent. Is this acceptable?

Even with the fan on the whole time I still get a considerable amount of moisture in the bathroom-5 minutes into a hot shower the entire mirror is frosted over and stays that way. Even after i take a shower and the shower is off, if i leave the bathroom door closed it will take 10 minutes for the exhaust fan to clear the moisture. Is this a problem because of placement of the fan, the actual fan not having enough "suction", or the ducting?

Thanks for the reply.


KUIPORNG 10-30-2006 01:43 PM

I would think the suction power is the caused, changing the fan should fix the problem, although I would recommend you to change the duct to metal one at the same time...

rjordan392 10-30-2006 02:08 PM

It sounds like your Exhaust fan is too weak to keep up with the increasing amounts of humidity. Look at the fan blades; are they propeller type or like a blower wheel? The blower wheel type is what you should have. The blower type can handle friction and back pressure much more then a propeller type. But it does not stop there because the length of the duct and the size of the room determines what size blower fan you need to remove moisture and odors in a timely manner. I have 535 cubic feet of bathroom area and my ductwork runs 18 feet. The exhaust fan is rated to 270 cubic feet of free air and has a horsepower rating of 1/20. It just barely does the job. I should have going to the 1/15 HP one with a higher free air removal rating. Now you have to keep two things in mind. First is that fans will exhaust much less then the free air rating because of friction loss in the ductwork and the back pressure from pushing the air in a vertical duct. Second, when air is exhausted, the fan is not removing 100% of the moisture or odors because both of these are being diluted by fresh air intake. So its the diluted air thats being removed. After so many minutes of this, then most traces of the moisture and odors are barely detectable.

I realize this is not the total answer you wanted and its just something to ponder. But based on my assumption of you having 400 cubic feet, I think a blower fan rated at 270 cu. ft. per minute of free air with a 1/20 horsepower rating should do the job. Also if you have a skylight, you may need to seal it up some.

Darylh 10-30-2006 10:42 PM

This may be a silly question and I appoligize but did you check that there is air coming out of the roof vent. Sometimes the flapper gets stuck otherwise your fan is not big enough.

AllGoNoShow 11-09-2006 05:44 PM

I found out the fan is the el cheapo $17 variety from lowes that only pushes 50cfm and is loud as all hell (6.5 sones). I picked up a 110cfm model that fits in the same opening and is rated for a bathroom up to 100 sq. ft....its still a little loud (4 sones) but i dont really care that much and the real whisper quiet ones were double the price. I'll let you all know how installation goes and if it clears up the problem.

mdshunk 11-09-2006 06:04 PM

No bath fan will substantially lower the static pressure in your bathroom. That said, you can only exhaust as much air from the bathroom as you bring in to replace it. If the bathroom door has a very small gap at the bottom, you're working against the fan. You either need to saw off the bottom of the door a bit (3/4" gap is great) or install a louver in the door. If your bathroom door has a more "normal" gap to the floor (around 1/4"), you'll continue to fight your fan.

AtlanticWBConst. 11-09-2006 06:13 PM

Also, check the venting hose to make sure that it isn't crimped and adding to the lack of exhaust....

rjordan392 11-10-2006 01:44 PM

Square footage is of no concern. Fans remove air by the cubic foot per minute. Look deeper on the packaging of your fan for this. Check also the cross sectional area under the door that is open for air intake. It should measure the same or greater then the square inch area of the fans exhaust.

AtlanticWBConst. 11-10-2006 07:38 PM


Originally Posted by rjordan392 (Post 23338)
Square footage is of no concern...


You want the proper 'rated' exaust fan dependant on the size of your bathroom (7'x7', 10'x10', 12'x12', +...)
...That means that SF is one of the points in determining the exhuast fan you will chose to install...

mdshunk 11-10-2006 07:53 PM


Originally Posted by AtlanticWBConst. (Post 23370)

He's saying use cubic feet and not square feet. Bathrooms have all different ceiling heights.

AtlanticWBConst. 11-11-2006 07:59 AM


Originally Posted by mdshunk (Post 23372)
He's saying use cubic feet and not square feet. Bathrooms have all different ceiling heights.

Yes, I do understand that bathrooms all have different ceiling heights...
...But, I guess I misunderstood what was meant. I was thinking 'dimensions' not SF.
(In my mind, I was grouping 'dimensions' and SF together as the same concept)

We deal with 'cubic feet' alot outside: when using concrete, demolition, excavation, etc... but inside the home we are used to always using and thinking in 'dimensions and SF'.

Thanks for clearing that up ...

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