My house has a 24" Dayton belt drive whole house fan that's at least 20 years old but looks exactly like the current model 2EAX5. It's nice and quiet but I want it to move more air. It has a 1/3hp two speed motor with a 2" pulley driving an 8" pulley on the fan. The motor doesn't have any markings to tell its speed, but I've found quite a few that run at 1140 & 1725 rpm. With the 8" pulley that works out to a fan speed of 285 & 430 rpm, about 75% of the currently advertised 370 & 560 rpm. I don't know how many cfm that comes out to but current ads with a 6" pulley say 2500-3700 cfm.
I recently acquired a Cool Attic / Ventamatic 30" belt drive whole house fan, model CX30BD2SPD. It also has a 1/3hp two speed motor, marked 1140 & 1725 rpm, with a 2" pulley, but driving a 6" pulley on the fan. This spins the fan at 380 & 575 rpm, very comparable to the current Dayton advertised speed. Because this fan is 6" larger and spins 25% faster it moves more than double the air the Dayton does, advertised 5400-7800 cfm.
To summarize…
Dayton 24":
6" pulley,
fan speed: 370 / 560 rpm,
air flow: 2500 / 3700 cfm (advertised)
Dayton 24":
8" pulley,
fan speed: 285 / 430 rpm,
air flow: ??? / ??? cfm (not enough)
Attic Cool 30":
6" pulley,
fan speed: 380 / 575 rpm,
air flow: 5400 / 7800 cfm (advertised, LOTS more than the 24")
Attic Cool 30":
8" pulley,
fan speed: 285 / 430 rpm
air flow: ??? / ??? cfm
Why not swap out for the larger fan with the 6" pulley? My attic doesn't have enough ventilation for it.
I've thought about swapping the pulleys, putting the 6" on the 24" fan and the 8" on the 30" fan. I'm reasonably certain that would get the 24" up to the advertised 2500-3700 cfm. How can I estimate how much air the slowed-down 30" fan would move? Is it a linear function of speed, blade area, or some combination of the two?
Once I know how much air is moved, I can figure out how much more ventilation my attic might need for either one.
Best Answer
unless the fan is shrouded or ducted and sealed to your ceiling it will be quite hard to tell how much air is moved because much of the air could just recirculate around the fan.
the simplest way is to
Area = pi * radius^2
B = fan pitch in inches / 12
C = fan rpm
1 rpm = air moved so many feet at 0% slippage or 100% efficiency
max theoretical cubic feet per min = A * B * C
i don't know what % slippage would occur, that would be based on how well the fan blade is designed. Then account for however much regurgitation of air around the fan due to no shroud gap and not sealed to ceiling. this also doesn't account for any varying blade pitch (i.e. more pitch near center less near tip).
the easiest and cheapest way might be to build a 1-2 foot duct on output side of fan and grab some cheap wind speed gauge to measure the output air speed from fan. then math would be Area * speed; with wind speed converted to feet per sec.