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Old 01-10-2012, 03:42 PM   #1
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Proper CFM rating for a bathroom fan


Hello all,

I'm going to be installing a new vent fan in my bathroom in the next week or two. I've read that the necessary CFM rating should be about 1-1.1 times the square footage of the bathroom. I don't remember the exact dimensions of my bathroom but it's approx. 50 sq ft.

My question is, when it comes to the CFM rating, is more always better or would I just be wasting my money by getting a higher CFM model than the above rule of thumb says that I need? Or is that rule of thumb even accurate?

Thanks in advance.

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Old 01-10-2012, 03:50 PM   #2
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Proper CFM rating for a bathroom fan


If my memory serves me correctly it's the total cubic square footage of bathroom, measured width x's hiegth x's depth divided by 8, the number of times it should be able to refresh the air in the room per hour. That's the cfm you will need.

I'm pretty sure you need to re-measure the space, my non-walk in closet is bigger than 50 square cubic feet and I barely fit in there.

And research "sones" as well, noise level of fan.

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Old 01-10-2012, 03:54 PM   #3
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Proper CFM rating for a bathroom fan


Here you go. Total volume of the room.
http://www.greatpossibilities.com/ar...aust_fan.shtml
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Old 01-10-2012, 04:14 PM   #4
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Proper CFM rating for a bathroom fan


Thanks for the link, Doc.

The suggestions I've seen otherwise have been relative to square feet. The volume of my bathroom is probably in the neighborhood of 400 cubic feet if I had to venture a guess. It looks like the recommended CFM rating using either formula would be a little over 50.
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Old 01-10-2012, 04:27 PM   #5
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Proper CFM rating for a bathroom fan


You're welcome and again, pay attention to the "sones" level. You don't want it sounding like an airplane flying through there every time it's on.
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Old 01-10-2012, 04:33 PM   #6
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Proper CFM rating for a bathroom fan


Quote:
Originally Posted by MitchTheButcher View Post
Thanks for the link, Doc.

The suggestions I've seen otherwise have been relative to square feet. The volume of my bathroom is probably in the neighborhood of 400 cubic feet if I had to venture a guess. It looks like the recommended CFM rating using either formula would be a little over 50.
The choices are very limited, we recently ( 09') rebuilt from a house fire, and I did alot of research and found that regardless of the size of the bath room, it came down to four fans to chose from, 50, 70, 80, and 110 cfm. i went with the 110 cfm and have not regretted it one bit. And yes it was over kill, but it gets the moisture, and the odor out in a hurry.
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Old 01-10-2012, 05:15 PM   #7
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Proper CFM rating for a bathroom fan


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Originally Posted by harleyrider View Post
The choices are very limited, we recently ( 09') rebuilt from a house fire, and I did alot of research and found that regardless of the size of the bath room, it came down to four fans to chose from, 50, 70, 80, and 110 cfm. i went with the 110 cfm and have not regretted it one bit. And yes it was over kill, but it gets the moisture, and the odor out in a hurry.
I did zero research before going to buy the fan so we just bought the one with the best combination of high CFM and low sones. I think it was 110 CFM, don't remember the sones. It wasn't until after the fact that I read the recommendations of a certain CFM per sq ft.

Now I'm trying to decide if I should return the fan for something with lower a CFM rating (and I'm assuming a lower price) or just keep the one I have.
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Old 01-10-2012, 05:47 PM   #8
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Originally Posted by Doc Holliday View Post

I'm pretty sure you need to re-measure the space, my non-walk in closet is bigger than 50 square cubic feet and I barely fit in there.
My bathroom is 6x8 and I barely fit in there.
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Old 01-10-2012, 06:01 PM   #9
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My bathroom is 6x8 and I barely fit in there.
6' wide by 8' foot deep by let's assume 6' high equates to 288 cubic square feet which is very tiny and would require a minimum exhaust fan of 50 cfm. Granted the minimum requirement would be 36 cfm but 50 is the smallest made.

My closet is maybe 4 x 4 x 6 which equates to 96 cubic square feet. I need a new closet.
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Old 01-10-2012, 06:36 PM   #10
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Proper CFM rating for a bathroom fan


Quote:
Originally Posted by harleyrider View Post
The choices are very limited, we recently ( 09') rebuilt from a house fire, and I did alot of research and found that regardless of the size of the bath room, it came down to four fans to chose from, 50, 70, 80, and 110 cfm. i went with the 110 cfm and have not regretted it one bit. And yes it was over kill, but it gets the moisture, and the odor out in a hurry.
yea harley we have a pr off kit and dinning room about 4x5 with high ceiling..110 cfm total air removal in no time....great choice..oh the wife thinks it great...too....
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Old 01-10-2012, 06:56 PM   #11
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Proper CFM rating for a bathroom fan


Quote:
Originally Posted by Doc Holliday View Post
6' wide by 8' foot deep by let's assume 6' high equates to 288 cubic square feet which is very tiny and would require a minimum exhaust fan of 50 cfm. Granted the minimum requirement would be 36 cfm but 50 is the smallest made.

My closet is maybe 4 x 4 x 6 which equates to 96 cubic square feet. I need a new closet.
So basically any fan is a certain degree of overkill for me?

This actually reminds me that I bought one many months ago. Panasonic, IIRC. I'll have to check the CFM/sones.

I think it's buried in a pile of crap somewhere. Despite my pleas, it refused to install itself.
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Old 02-10-2012, 01:32 PM   #12
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Proper CFM rating for a bathroom fan


Hey All,

I finished the project a couple of weeks ago and have had a chance to use the fan for a while, so I thought I'd share my thoughts on it.

First, you can calculate the CFM based on either the square footage or the cubic footage of the room.

Quote:
Originally Posted by Doc Holliday View Post
I'm pretty sure you need to re-measure the space, my non-walk in closet is bigger than 50 square cubic feet and I barely fit in there.
Doc, you're referring to cubic feet, where the original formula I posted refers to square feet.

Looking around at a few websites, it seems that the recommended CFM rating would be square feet x1 or 1.1, or cubic feet /8. Obviously, calculating your cubic footage will give you more accurate results, but the square footage formula should work fine unless your ceilings are particularly high or low. In my case, my ceilings are exactly 8 feet high, so square feet x1 or cubic feet /8 will yield the exact same number, in my case in the mid 50's.

I ended up returning the 110 CFM fan I bought for about $120 and purchased a 60 CFM fan for a little more than half that.

The most important question is, does the 60 CFM fan provide adequate ventilation? Yes, it is adequate. After a hot shower, all steam is gone within a minute or less of turning off the shower, and there doesn't seem to be any condensation on the ceiling at any point.

The question I was looking for an answer for was, is more than the minimum better? I would say yes. I don't know what the CFM rating for the old fan was, but it definitely moved more air. During a hot shower with the old fan, there was almost no visible steam whatsoever, and it cleared odors maybe a little less than twice as fast, in my estimation.

While the old fan was functionally fine, we wanted to replace it because it was super old, looked terrible, and was super noisy. I don't remember the exact sonne rating on the new one, but it was less than 1 and you can barely hear it in the bathroom, and you can't hear it at all even 5 feet outside the bathroom with the door open.

So in short, the minimum CFM rating is adequate, but higher is noticeable and nice. The drawbacks to a higher CFM fan are 1) expense, and 2) most fans 100 CFM or higher are really big, and would look a little weird in a small bathroom. Also, I would think that a fan with a higher CFM rating would use more energy, but I don't know if the difference is significant.

I hope this proves helpful to someone in the future.

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Old 02-10-2012, 01:44 PM   #13
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Proper CFM rating for a bathroom fan


I think the actual energy (amps) used by the larger fan would be insignificant. More significant may be the cost of conditioning the air that you are expelling thru the bath fan. I'm thinking that once the room is dry, it's dry. I just installed 90 CFM fans in my bathrooms. One has had none for 50 years. Just a window. (The other bathroom is new construction and hasn't been used yet.) The difference is noticeable. I also put timers on the fans. Usually set the timers to run for 10-15 minutes after the shower is turned off.
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Old 02-10-2012, 01:53 PM   #14
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I think the actual energy (amps) used by the larger fan would be insignificant. More significant may be the cost of conditioning the air that you are expelling thru the bath fan.
Good thought. That hadn't occurred to me.
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Old 02-10-2012, 10:10 PM   #15
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Proper CFM rating for a bathroom fan


Isn't the cfm rating of these fans without any ductwork connected to them? So if you need to duct them 6 or 8 feet out you're probably decreasing the cfm of the fan by a significant percentage.

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