When outdoor air drops below your comfort setpoint (~72°F), a whole-house fan can replace hours of central A/C — at a fraction of the wattage.
System & Climate
Operational window: Evenings when outdoor temps fall below ~72°F — swap compressor cooling for attic ventilation to flush heat and pull in cool air.
Evenings per year when outdoor air can cool the home below 72°F.
Running Cost Comparison
Net seasonal utility savings
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Hours shifted from A/C compressor to ventilation fan.
Central A/C draw
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Whole-house fan
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| Period | Central A/C | Vent fan | You save |
|---|---|---|---|
| Calculating… | |||
Practical tips & common mistakes
Only when outdoor < indoor
Run when outside air is cooler and drier. Humid nights can increase A/C load the next day.
Running with windows cracked on one floor
You need enough open window area (typically 2–4× fan CFM in sq ft) or the fan pulls from attics and wall cavities.
Seal the damper in winter
Whole-house fans are giant holes in your thermal envelope. Insulated cover or damper leakage negates seasonal savings.
Not a whole-home substitute
On 90°F afternoons the fan adds heat. Model shoulder evenings only — exactly what the favorable-days slider represents.
Note: Assumes each favorable evening the fan fully substitutes A/C runtime (no mixed-mode hours). Central A/C watts scale 3,500–5,000 W by tonnage. Fan-only ventilation requires safe outdoor temps, low humidity, and adequate attic venting — not a replacement for A/C on hot, humid days.