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I need ~8-10 sq ft of additional exhaust vents to accommodate a new 7600 cfm whole house fan. Uncertain if add'l soffit vents needed for adequate ventilation flow through attic. Planning to install 3 dormer vents near ridge. Two screened gable vents (2 ft x 1 ft) installed when house was built.

  • Is there a reason you're going for exhaust-only ventilation? Drawing a negative pressure on a building gets you uncontrolled infiltration (it's a good way to mine radon, too, haha) – ThreePhaseEel Jun 3 '17 at 17:46
  • "Whole house fans are designed to pull air from the hottest point in the house (the highest point in the living space), and replace it with cool air pulled in through the windows." –Whole house fan location. What's the question? There'll be so much air flow through the attic when the fan runs that additional intakes wouldn't matter. You need sufficient attic vents for when it isn't running though. Also, too many intakes in the attic would cheat the airflow from the rest of the house. – Mazura Jun 3 '17 at 18:17
  • I've read where too many exhaust vents vs soffit vents could disrupt pattern of airflow, causing air flow into attic via exhaust vents instead. Currently have ~3 sq ft exhaust vent vs ~1 sq ft soffit vent. Can't imagine that I'll be able to add add'l soffit vents to maintain same ratio if I'm adding ~8-10 sq ft add'l exhaust vents to satisfy the whole house fan requirement! – Grant Jun 3 '17 at 18:59
  • Is the fan ducted to a grill in the ceiling or what? How is air going to get from inside the house and into the attic? I'm still not sure what the question is. Is it, Do you need additional vents? - Do you want it to act as a whole house fan, as an attic exhaust, or both simultaneously? – Mazura Jun 3 '17 at 19:27
  • Fan will be installed over a ceiling joist in the attic. Windows and/or doors are cracked open in house for fan to move cooler outside air up into attic and forced out of attic via the exhaust vents. Hence the need for additional exhaust vents near roof ridge. BTW, considering O'Hagin "Tapered Low Profile" vents to be installed on asphalt shingle roof. Any feedback on these vents appreciated! – Grant Jun 3 '17 at 20:37
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If the TOTAL attic venting (ridge, soffit, gable, turbines, vent cans, etc. combined) is more than the fan requires, you're fine. If it's not enough, figure how much more you need to add to satisfy the fan, then balance that to provide 50-60% of the total attic ventilation as intake (soffits or low on the roof) and 40-50% as exhaust (gables or high on the roof). When you run the house fan, it can pressurize the attic and exhaust out of all the vents, including the soffits. Since you only run the fan a short time in the morning, before the attic heats up, there won't be any issue with air going out the soffits instead of in. When you turn off the fan and the attic heats up, the airflow will return to normal - in the soffits and out the ridge and gables.

A whole house fan needs 1 ft^2 NFVA per 750 CFM. A 36" whole house fan moving 7600 CFM from the house into the attic needs a total of 7600/750 = 10.13 ft^2 or about 1460 in^2 of net free vent area (NFVA) from the attic to the outside.

1460*40% = 584, 1460*50% = 730, 1460*60% = 876. Thus, you should aim to have 730 to 876 in^2 NFVA in your soffits and 584 to 730 in^2 for exhaust as high as practical.

FWIW, a combination of soffit & ridge vent (or near the ridge) is better than soffit & gables. The gables don't remove the hot air from above them or as effectively from the center of the house.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming! – Daniel Griscom May 2 at 16:15
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Hmmm...I think we're discussing two different things. Attic vents are in terms of "square inches." Room vents are discussed in terms of cfm.

Attic vents, Soffit vents, etc. are placed for "cross-ventilation" and generally not motorized.

Equipment measured in cfm is sized for make-up air, exhaust ventilation, etc. The 7600 cfm fan you mention should not be bringing in air from the attic nor exhausting air from the house into the attic.

If you have a ventilation system (HVAC system) that can bring in 7600 cfm "fresh air", then you need to exhaust about the same.

If you have an exhaust fan that exhausts 7600 cfm, then you need a "relief" air vent...it does not need to be motorized...just a relief grille (with damper).

  • No, it's not part of the HVAC, it's called a "whole house" fan and is popular in the western US where very low humidity, as in California where I reside. The fan is turned on manually early in the morning to pull cool air via open windows/doors to cool the entire house and evacuate the hot air within the attic. However, I need adequate exhaust vents in attic to handle 7600 cfm while the fan is running for 20 - 30 minutes each morning! – Grant Jun 5 '17 at 18:32
  • @Grant - What you need IMO, is the fan secured to the gable wall, or a duct running from it to that. You should not be "exhausting air from the house into the attic" (+1). You can pull air through the attic, but you shouldn't push air into it. – Mazura Jun 5 '17 at 20:29
  • @Mazura I don't get it...if the fan is mounted in the attic (widucting to house,) then the "exhaust fan" will pull air from the outside through the Soffit vents, roof vents, etc. I'm not sure any air would come from the house, even with vents between the house and attic, because the resistance will be less because it has to draw from around doors, windows, etc. If the fan is mounted in the attic WITH DUCTING – Lee Sam Jun 6 '17 at 0:09
  • WITH DUCTING then it will draw only from the house and it could be mounted anywhere...there is no advantage to mounting high in attic. Oops...I accidentally sent the first comment when I was trying to say: (without ducting to house)... – Lee Sam Jun 6 '17 at 0:14
  • Prefer not to duct the exhaust from whole house fan directly to exterior. Much better to pressurize the attic to force the hot air out! – Grant Jun 6 '17 at 22:12
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Okay looks like several folks have no idea what a whole house fan is or how it works. You need to check your fan rating to see how many CFM it will move. Then you need to take that number and divide it by 750 to see how many square feet of vent openings you need. The square feet of opening depends on the type of vent you have. The vents could be ridge vents, gable vents, turbine vents or any combination thereof. Check the square feet of opening for each vent you have and add it up. If it’s not more than the fan rating divided by 750, you need more. If you are installing a 2 speed fan, make sure you are using the higher cam rating.

  • Very true that it doesn't seem like people know what a whole house fan is. – swade Aug 7 at 17:20

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