How do attic fans work?
Short answer. A powered attic fan uses a thermostat to switch on a motor when the attic gets hot; the blade pushes attic air out through a roof or gable opening, which lowers the attic's air pressure and pulls replacement air in. Whether that air comes from the soffit vents or from inside your house decides whether the fan helps or hurts.
The parts
- Thermostat: a sensing control mounted on the fan or a nearby rafter. When attic air passes the set temperature, it closes the circuit.
- Motor and blade: the motor spins a blade inside a housing under a dome on the roof, or behind a gable louver.
- Dome or shroud: keeps rain out while letting air escape around the edges.
- Optional humidistat and a high temperature shutoff that stops the fan in a fire.
The airflow
- The blade pushes air out of the attic.
- Attic pressure drops below the outdoors and below the house.
- Air rushes in through every available opening to replace it.
With generous soffit vents, outside air comes in at the eaves and sweeps the deck. With little intake, the lower pressure pulls air from the house through ceiling gaps, light fixtures and duct leaks, and a fan near a ridge vent can pull air in through the ridge itself.
Solar and gable versions
Solar fans use a panel and a DC motor, running only when the sun is strong. Gable fans mount behind a gable louver and pull air through the attic toward that end, avoiding a roof penetration.
Not a whole house fan
A whole house fan pulls air from inside the house through open windows and dumps it into the attic, a different job.
Compare fans with passive venting in ridge vent vs box vents and roof ventilation basics.