Testing a bathroom fan

House Repair Talk

Help Support House Repair Talk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Jun 19, 2020
Reaction score
We have two small bathrooms that are side by side. One is a 1/2 bath, one is a full (but small) bath. I bought two 50cfm fans for them (in 2015- they are older). We had the ducting installed, and they connected both together and the output went to a soffit. We had the problem that when someone took a shower in the full bath, humidity came into the 1/2 bath. So, we had the ducting split to two soffit vents.

We have someone using the full bath. Within minutes of turning on the shower, it gets moist and the glass fogs up. The fan is working. This is in an in law apartment we're renting, so I only have access when she's there.

I went to the 1/2 bath (in our main house) and I tested the fan suction by using paper that got sucked up against the fan. I then went outside and took off the soffit vent. I tested the fan again using the paper trick. It SEEMED like the paper was on there stronger, but it's difficult to know.

My theory is that the soffit vent they used is restricting flow. I could get a better soffit vent. But I can't figure out how to test (in the 1/2 bath) that the soffit vent is restricting flow.

Is there any way to perform a better test of a bathroom fan?

(I will ask the renter to get in there, take off the soffit vent, turn on the shower, and see what happens. But I'd like to see if the soffit vent is an issue, and replacing a fan is harder, because I have to figure out the volume of the bathroom, the length and type of ducting for 4 inch ducting, etc., to ensure the fan and ducting are suitable.)
Are you sure there are no kinks in the ducts? It sounds like you may have had a bit of work done when they divided the two systems.

To be clear; there is now no cross-venting problems between the two bathrooms? Just not venting well to the outside?
You could buy smoke candles to watch the smoke exiting.

But if you see very little smoke exiting, what does that really tell you ? Is the flow restricted because the duct is restricted, the baffle is restrictive or the fan is too small ?

I don't think it's even possible to buy a bathroom exhaust fan smaller than what you have. I think your fan is too small.
If you are worried about sound, the Panasonic fans really are quiet.
Thanks, both of you. I do have Panasonic "squirrel cage" fans. They are very quiet.

I will go into the attic and look to see what the ducts look like. I can't do that until this weekend.

When I bought these, I went through the calculations: volume of the bathroom; length of the ducts (and there are calculations for both smooth and not smooth ducts), etc. 50 cfm came out fine, a bit larger than necessary for the bigger bathroom. But the duct makes a big difference, and I don't know what they used when they split it.
I also wonder if the termination of the ducts is limiting flow? I took it off of the smaller bathroom, but I can't figure out what difference it made.

My wife also reminded me that this is where my mother kept the cat's litter. So, I'll clean the squirrel cage, take off the outer termination for the fan in the in law (with shower), look at the ducts, and see what happens. I may also disconnect the fan in the attic from the duct to see what happens. If that works, then the duct is too long/wrong kind/ the termination is wrong/ or the fan needs to be bigger.

The problem with these fans is they go from 50 cfm to 100. I'm sure 100 is way too much.

I'll report back.
I've found a few 'flex' ducts which looked OK but were 'sagged' in places and those contained water which was impairing airflow. Best practice with these and dryer vents is to use the minimum "flex" pipe necessary and run the rest solid pipe with downhill sloping to the exhaust..
What I figured out is the following. The bathroom is less than 50 cubic feet, which supposedly means you use about the same size of fan, which is what is installed (50 cfm, cubic feet per minute).

This fan uses a squirrel cage fan system. I vacuumed it out, and then blew it with compressed air. I then took the plastic tube and scratched one of the fins of the cage. Covered in soot. Ugh. Took the soffit vent cover off, ran the fan while running the shower. Took longer to fog up the mirror than before, but still fogged up the mirror.

I tried to remove the cage to clean, but I couldn't see how to get it out of there. Might be possible, but there was a plastic shroud that covered the edge of cage. The cage seems to only be held by a nut, but I can't see how to get the cage past the shroud.

Went into the attic to see how long the ducting was. About 2 feet corrugated from fan to smooth ducting, about 5 feet smooth ducting, then 2.5 feet corrugated that takes a plunge from flat to the soffit outlet.

The last time I did this, I found some formula for calculating the fan size, which included length and type of ducting and also added in for the vent. I can't find that now, but about 10 feet of ducting is a bit, with two turns (one from fan upward, one plunge from smooth ducting to soffit). The formula I previously used also deducted more for corrugated ducting. And the soffit vent isn't a great one. Panasonic sells a nicer one, but given where the soffit vent is, it might be a bear to install.

So, researched Panasonic fans. Now, they sell one that has 50, 80, or 100 cfm, adjustable. I'm going to buy that, set at 80 cfm and see what happens.

As suggested by @Mastercarpenty, I'll also make sure the ducting is angled toward the soffit (not sure that it is now).