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Community and Q&A

Bathroom fan not as effective as expected

gbavhm | Posted in Mechanicals on

Our bathroom fan seems to be underperforming:
– it does not clear the mirror fog very quickly (takes over 20 minutes after the shower)
– the walls are wet during baths/showers to the point of dripping and are still wet 20 minutes after the bath
– from inside the bathroom, it fails the tissue test

But I can feel a fair amount air coming out from the gable outlet when the fan is on, and the flap is opening.

Am I just expecting too much, or is this a problem?

– bathroom is 5X10X8′ with a 3/4″ undercut on the door, which is the ~8′ length of the bathroom from the fan.
– bathroom walls are insulated – plaster on cement board
– baseboard heater below the fan
– typically room temperature right now is ~16*C
– house is ~830 ft2 with no other ventilation than windows/doors but is 1949 tested as [email protected]:.
– fan is Delta BreezeSignature VFB25AC 80 CFM chosen for its low noise level, installed in 2011
– duct is 4″ rigid metal with a single 90* bend about 1/3 along the ~20′ total length toward the gable; the vent is a hood with a flap.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Gbavhm,

    After watching a less powerful fan struggle to clear the air in a bath a bit smaller than yours, I installed a 100 cfm fan and it still didn't clear the air in real time and took a few minutes after a shower for the mirror to defog, etc. But it did work pretty well and did a great job if I cracked the window when running it.

    I'm surprised that the fan failed the toilet paper test but moves the vent damper 20 feet away, but I know little of static pressure and all that mechanical jazz. One thought: Was the fan recently installed? Sometimes the internal dampers are taped shut for packing and shipping and the tape needs to be removed before operating the fan.

  2. Jon_R | | #2

    You will get better performance with less replacement air restriction (like a jump duct). You can easily test how much by leaving the door open an inch.

    I've found that locating the fan immediately above the shower helps. I'm sure that a better sealed up shower would help even more. Mirror heaters are available.

  3. GBA Editor
    Brian Pontolilo | | #3

    I agree with Jon. Fan location makes a big difference on how well it clears shower moisture.

    Also, if you aren't able to improve the situation, you could add a timer switch, so the fan runs as long as it needs to to clear the bathroom and shuts down on its own.

  4. Yupster | | #4

    A simple single duct calculation using your described ductwork says you need a 5" duct to deliver 80 cfm at 0.1 in.wc. (the available fan power to produce 80 cfm). With a 4" duct in your system, that fan can deliver 56 cfm at 0.25 in.wc.
    I wouldn't worry about the free area of your door inlet, at 56 cfm and a 3/4" undercut on a 32" door, that's only about 0.02 in.wc. pressure difference or 4.7 pascals. Not too bad.

    Basically, you want more flow, you need larger ductwork.

    Cracking the window might result in a small improvement to fan capacity but more likely will let lots of fresh air move into the bathroom through wind, convection, etc. So that would definitely help.

  5. Yupster | | #5

    As another note, your room is about 400 ft³. 56 cfm should result in 1 air change every 7 minutes or so. Enough to clear the bathroom relatively quickly. Certainly in 20 minutes. So if I had to hazard some guesses over the internet, I would say your ductwork has more restrictions than you think, maybe some flex buried under insulation that you can't see or a mouse nest or a crushed section of duct from someone stepping on it. It definitely shouldn't fail the tissue test at that flow rate.

  6. Yupster | | #6

    Also check the fan blades and the grille for dirt, dirty fan blades can substantially reduce flow, as can a restricted opening.

    Okay, I'm done now :)

  7. Expert Member
    AKOS TOTH | | #7

    I think the bigger issue the bathroom temperature. At 16degC it sounds like the bath is colder than the rest of the house, if take in 50% RH air at 20C from the rest of the house it is close to 75%RH at 16C, no matter how much air the bath fan moves, you'll get very little drying.

    Increasing the bathroom temperature a bit should solve the problem. With plaster/lath it will take a while to dry out the walls, heating it won't be an overnight fix but should get better in a couple of days. Check with a decent RH meter, you don't want more than 60% RH anywhere in the house.

  8. gbavhm | | #8

    Wow thank you so much for all the answers! I will try your recommendations.

  9. Deleted | | #9


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