Why is my attic fan constantly running?
Short answer. Most attic fans that never shut off have a thermostat set too low for a Georgia summer, a humidistat that keeps calling for the fan because the outside air is humid, or a failed thermostat or relay stuck on. A fan that runs constantly may also be fighting a ridge vent or weak intake and never managing to cool the attic.
Check the controls first
- Thermostat setting: most fans have a dial on the unit or nearby in the attic. Manufacturers commonly suggest a setting around 100 to 110 degrees. If it is set near 70 or 80, it will run all day and much of the night.
- Humidistat: many fans include one so the fan runs when attic humidity is high. In Georgia, outdoor humidity often stays high, so the fan keeps pulling in more humid air and never satisfies the setting. Many owners turn the humidistat off or set it very high in summer.
- Failed control: thermostat contacts can fail closed. If turning the dial all the way up does not stop the fan, the control is likely bad.
- Wiring: some fans were wired straight to a switch, bypassing the thermostat entirely.
Then check the attic
If the fan runs constantly at a sensible setting, it may be unable to cool the attic. That happens when intake is too small, so it moves little air, or when a ridge vent next to it offers a short path, so it just recirculates outside air near the peak. A fan running against weak intake can also pull conditioned air out of the house, which shows up on the power bill.
Safety
Shut off power at the breaker or the fan's switch before touching the controls. The attic can be dangerously hot on summer afternoons, so check it in the morning. A motor that runs nonstop wears out sooner, which is one reason most units include a high temperature cutoff. See attic heat and shingles and roof ventilation basics.