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
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 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 equaltion 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.