calcs.homes
calcs.homes
Enter your home's square footage and ceiling height to size an attic-vented whole house fan — a much bigger airflow job than a single room's exhaust fan.
Estimate only — also confirm your attic has enough vent area to exhaust that airflow.
We multiply house square footage by ceiling height to get house volume in cubic feet, then multiply that by your target air changes per hour and divide by 60 to convert from changes-per-hour to changes-per-minute — the unit CFM (cubic feet per minute) requires. The default target is 2 air changes per hour, meaning the fan should be able to replace the home's entire air volume twice every hour.
2-3 air changes per hour is the standard sizing target for whole house fans because it's fast enough to meaningfully cool a home by pulling in cooler outside air once evening temperatures drop, without oversizing the fan beyond what the attic can safely exhaust. This is a different calculation from a single-room exhaust fan, which only needs to clear the air in one space — a whole house fan has to move the volume of the entire home.
2-3 ACH is the standard target — enough to fully replace the home's air every 20-30 minutes, which is effective for cooling in the evening once outdoor temperatures drop.
Yes — insufficient attic ventilation can cause backdrafting or reduced airflow. As a rule of thumb, aim for at least 1 sq ft of net free vent area per 750 CFM of fan capacity.