Bathroom fan not as effective as expected
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?
Details:
– 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 ~8.30ACH@50Pa:.
– 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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Replies
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.
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.
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.
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.
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.
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 :)
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.
Wow thank you so much for all the answers! I will try your recommendations.
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