ECN Forum Forums General Topics of the Trade General Discussion Area Whole house fans

Assuming proper free vent areas, what size whole house fan would go in a 3500 sq. ft. house?
What about the amperage? Dedicated circuit?

Thanks!

Redsy:
Used to get that info from Chelsea Fan Company, don't even know if they are still around.
The advent of central air kind of killed the whole house fan thing.

Another source used to be in the Grainger catalog. They have a lot of interesting stuff in there.

John

Choosing the fan will depend on a lot of things, CFM you want to remove, duct and appature sizes, even the make up air sizes, etc. After you get that sorted out, you can determine wether or not it is in need of a dedicated circuit. But off the top of the head I would say so.... You might also want to think about how these would be controlled, heat, (FART), manual, zoned?

Oh, avoid getting into the duct-work or specifying the fan for them. I made that mistake for a few bath rooms in the past and ended up with a fan that really SUCKED! In fact it would pull doors shut. It also sounded like a jet engine.

I have 2 whole house fans we use a lot. They are 30" fans with 2 speed 1/4hp motors for ~1700 sq/ft but, both on high, they whistle the air through here.

Proper free vent areas is an assumption I wouldn't make. It's rare to have enough & without it the fan will fight the back-pressure. It will also force attic dust into the house, a real problem with allergies.

I tie a whole house fan in with a couple power roof vent fans. It cools the attic & house quickly. I did one a few weeks ago on the lighting circuit with no problems. The motors were all small.

Dave

Redsy -

I can't remember where I got the below, but it is a pretty good guide of how to determine what size of fan (CFM) you would need and the amount of attic vent area you would need.

"SIZING A WHOLE HOUSE FAN

Determining the amount of airflow in cubic
feet per minute (cfm) that the whole house
fan should provide involves a simple
calculation. Multiply the total gross square
footage of the house (include upstairs area)
by the ceiling height (typically 8 feet).
Select a fan that delivers between one-half to one time that amount of cfm at 0.1" static pressure.

For example, a 25' x 40' one-story home is
1,000 square feet and would need an 8 x
1,000 x 1/2 = 4,000 cfm fan or better. A
two-speed unit that delivers 4,500 cfm at the
high setting (240 watts) and 3,200 cfm at low
(120 watts) should be adequate.


A guideline for sufficient attic vent
area is one square foot of net-free
vent area per 750 cfm of fan airflow
(4,500CFM / 750 = 6 square feet attic vent area).

Continuous ridge and soffit vents are usually more than adequate. Always make sure that the vent area ae not blocked by insulation or things stored in the attic.

Vents with insect screens may have a net-free area equivalent to one-half of the total open area, depending upon the size of the holes in the screen area. Insulation should be installed directly against the fan box frame. Blown-in insulation may require the sides of the fan box to be raised (with baffles) to prevent interference."

Steve

Very good information Steve. IMO the continuous ridge and soffit vents are inadequate though. I matched up a 4500 cfm whole house fan (24") with two 1250 cfm power roof vent fans & it worked very well.

Dave

I ended up having to put gable vents in (added to the ridge and soffit vents) for my fans.
Even with that, I still overpressurize the attic enough to get that familiar attic smell from any other opening in the house when the fan comes on.
In our next renovation, after the rainy season we will have the roof open and I am putting in a coupola. The fans will probably get relocated there.

Thank you all for the input!