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.
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.
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).
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.