My bathroom ventilation fan broke up and I am shopping for a new better one. I noticed the newer fans are higher CFM through-put and are quieter, I have narrowed down my choice to this one.

My question is that my condo was built in 1980 so its older, will this fan still do a good job and be quiter? I have read in some product reviews/youtube installation that high CFM fans require bigger ducts. I live in 3 floor condo building. I only have access to the bathroom and nothing outside.

Will this 150 CFM be still a good option for me or I just need to go with the lower ranking basic fan?

The bathroom size is 7 ft by 5 ft. I have no idea of about the ducts size.


I end up asking my condo management and I am told the duct is 6 inches which is great. Now I am gonna go with 150 CFM and I am buying this from Amazon which is way cheaper than others but I worry if this will fit? I took my current one dimensions (from outside) and is 12 by 10.5 inches. The one at amazon shows bigger but I think all new ones will be standard sizes. Should I be worrying about physical dimensions? Thanks!

  • Some info that would be helpful for someone to answer your question are the size of the ducts and the size of the bathroom in cubic feet. Feb 2, 2015 at 23:09
  • I added that to question but I don't have any of current ducts size, is that something I can find out from under? Needless to say there is no way I can replace them so maybe assume standard size or perhaps if anyone has knowledge what were they using in late 70s!?
    – zar
    Feb 2, 2015 at 23:13

3 Answers 3


It's very difficult to determine what type of ducting was used and if that ducting was properly installed or has an obstructions just by the era the building was built. Since the fan is broken you can try removing it to see what size ducting is currently in place.

If your current fan was doing a good job before it broke... meaning you didn't have moisture or mold issues, fogged mirrors, etc... then sticking with the same CFM fan would be a good idea. The new fans should generally be quieter and more energy efficient.

The Home Ventilating Institute recommends a minimum 1 CFM per sq foot in bathrooms smaller than 100 sq ft. That will give you the minimum 8 air changes per hour in a bathroom with standard 8' ceilings with a 50CFM minimum.

For your bathroom you'd need a minimum of 35 CFM. Panasonic makes those WhisperCeiling Fans in various sizes. Larger CFM fans will have a 6" duct and smaller fans have a 4" duct. The WhisperGreen fans use less energy and the smaller sized ones come with the option of connecting to 4" or 6" ducts. Larger CFM fans have a 6" duct. The larger duct size is recommended if the ducting is very long. The ducts create resistance so a larger duct mitigates that. If you have 4" ducts and the ducting isn't very long you should be fine but I don't think you need the 150 CFM fan.

If you can take the old fan out you might be able to tell if the duct is a short straight run if you can see daylight through the duct.

  • My current one was horrible, I am glad its toast. It was noisy and I never felt it ever did anything good so going with same model or power is nearly not an option for me.
    – zar
    Feb 3, 2015 at 1:36
  • Panasonic fans up to 110cfm use 4" duct, and come with a 4"-3" adapter. 150cfm and up use 6". Feb 4, 2015 at 2:33
  • @SpeedyPetey Mixed up the ceiling with the WhisperGreen fans. Made the changes, thanks. Feb 4, 2015 at 17:14

Calculate the CFM Needed. (Length X Width X Height)/7.5 (7 x 5 x 8)/7.5 = 37.3 CFM Did the 7x5 include a Shower/tub area too? If so, the 50 CFM should be fine.

That Fan is a bit overkill for your bathroom, that said Mine is a 190CFM for the same size. The issue you are going to run into is that the fan you chose has a 6" Duct coming from it.

Typically fans from a Bathroom are 3" or 4" for something that size. It would be Similar to your Dryer Vent where it Exits the house, so that should give you an Idea. If you can get the Part number of the Existing Fan and look it up you should be able to google it to determine its Duct Size. There are Downsize collars that would downsize the 6" to 3 or 4" though that is not recommended as you will cause back pressure on the Fan as it can't push the required CFM out the smaller hole. If your Duct run was less than 2' you might get away with it, but if not you would probably void the fan warranty.

Find the Duct you have and then get a Fan that Supports its.

  • Thanks my last one (NuTone but don't know the rating) was quite useless so I do want to go with high CFM. They have 110 CFM in the same model but I think it also has 6 inch duct.
    – zar
    Feb 3, 2015 at 1:32
  • @zadane, the size of the duct is going to be a fairly important factor. Using a reducing bushing could result in reduced motor life and overheating of the motor causing the thermal break to be replaced. @ 6" you're probably looking at a 50% size reduction; too much. Feb 3, 2015 at 5:07
  • @zadane, a 110cfm fan, regardless of make will NOT have a 6" duct fitting. Feb 4, 2015 at 2:35
  • Old school Nutone fans were absolute garbage, and are still available today. You cannot compare them to a mid-grade or better fan of today. As stated, a 50cfm will meet the minimum recommended size. An 80cfm will be much better. A 110 will also be fine and do a good job, but is overkill IMO. Feb 4, 2015 at 2:38
  • @SpeedyPetey thanks for comment, I updated the post with additional info. I am so bitten by the junk Nutone that I just wanna go with 150 CFM in hope it will really work.
    – zar
    Feb 4, 2015 at 4:21

A 50 cfm should be sufficient for a 35 sf bathroom. 1 cfm per 1 sf is a good rule of thumb.

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