How does an attic ventilation fan with a thermostat work, and how should it be set?
Short answer. A thermostat on an attic fan turns the motor on when the attic reaches a set temperature and off when it cools below it; many installers set it somewhere around 100 to 110 degrees Fahrenheit. A humidistat, often in the same control, can also run the fan when attic humidity is high, which needs care in a humid climate like Georgia's.
How the control works
The control box mounts on a rafter or on the fan housing, with a sensing element exposed to attic air. You turn the dial to the trip temperature. When attic air exceeds it, the fan runs; it shuts off a few degrees below the setting so it does not cycle constantly. Combination controls add a humidity dial that runs the fan above a set relative humidity, regardless of temperature.
Practical settings
- Too low, for example 80 to 90 degrees, and the fan runs most of the Georgia summer, including mornings, adding wear and electric use.
- Too high and it seldom runs at all.
- Humidistat: be careful in summer, when outdoor air is humid. A humidistat set too low can run the fan through muggy nights and pull in damp outside air without much benefit.
Signs the thermostat has failed
- The fan never runs on a hot afternoon, but starts when the dial is turned all the way down.
- The fan runs all night or all winter.
- Clicking or humming at the control without the fan starting, which can point to the motor or capacitor instead.
Replacement controls are inexpensive, but they are line voltage devices, so have an electrician handle the wiring if you are not qualified. Also keep in mind that a thermostat cannot fix the bigger issue: without enough soffit intake, the fan pulls conditioned air up from the house. See roof ventilation basics and attic heat and shingles.