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Small Cabin Forum / Cabin Construction / Whole House Fan sizing
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# Posted: 14 May 2023 09:50

Hi Friends!
I’m looking for some information on how to size a whole house fan for COOLING. I know how to use Google and the only thing I seem to find there is information stating you need 2-3 air changes per hour for ventilation. I don’t want ventilation air changes, I want to know the recommended number of air changes for cooling. I want to feel air being sucked in through the windows. I know this is possible because I grew up in a house (and still part time live in said house) who’s whole house fan will suck the doors closed as it rips an air current through the house. On nice cool evenings when the house is like an oven, that fan will draw in cool air and cool the house down in 5 minutes.

Anyhow, I currently have a smaller gable end exhaust fan. It’s rated for 1050 cfm but it’s really meant for gently pulling cool air into a hot attic. I need a larger cfm fan. I just don’t know what that size should be because I’m looking for a recommended number of air changes for cooling.

I did some reverse calculations using manufacturers specs on a large fan I found; a 9,700 cfm whole fan claiming it’s for a 3,000 sf home will yield 24.25 air changes per hour on high. Is this too many air changes for my little cabin?

I know I need more that 3 to 6 air changes for cooling I just don’t know where to draw the line. I’m hoping y’all may have some input.


# Posted: 14 May 2023 11:21 - Edited by: ICC

good question and I don't have a good answer.

Here in the southwest people used a lot of evaporative coolers for decades. Many have switched to refrigerated air and in recent years to heat pumps for cooling. BUT we used to install lots of swamp coolers, as we call them. We sized those by taking the cubic feet volume of the room or building and dividing by 2 and used a swamp cooler that could supply that CFM, or bigger.

20 x 20 foot house with 9 foot ceiling = 5400 cu ft. Divide that by 2 = 2700 CFM for a swamp cooler. We'd round up and use a three-speed blower motor if possible.

Swamp coolers don't work very well when the relative humidity gets high. At those times we would sometimes run only the blower, and leave the water turned off. At those times the highest speed was a help but I do remember wishing for more airflow, a higher volume rate.

That's probably not a big help but it is all I know.

# Posted: 14 May 2023 14:25 - Edited by: gcrank1

Screening really cuts the air flow though doors and windows so it isnt just a cf thing as it would be with open holes.
Ideally I think Id want a multi speed or rheostat motor control.
Fwiw, for our 16'x24' with open room, cath ceiling And an open door along with some screened windows we vent one peak with a length of 6" stove pipe that has a 10"? bell on the inside and a small fan pointed at the bell. We dont really feel the draft but it works surprisingly well.
I rigged it up with what I had, quick & dirty, intending to learn something and upsize. Maybe someday, lol....

# Posted: 15 May 2023 06:10

Hole house fans only work a few weeks a month in the northeast. Typicaly if it cooler outside and the days heat is still in the house it's going to be a very humid night. Cool and humid is fine until the next day when the heat builds up and the humidity is still there.

Have you tried a box fan in a upper window?

# Posted: 15 May 2023 10:02

I grew up in a brand new house with only a whole house fan as our "air conditioning" so I'm familiar with the exact scenario LOL. It wasn't until I went to college that my parents finally put central air in. This time of year and later fall evenings they work really nice to pull the day's heat out.

Never thought to consider screening. I mean yeah, of course it obstructs the air...and I do have a screen in front of the fan as it exits out out a window. DOH!

swamp coolers are pretty cool but here in NJ they don't work very well unless you WANT to add more humidity to the air LOL.

Doing some reverse calculations, it looks like most whole house fan manufacturers rate their fans on an air change/hour rate of 22-24.

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