Whole-House Fan CFM & Attic Vent Calculator
Size a whole-house fan by air changes per hour (5–40 ACH) or by 2–3 CFM per square foot, split it across several fans, check the attic net free vent area it needs (1 ft² per 750 CFM) from your vents, the window area to open (4 ft² per 1,000 CFM), minutes per air change, and the running cost, savings and payback against air conditioning.
How it works
CFM = floor area × ceiling height × ACH ÷ 60, or floor area × 2–3 CFM per ft² · attic net free area = CFM ÷ 750 ft² · open window area = 4 ft² per 1,000 CFM · minutes per air change = volume ÷ CFM · nightly saving = (AC kW − fan kW) × hours × price
A whole-house fan pulls cool evening air in through open windows and pushes the day's hot air out through the attic. It works by volume: a large fan replaces all the air in the house every two to four minutes. That is why it is sized in air changes per hour, usually 15 to 30, or by the manufacturers' shortcut of 2 to 3 CFM per square foot. Quiet insulated-damper fans that run all night can be as small as 5 ACH. All that air has to leave the attic, so it needs about one square foot of net free vent area for every 750 CFM. Without it the fan fights back-pressure and pushes attic air and dust into the house. The air also has to come in, and the U.S. Department of Energy's guide asks for at least 4 ft² of open window per 1,000 CFM. Count your soffit, ridge and gable vents to see whether the attic has enough. A large house can split the airflow across two or more fans. Because the fan moves air instead of removing heat, it costs a fraction of air conditioning to run. It only helps when the outdoor air is cooler than indoors and dry enough that it does not bring in humidity.
Worked example
An 1,800 ft² house with 9 ft ceilings holds 16,200 ft³. At 20 air changes an hour that is 5,400 CFM, one air change every 3 minutes. It needs 7.2 ft² of attic net free vent, so an attic with 6 ft² needs 1.2 ft² more: 3 more soffit vents or 10 ft of ridge vent. It also needs about 21.6 ft² of open window, for example 8 windows opened 3 ft² each. The fan draws about 0.75 kW. If it replaces a 3.5 kW air conditioner for 4 evening hours, it saves about 11 kWh a night, or roughly $59 a month at $0.18/kWh.
Sources: U.S. DOE Building America — Measure Guideline: Ventilation Cooling (ACH, 1 ft² NFA per 750 CFM, 4 ft² of window per 1,000 CFM, back-drafting) · QuietCool — whole-house fan sizing (2, 2.5 or 3 CFM per ft²) and attic venting
Frequently asked questions
Can I run it with the windows closed?
No. It will pull air down flues, chimneys and the water heater vent, back-drafting carbon monoxide, and it starves itself of air. Open windows first, every time. Interlocked models will not start otherwise.
How many CFM do I need per square foot?
Manufacturers use 2 CFM per ft² as good, 2.5 as better and 3 as best, assuming 8 ft ceilings. That is about 15, 19 and 22.5 air changes an hour. With taller ceilings the volume grows, so size by air changes per hour instead.
How much attic venting does a whole-house fan need?
About 1 ft² of net free area per 750 CFM, on top of normal attic ventilation. A 5,400 CFM fan needs 7.2 ft². A 16 × 8 in soffit vent gives roughly 0.45 ft² and ridge vent about 0.125 ft² per foot, but use the NFA printed on your vents. Pick Count vents to add them up.
Should I use one big fan or several smaller ones?
Either works if the combined airflow meets the target. Two-story or spread-out homes often do better with one fan per zone, such as one over the bedrooms and one over the living area, so each area gets its own breeze. Enter the number of fans to see the size of each.
Whole-house fan or attic fan?
They do different jobs. An attic fan only ventilates the attic space and does little for the rooms below. A whole-house fan ventilates the living space through the attic. In humid climates neither one replaces AC.
Related calculators
Sizing rules from HVI, ASHRAE 62.2, IRC/IMC guidance and common UK/EU building-regulation values. Local codes, manufacturer data and a heat-loss survey take precedence; ventilation and heating installations may require a qualified installer.