Exhaust fan not pulling steam out

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brantmacga

brantmacga

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Former Child
So this is at my GF’s house; we’ve been doing some minor remodeling and repairs getting it ready to sell.

Her shower and toilet are in a room approx 5’x10’; the existing 50cfm exhaust fan was not adequately pulling steam out and leaving the walls wet. We’re about to repaint that room so I want to get it fixed.

I put a 110cfm fan in yesterday, and it’s still not pulling the steam out. I picked up a Broan 744 70CFM exhaust fan recess light combo thinking I’d put that directly over the shower to replace the can light I added.

But thinking it over today, part of the issue could be the room is too air-tight; it seems to me that 110cfm should have done the job. Anyway, I could easily add a 4” fresh air duct in the ceiling, but would that work? Or would I be better off to cut an inch or so off the bottom of the door? I just don’t know enough about hvac to know what the best route is. I’d rather add the fresh air duct if that’s a good solution.


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Are you closing the door and the fan still doesn't take out the steam? There usually is plenty of air that comes in under the door but you may need to plane down the door.

Don't put a grill in as the idea is to pull air from the floor up thru the room and out the exhaust. The duct in the ceiling will just move air from the duct to the fan and that is worthless.

I assume the vent is not blocked up.
 
Sometimes the flappers on the fan get stuck in the closed position. Either remove the flapper, if you have one on the exterior or make sure it is free and clear. You can check it from inside the housing.
 
I agree with Dennis, to test the fan try a shower with the door open. If it remains dry then you have an air intake issue with the room. IMO altering the door is the best option.
 
The 110cfm I just put in is not blocked. I replaced the duct as the original was 3” and this new one needed a 4”. I think maybe then I’ll just take my table saw over there tomorrow and start cutting the door down. I just need to make sure she is gone when I do


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vol = 5 x 10 x 7.5 = 375 cuft
60 min/hr x 110 cuft/min / 375 cuft = 18 ac/hr
should be plenty
not enough inlet air

had similar problem in my kitchen
exh hood worked fine but cooking smells lingered
added a small interlocked motorized make-up air damper in a kitchen closet and undercut the door
works perfect now
 
A 'fresh air' vent in the ceiling simply turns into a chimney to let all the heat out in the winter.

If anything, put a vent along the bottom of the door.
 
if it happens with the door open you are living with it
sometimes the room surfaces are cool and the warm moist air condenses
the solutions are to warm the room up or run the fan on a timer when done

recommended ac are 8-10
you have 18
 
brantmacga said:
So this is at my GF’s house; we’ve been doing some minor remodeling and repairs getting it ready to sell.

Her shower and toilet are in a room approx 5’x10’; the existing 50cfm exhaust fan was not adequately pulling steam out and leaving the walls wet. We’re about to repaint that room so I want to get it fixed.

I put a 110cfm fan in yesterday, and it’s still not pulling the steam out. I picked up a Broan 744 70CFM exhaust fan recess light combo thinking I’d put that directly over the shower to replace the can light I added.

But thinking it over today, part of the issue could be the room is too air-tight; it seems to me that 110cfm should have done the job. Anyway, I could easily add a 4” fresh air duct in the ceiling, but would that work? Or would I be better off to cut an inch or so off the bottom of the door? I just don’t know enough about hvac to know what the best route is. I’d rather add the fresh air duct if that’s a good solution.


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Click to expand...


I painted over water stains at my place before putting it on the market as well. I used Kiltz latex primer then still had to use two coats of paint. My stager recommended Agreeable Gray. It's a Griege and is all over HGTV. I got sick of it after two rooms and switched over to Passive. Good luck on the sale!
 
Be careful when you cut the bottom off a door. The hollow core doors they sell now have very little wood at the bottom, and it wouldn't be too hard to take it all off.

I was recently replacing a door. They now have enough blocking for the knob on only one side, and not all the way top to bottom. I was able to use a stud finder on that cheap 1/8 ply to "see" what was inside.

Another way would be to drill w/ a small drill up from the bottom to check the thickness.
 
The 4-inch exhaust duct has a cross-sectional area of 12.6 square inches.
The gap under the door need not be any bigger than that. (0.42 inches high for a 30-inch-wide door)

The fan's published flowrate, in cubic feet per minute (cfm) was measured with no pressure drop.
When you add pressure drop by adding ductwork, fittings and anti-backdraft valves, the actual flowrate will be less. Not catastrophically so, but you won't actually achieve 18 air changes per hour.

For a quick check, put a piece of paper on the fan's intake grille. It should stay there. If not, there's something wrong. Exhaust duct blocked, adverse pressure difference between indoors & outdoors, (is there a big, powerful whole-house fan?) fan turning the wrong direction, yada, yada, yada.

Is it actually not removing steam, or is the problem that it's not removing liquid water from the walls? If the house is not air-conditioned, (or if it is, but the a/c doesn't control humidity well) the bathroom fan might just be drawing moist air in, which doesn't have much capacity for evaporating water and removing it from the room.

If you'd like to dive in and get geeky, I recommend Crane's Flow of Fluids.
https://store.flowoffluids.com/shop/publications/TP410-US.html
 
drcampbell said:
For a quick check, put a piece of paper on the fan's intake grille. It should stay there.
Click to expand...

That was the first thing I did; it held on tight. I watched the steam just accumulate up at the ceiling and fall back to the walls. The fan is roughly 3’ outside the edge of the shower.

I suppose a circular saw with a guide on it would be easier. I just know I can’t free hand it in a dead straight line. But I do have some clamps; good call.

The existing gap under the door is about 1/2”. And it is a hollow door so I could maybe get another 1/2” off. I think maybe I could drill a small hole in the bottom to see just how much solid material I have to work with.

Thanks for all these replies.



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Before modifying the door maybe try opening door just a little and see if that makes any difference. If it doesn't, try opening an exterior door or window and see if you then get enough make up air. Anything you pull out has to come in somewhere, and they seal these places up so tight anymore it can create this kind of problem at times.
 
Barbqranch said:
Be careful when you cut the bottom off a door. The hollow core doors they sell now have very little wood at the bottom, and it wouldn't be too hard to take it all off.
Click to expand...

Very true. But it’s not difficult to replace the wood in the bottom. I have reused the piece that I cut off - remove the skin, coat with wood glue, slide it into the cavity, and shoot a couple pins in it to hold until the glue dries.
 
brantmacga said:
The existing gap under the door is about 1/2”.
Click to expand...
If you've got a 1/2" gap under the door, don't cut it, it's almost certainly enough already. And as others have mentioned, you can test if it would help by just leaving the door ajar to provide more inflow area.

If that doesn't work (and it likely won't), the only fix will be to relocate the fan intake (or add a second fan as you mentioned). Unless there's some other reason that the bathroom walls are colder than they should be, e.g. an oversized AC vent, or in the winter, inadequate insulation in an exterior wall.

Cheers, Wayne
 
Try a bath in the tub. ;)
Are you seeing the out door flap open?
How long a run?
How many 90s?
Is it corrugated hose?
PVC, aluminum?
 
I can't imagine adding another fan in a 5x10 bathroom. Either the water is way too hot and the GF likes to boil under the water or something is not right as we stated above. Are you sure the flap isn't stuck on the inside. I have seen those flaps come with tape on them to keep them shut during shipping.
 
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