Compensate inadequate exhaust vents for whole house fan by installing a power vent in attic?

Question

I would like to install a whole house fan in my 2400 sf house. However, with only three 1 ft x 2 ft gable vents, I'm looking at an NFVA of only 3 sf of exhaust vent when taking into consideration that they are screened!

I'm assuming that I would need ~10 air exchanges per hour to get a "breeze effect with the fan turned on. However, I would need an additional 4sf of exhaust vent: 3500cfm/500cfm/sf = 7sf - 3sf existing exhaust vent.

I currently have 1.17sf of soffit intake vents (84 3" holes under eaves) vs 6sf exhaust vents(3 gable vents). Since these vents should actually be same ratio, by adding 4sf of additional exhaust vents, the ratio would be even more skewed!

The Quiet Cool website suggested that an attic power vent could be run in tandem with a whole house fan and eliminate the need for additional exhaust vents to be installed on the roof. With that said, would this be the best solution for my dilemma?

Below is a picture indicating the type house ventilating fan I am considering

Answer 1

It sounds like you’re trying to cool your house without increasing your air conditioner size (or add air conditioning) by 1) adding air movement in your house, and 2) lower the air temperature in your attic, which could be “heating” your house.

1. Yes, adding air movement (even warm air) will give you the feeling of cooling. That’s why sitting in front of a fan feels cool. However, that air needs to blow across your skin to be effective. (I think you’ll need to get in your shorts and wear short sleeves. )

2. Stagnant air in the attic will heat up throughout a warm day, so increasing the air movement in the attic is probably good.

However, your notion about “intake vents” and “exhaust vents” is not quite correct. Actually, the small 3” round vents in the eaves are not just “intake vents”...they’re “exhaust vents” too. Your house (especially where you live) has a windward and leeward side. This creates positive air pressure and negative air pressure on your house. Openings (vents) allow air in and out. Generally, we like these openings balanced. That is to say about equal sized.

However, the Code has established minimum sizes to prevent moisture condensation...not for cooling requirements.

The minimum amount of venting by Code is 1/150th the area for CROSS VENTILATION or 1/300th the area if you have cross ventilation AND between 40% and 50% the area is located not more that 3’ below the ridge. (See IRC R806)

It sounds like your attic ventilation meets this requirement. (A 2400 s.f. House will require 2400/300=8 s.f. and you have about 4 s.f. Ridge vents. I know you think you have 3 s.f., but I think you’ve compensated too much for the screening in the vents.) (BTW, I think you have 4.1 s.f. of venting from the 3” holes not 1.17s.f. that you think you have. Did I do that right?)

In summary,

1. I think you have adequate attic ventilation and increasing it will give you a minor benefit
2. power venting your living space can help, but you’ll need to sit close to the fan.

Alternate idea: In Hawaii and the Caribbean, they remove the exterior walls and rely on the tradewinds. Now, you probably can’t remove your walls, but you live where there is plenty of wind. Can you enlarge windows/doors to capture these winds? If not, then add a fan...but provide the same amount of “intake” as “outlet” available.

Answer 2

I would increase the size of the gable vents. I made about a 32" X 7' gable vent ( can't remember the roof pitch) for a 2400sf size house; It worked fine.I would not consider adding a power vent to the attic, the house fan has sufficient power. I have had 3 houses with attic/whole house fans and I strongly recommend a belt drive rather than a direct drive to reduce fan noise. In my present house ( same size) there is 2" wide strip in most of the soffit and traditional plastic ridge vents. They provide enough venting for the fan .

Answer 3

Thanks for the picture, it helps.

There are two issues: 1) attic ventilation, and 2) house ventilation.

1) Attic ventilation is based on CROSS ventilation. If you have the correct amount of vents installed and the attic is still hot, you can add a mechanical ventilator and “exhaust” air out of your attic.

Yes, it appears you have the correct amount of ventilation in your UPPER roof (if all the vents are open and not blocked). However, you do NOT have CROSS ventilation in your LOWER roof. Also, I doubt if you have the correct amount of vents (in square feet) in your lower roof, much less it’s not CROSS ventilation.

I’d install a thermometer in each attic and test for a few days. I think you’ll find that the lower roof is the problem. (That lower roof should not be venting into the garage. )

I’d stop referring to exhaust vents too. It’s all about cross ventilation so some are intake one day and exhaust vents another. If you install a mechanical vent in the attic, it’s an EXHAUST vent. It does not push air into your attic so you don’t need to worry about “exhaust” vents.

2) House ventilation is SEPARATE from attic ventilation and has very little to do with each other.

Yes, if you cool the attic, the house should cool too. However, with attic insulation it would need to get really hot in the attic before it could affect the temperature in the House very much.