About Whole House Fan Sizing Calculator (CFM & Attic NFA)
The whole house fan calculator sizes the classic night-flush cooling machine: a large ceiling-mounted fan that pulls cool evening air in through open windows, sweeps it through the living space, and pushes the hot air out through the attic vents. Sizing follows the rule the industry actually publishes — 2 to 3 CFM per square foot of living area, the "good / better / best" ladder in QuietCool's sizing guide, which sits inside the 1.5–3 CFM/ft² band in the DOE-backed Building America Solution Center guidance. Enter your floor area, pick an intensity, and the calculator returns the target airflow plus the number most buyers miss: the attic exhaust area. A whole house fan can only blow out what the attic can vent, and the published requirement is 1 square foot of net free vent area for every 750 CFM — typically 2 to 4 times the attic venting a house already has.
The air-changes-per-hour readout is a cross-check, not a second rule. At standard 8-foot ceilings, 2–3 CFM/ft² works out to 15–22.5 air changes per hour — squarely inside the 12–23 ACH range the Building America guide calls effective (the DOE Energy Saver page cites 15–23). The two published rules are the same rule seen from different sides; the ACH view earns its keep when your ceilings are not 8 feet. Tall great-room ceilings sink the ACH for the same CFM-per-square-foot, and the calculator flags it so you can size up a step, exactly as the manufacturer guidance suggests. One thing this tool deliberately is not: a cooling-load calculation. A whole house fan trades outdoor air for indoor air — it works when the outside air is cooler than inside, and it is not a substitute for sizing air conditioning against a real load.
How It Works
- Enter the conditioned floor area the fan serves and your average ceiling height, then pick a sizing intensity: 2 CFM/ft² is the published "good" baseline for milder climates, 2.5 the "better" all-around choice, 3 the "best" rate for hot climates or fast cool-downs.
- Target airflow = floor area × sizing rate. This is the CFM rating to shop for; whole house fans are sold by CFM, and many models offer two speeds or a CFM range, so treat the target as the high-speed figure.
- The cross-check converts that airflow into air changes per hour: ACH = CFM × 60 ÷ (floor area × ceiling height). The badge shows where you land against the 12–23 ACH effective band — below it (usually tall ceilings: consider the next preset up), within it, or above it.
- Attic exhaust: the fan pressurizes the attic, and the published requirement is 1 ft² of net free vent area (NFA) per 750 CFM of fan capacity. The calculator reports the NFA in ft² and in², plus the gross vent area to provide when vents are screened or louvered (about double, per the Building America guide) — take the NFA number to the attic ventilation calculator to turn it into ridge, soffit, or gable vents.
- Window opening: the intake side matters too — open windows totaling at least 4 ft² of screened area per 1,000 CFM while the fan runs, ideally in the rooms you want cooled.
Worked Example
A 1,500 ft² single-story house with 8-ft ceilings, sized at the "better" 2.5 CFM/ft²: target airflow = 1,500 × 2.5 = 3,750 CFM. The cross-check: house volume = 1,500 × 8 = 12,000 ft³, so ACH = 3,750 × 60 ÷ 12,000 = 18.75 air changes per hour — within the 12–23 effective band, as the 2–3 CFM/ft² rule always is at 8-ft ceilings. Attic exhaust: 3,750 ÷ 750 = 5 ft² of net free vent area (720 in²), or about 10 ft² of gross vent area if the vents are screened. While the fan runs, open windows totaling at least 3,750 ÷ 1,000 × 4 = 15 ft² of screened area.
Formulas
- Target fan airflow
CFM = floor_area_ft2 x rate- Air-change cross-check
ACH = CFM x 60 / (floor_area_ft2 x ceiling_height_ft)- Attic exhaust net free area
NFA_ft2 = CFM / 750- Gross vent area when screened
gross_ft2 = NFA_ft2 x 2- Open window area
window_ft2 = CFM / 1000 x 4
Standards & References
- Sizing rates are published industry practice, not code: 2 / 2.5 / 3 CFM per ft² of living area is QuietCool's good/better/best sizing ladder, and the DOE-backed PNNL Building America Solution Center guide "Whole-House Fans" brackets effective sizing at about 1.5–3 CFM/ft² of floor area (equivalently 0.2–0.375 CFM per ft³, or 12–23 air changes per hour)
- The attic exhaust requirement — 1 ft² of net free vent area per 750 CFM, roughly 2–4 times normal attic venting — is published by DOE Energy Saver ("Cooling with a Whole House Fan"), PNNL Building America, and manufacturer sizing guides alike; screened or louvered vents need about double the gross area
- A common marketing claim of "a complete air change every 1–2 minutes" would be 30–60 ACH — that is a turnover boast, not a sizing rule; every published sizing source lands in the 12–23 ACH range instead
- A whole house fan is ventilation cooling, not thermal design: it only helps when outdoor air is cooler and drier than indoor air, and it does not replace a heating/cooling load calculation for equipment sizing
Frequently Asked Questions
How does a whole house fan actually work?
It is a large fan mounted in the ceiling between the top floor and the attic. With several windows open, the fan pulls outdoor air in through the windows, sweeps it across the living space, blows it into the attic, and forces it out through the attic vents — cooling the house and purging the superheated attic in one move. Two operating rules follow directly: windows must be open while it runs (running it buttoned-up can depressurize the house and backdraft combustion appliances), and it only helps when the outside air is cooler than the inside air — which is why the classic use is the evening and overnight "night flush" in climates with cool nights, storing coolness in the house structure for the next day.
Why does the attic vent area matter so much?
Because the attic is the fan's exhaust pipe. Every cubic foot the fan moves has to leave through the attic's ridge, gable, or soffit vents, and if the openings are too small the attic pressurizes, the airflow chokes well below the fan's rated CFM, and the motor works hot for less cooling. The published requirement is 1 square foot of net free vent area per 750 CFM of fan capacity — for most fans that is 2 to 4 times the venting a code-minimum attic already has, so plan on adding vents. "Net free" area is the actual open area after subtracting grilles, screens, and louvers; insect screening roughly halves an opening, which is why the calculator also shows a doubled gross-area figure. Use the attic ventilation calculator to turn the NFA requirement into a vent count.
What is the difference between a whole house fan and an attic fan?
They sit near each other but do different jobs. A whole house fan moves air through the living space — windows open, air drawn through the rooms, exhausted via the attic — and cools the people and the house itself. An attic fan (powered attic ventilator) sits on the roof or gable and only exchanges attic air with outdoor air; the living space is untouched except by reduced heat gain through the ceiling. If your goal is to cool the house on cool evenings, you want the whole house fan. If your attic overheats in summer, passive attic ventilation sized correctly usually beats a powered attic fan. This calculator sizes the first kind; the attic ventilation calculator handles the venting for either.
Should I just buy the biggest CFM I can? What is the noise trade-off?
Bigger moves more air but is not free: high-CFM fans cost more, need proportionally more attic vent area (another 1 ft² of NFA per 750 CFM), and — with traditional direct-mounted fans — get loud, since noise rises with airflow and blade speed. The published good/better/best ladder exists precisely so you can pick the trade: 2 CFM/ft² cools effectively on mild evenings, 3 CFM/ft² gives the fast pull-down that hot-summer climates want. Two common ways to have both: a two-speed or variable-speed fan sized at 2.5–3 CFM/ft² that runs high for the initial flush and low (and quiet) overnight, or the modern ducted/insulated-damper style fans that hang from the rafters and are dramatically quieter than old direct-mount units at the same CFM.
Can a whole house fan replace my air conditioner?
Sometimes on the mildest days, but that is the wrong question to settle a purchase with. A whole house fan can only ever bring the house down toward the outdoor temperature — it is superb in climates with hot days and reliably cool, dry nights, and nearly useless on humid nights that stay warm, because it imports whatever humidity is outside. Air conditioning is sized against an actual heating/cooling load calculation, and nothing here substitutes for that — see the HVAC load calculator for equipment sizing. The honest framing: a well-sized whole house fan slashes the hours the A/C needs to run in the right climate, often paying for itself quickly, but it is a complement to cooling design, not a replacement.