Whole house fans pull air from inside the house into the attic. Since air is pushed into the attic, it is important that the attic is equipped with enough exhaust vents, like ridge vents to avoid creating excess air pressure. Therefore, it is important that a house has either enough ridge ventilation or attic fans when installing whole-house fans.
Whole house fans are compatible with ridge vents as long as there is 1 square foot of ventilation for every 500 CFM capacity of the fan. For example, if you have a whole house fan with a rating of 4000 CFM the ridge ventilation should be more than (4000/500) = 8 sq.ft. If you have ridge ventilation for your attic I am pretty sure it has more area for attic vents than this.
Ridge ventilation needed (sq.ft) = CFM of whole house fan/500
It is also important to note that whole house fans are different from attic fans (Read attic vs whole house fans). Whether attic fans are compatible with ridge vents is an entirely different question. (See are attic fans compatible with ridge vents).
How did I reach this ridge vent area calculation?
According to Energy.gov, the attic ventilation needed for a whole house fan is 1 sq.ft per 750 CFM capacity of the fan. So for a fan of 4000 CFM, the attic should have a vented area of 5.34 sq. ft (4000/750). But in the case of ridge vents, the available ventilation is limited since they come with screens. Therefore we need to add 50% more area for the ridge vents to match the ventilation requirements of whole house fans.
That means; Ridge ventilation needed = (CFM of whole house fan/750)x1.5
Since 1.5/750 is 500 the above equation becomes;
Ridge ventilation needed = CFM of whole house fan/500
Here is a table listing the size of the whole house fan, the area it can cool, and the ridge ventilation needed;
|Whole house fan size||Area of house||Ridge ventilation needed|
|2500 CFM||1000 Sq.ft||5 sq.ft|
|4000 CFM||1500 sq.ft||8 sq.ft|
|5500 CFM||2000 sq.ft||11 sq.ft|
|7000 CFM||2500 sq.ft||14 sq.ft|
|9000 CFM||3000 sq.ft||18 sq.ft|
Below is a graph showing the ridge ventilation needed, based on the CFM of the whole house fan. As you can see it is a linear graph.
You can check my article whole house fan sizing guide to check what CFM your whole house fan needs to be.