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I'm in the middle of a home addition and am thinking about adding a whole house fan. The finished size of the house will be about 2000 square feet - approx 1500 on the main level, and 500 on a lower level. I'm currently looking at the Quiet Cool Line of fans.

My question: How do I size the fan? Is the relevant number to use the total square (really cubic footage, but the marketing material usually uses square) footage of the house, or just the upstairs square footage? Seemingly it would just be the upstairs square footage, since the downstairs warm air would be rising anyway. Or is there some other criteria I should be using?

  • I'm not sure square footage matters with a whole house fan. It matters in air conditioning because you have to cool every cubic foot of air, and that takes energy, but a whole house fan just makes air move. I think what really matters is how many windows you want to suck the air through, which could depend on the shape of the house and location of the fan. More windows = bigger fan. – JPhi1618 May 2 at 17:28
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The guidelines I've seen (Dayton?) figure the cubic footage of the living area of the house (sum of all room & hallway height * width * ceiling height), then multiply that by 30 (assuming an air change every 30 minutes), divide by 60. That will give you a suggested CFM capacity for the fan. It'll be most effective if you can have air intake at the lowest level and at the extreme ends of the house on the larger upper level.

Also check your attic ventilation to make sure the air can get out. Divide the fan CFM rating by 750 to get sq ft of net free vent area (NFVA) needed. If your attic is ventilated to the 1:150 standard, that's probably enough. If it's ventilated to the 1:300 standard, or less, you'll need to look at adding more.

The Quiet Cool fans are very quiet, but very expensive. A much cheaper way to get the same effect is to suspend the fan from the roof joists on nylon webbing (at least 18-24", preferably 1.5x blade diameter or more below the roof deck), then use rubber backed carpet to make a tunnel from the fan to the ceiling louver. (Make sure to seal it airtight at both ends to draw air from the house, not from the attic.) This isolates the fan so it doesn't vibrate the interior walls and ceiling. Doing this cut the noise directly under my fan from 92db to 65db, quiet enough I can have a quiet conversation directly under it and can't hear it at all from 2 rooms away.

As others have suggested, if you're going for a high efficiency, airtight house an ERV/HRV would be better, but if your house is at least 10-15 years old it probably leaks enough that a house fan would be more cost effective.

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When I sized the ventilation (with heat recovery) for the house we built, I used numbers based on the replacement rates for the activities in each room combined with the number of people.

So, bedrooms tend to have the lower requirements, kitchens etc highest.

We had extraction in kitchen / bathroom and input in bedroom / living areas etc The unit we fitted, that was designed to run 24/7, was a unit 2ft * 2ft * 1ft deep. We also took the opportunity to bring the inlet air underground for 100 feet to help mitigate the summer high and winter low temperatures of the air.

We used this supplier : https://www.stiebel-eltron.com/en/home/products-solutions/information-engineering/ventilation-systems.html

Have a look at this link as that will give some relevant values:

Room air changes

Once you have the total amount of air to move, then you can work out the amount per hour and from that the fan size needed.

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In my previous house which was about the same size as yours I installed a whole house fan that I bought from Graingers. I did this about 40 years ago and it worked quite well. I "GOOGLED" Graingers and got these model numbers: Fan- Item # 1LXN7 30" 2 speed belt drive fan and Item 4C225 a 30 inch blow open multi louver vent. A 24" would have been too small and a 36" would have been too large. This fan worked well for me until we got whole house A/C. We still used the fan in the milder months. Hope this helps.

  • How loud is it? The primary benefit of the QuietCool line of fans is that they are (supposed to be) both recessed into the attic and quiet – Steve French May 2 at 15:04
  • My fan was mounted in the ceiling of the 2nd floor where the bedrooms were. We never ran the fan on high during the night so it was quiet enough to allow us to get a good nights sleep. There are many ways to install an exhaust fan and I chose this way. since I could do it myself and it did not cost a lot. If you want a whisper quiet fan this is probably not the installation you will want. It worked for us and the price was right. – d.george May 2 at 17:33
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You might to look at an ERV or HRV depending on your climate. One of the best is the Zehnder.

The passive house standard where you build a really good airtight house and rely on mechanical ventilation recommends a total house ACH of .33. As other posters mentioned the rates vary between the rooms. To properly figure out the ACH as you've noted you use cubic feet (square feet likely assumes average ceiling heights and will probably get you close enough that the fan works).

ERV/HRV has the added benefit of capturing the conditioned nature of your air and recovering 90% it but they are more expensive and harder to install. If you want something cheap and easy go with your whole house fan.

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Having had 4 whole house fans, I would say the most important factor is noise; a belt drive is the most practical way to reduce noise. I presently have a 24" belt drive which I always use on "Lo" speed for a 2500 sq ft house giving very high air flow, so size is not very important. Another fallacy is central location ; my fan is in a corner of the house and air flow is easily controlled by opening windows in various locations. My first fan was a Hunter which was excellent, I don't remember the names of the others so, not much difference as long as they are belt drive.

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