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What size bathroom ceiling fan should I get?

14 years ago

Just remodeled our bathroom shower and we were told that we definitely should consider getting a bathroom ceiling fan/light. The bathroom dimensions are roughly about 8'x10.5'. We are presently looking at the Panasonic Whisper Lite series of fans but aren't sure if we should get the 80 cfm or the slightly larger 110 cfm fan. Would appreciate your help on this one, thanks!!

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Comments (5)

bradleyj
14 years ago
Remember CFM refers to the cubic feet per minute of air / vapor the fan can remove from the room. The higher the CFM, the quicker it will "clear the air" so to speak, but it will also create more noise and air currents / drafts. So there is a tradeoff. I have attached a link to a simple calculator for you. I have a 40 cfm PWL in my 3x5 shower, and a 110cfm WWarm in my 7x8 main bath area.

Here is a link that might be useful: cfm calculator
lazypup
14 years ago
The general rule of thumb is that a bathroom exhaust fan should provide 4 complete air exchanges per hour.

You begin by computing the total cubic feet in the room, then multiply that by 4.

By example, your bathroom is 8'W x 10.5L which is 84sq.ft You would then multiply that by the ceiling height to get cubic feet in the room.

For the purpose of this example I will assume an 8' ceiling, thus we get 84sq.ft x 8' height equals 672cubic feet.

The total volume of the room in 672cu.ft and we want 4 air exchanges per hour so we would need to move 672cu.ft x 4 = 2688cu.ft per hour.

The fans are rated in CFM(cubic feet/min) so we now divide the total cubic feet per hour by 60min/hr to get the required CFM to meet your need.

2688cu.ft/hr divided by 60min/hr = 44.8cfm.

Once you know the theoretical CFM you select a fan that is rated as close as possible. In our example the theoretical CFM was 44.8cfm so a fan rated at 45 to 50cfm would do fine.

P.S. if your ceiling is not 8' you will have to recompute the total cu.ft for the room but you should get the idea from the example.
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Hi Carol, I just got mine installed. They are not hooked up yet. My electrician, who is a wonderful guy, has been MIA for a month. I guess the schools electric upgrades come before my modest remodel :) Anyway, now school is about to start and they will be more available so that "Nate" can get over here and wire my stuff. The fan is a 170cfm unit if I recall. The fan is actually made in Germany. The grills are about 8" diameter or so and have a halogen light. I don't think it's too weird that the neighboring bathroom will be vented when the one in use is vented, really. It is a new concept but I think it is ok. I was able to keep the roof vent and hook everything together just fine. I had my brother do the work and he did all ridged ducting because it is more efficient. The vents are about 5' apart. I will probably have 2 switches to the unit because I have 2 doors to enter the room. Fantech is a unit that I have seen referenced many times on GW, with great reviews. When I talked to the tech guy he was very helpful.
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A bathroom exhaust fan should produce 4 air exchanges per hour. Your bathroom is 7 x 7 which is 49sq.ft Assuming an 8' ceiling the total volume would be 49 x 8 = 392cu.ft We want four complete air exchanges per hour so that would be 392cu.ft x 4 = 1568cu.ft per hour. There are 60 minutes in an hour so we divide 1568 by 60min and it equals 26.13cfm. You currently have a 50cfm fan, which is twice the size you really require yet your having problems with the mirror fogging up. I suspect your problem is not the fan. Nature abhors a vacuum therefore a fan cannot push air out of a room unless there is provision for an equal amount of air to enter the room. You can test the theory by running the shower and leaving the bathroom door slightly ajar to allow more air to enter the room. Generally they make a slightly higer gap under the bottom of a bathroom door to allow more space for makeup air to enter the room but in some circumstances that does not provide enough makeup air. The solution then is to install a duct through the wall to an adjacent room and put a grille on either side. That will allow the additional air.
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fandlil
14 years ago
When we replaced the junk exhaust fans provided by the builder of our house, we used the Panasonic website to calculate the cfms we needed for each bathroom. We selected the Panasonic because of their claim that they are quiet. They are in fact whisper quiet, and our mirror does not fog over anymore when we shower.

There are of course other brands out there that may be as good as Panasonic or even better, but that's what we got, and we're very satisfied. The other issue you should consider is the installation. Where the exhaust duct goes to the outside, how long it is, how many turns it makes and how sharp the turns are all figure in the fans capacity. Make sure you get a really competent installer.
pegandken
Original Author
14 years ago
Thanks guys!!! Great advice and again I really do appreciate the help!!You sure know your "stuff".
budge1
14 years ago
We used the Panasonic site to calculate but wanted to make sure it really worked so we went up one size from what it recom'd. It works really well but as bradley says, it does create a bit of a current. I find when I'm in the tub, I can feel a bit of a breeze. IF you have heat in the bathroom I doubt if you would notice this, but we don't so I find it cools off the tub faster.

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