Most powered attic fans are factory-set around 95°F to 100°F, and that documented setting is usually the right starting point — not the lowest number the dial allows.
A thermostat set too low makes the fan run constantly in mild weather, and Broan-NuTone’s Model 345 manual states plainly that lower settings can shorten motor life. The correct number is the one specified for your exact fan model, so the first move is to find your model’s label and its installation guide. What follows covers the documented ranges, why the factory setting usually wins, and the installation factors that matter more than a few degrees of dial position.
What Temperature Should An Attic Fan Thermostat Be Set At?
The documented setting depends on the fan: Broan-NuTone’s Model 345 ships set to 95°F, while pro-grade attic fans are commonly adjusted between 80°F and 100°F in summer. Check the model manual before turning the dial.
The Broan-NuTone Model 345 installation guide calls 95°F the factory setting for optimum operating efficiency and explicitly warns that lower settings shorten motor life. That manual’s thermostat scale includes markings at 75°F, 86°F, 95°F, 104°F, and 111°F, plus additional positions between them. The fan shuts off automatically once attic temperature drops 10°F below the selected setting — so a 95°F setting means the fan runs until the attic hits 85°F.
Pro attic fan documentation takes a wider view. It recommends 80°F–100°F during summer and 60°F during winter, where the winter setting exists for ventilation and ice-dam protection rather than cooling. Those fans have a thermostat adjustable from 60°F to 120°F, turning on at or above the selected temperature.
A general range of 100°F–110°F circulates widely online, and some guidance puts 110°F forward for maximum energy savings. Treat those numbers as rules of thumb, not specifications. One brand’s thermostats reportedly ship set to turn on at 100°F and off at 85°F, which is a different control scheme entirely — a fixed on/off spread rather than a single activation point.
| Fan / Thermostat | Setting Or Range | Note |
|---|---|---|
| Broan-NuTone Model 345 | 95°F factory setting | Shuts off 10°F below setting; lower settings can shorten motor life |
| Broan-NuTone Model 345 dial | Marks at 75°F, 86°F, 95°F, 104°F, 111°F | Additional scale positions between marks |
| Pro attic fan (PRO-1.3 / gable fan) | 80°F–100°F summer; 60°F winter | Adjustable 60°F–120°F; fire fuse cuts power above 182°F |
| Cool-only attic-fan thermostat | Adjustable 80°F–130°F | 120 V AC, SPST, 10 A, close-on-rise, bimetallic sensor |
| One brand’s reported factory setup | 100°F on / 85°F off | Adjustable 60°F–120°F; different control scheme |
| Common general guidance | 100°F–110°F | Not universal manufacturer guidance |
| Minimum inlet venting | 2–4 sq ft per fan | Separate from thermostat setting |
How Do You Adjust The Thermostat?
Adjustment is mechanical on most units: insert a screwdriver and turn the dial until the pointer sits at your target temperature. The selected temperature becomes the fan’s activation temperature, so the fan runs when the attic reaches or exceeds it.
The steps look like this:
- Cut power to the fan at the breaker before opening the thermostat housing.
- Locate the thermostat dial on the fan housing, usually a small recessed dial with a temperature scale printed around it.
- Insert a flathead screwdriver into the dial slot and turn it to the target mark.
- Restore power and confirm the fan kicks on once attic temperature passes the setting — or wait for a hot afternoon and listen.
The manual recommends safety glasses and gloves for any installation, servicing, or cleaning work around the unit. If you’d rather replace a failing thermostat than wrestle with an old dial, a tested roundup of attic fan thermostat models worth comparing covers the common fitments and electrical ratings.
Electrical compatibility matters more than most people expect. Thermostats differ in voltage, switching configuration, amperage, and mounting. A typical cool-only attic-fan thermostat is a 120 V AC, 10 A, SPST device with a bimetallic sensor — substituting a different rating or a heating thermostat can fail outright or create a hazard. Verify the ratings on your existing unit before buying a replacement.
What Are The Common Setting Mistakes?
The costliest mistake is setting the thermostat too low in hopes of a cooler house. Excessive runtime raises electricity use, and on Broan-NuTone’s Model 345 the manual links low settings directly to reduced motor life.
The second mistake is applying a generic number without checking the manual. Documented recommendations span 60°F winter operation to 110°F energy-saving operation, and the thermostat hardware itself may only adjust across part of that span. A setting that’s right for one fan can be wrong or unreachable on another.
Insufficient intake ventilation quietly defeats any setting. The manuals specify 2–4 square feet of inlet vents per fan — an exhaust fan with nowhere to draw air from just strains without moving heat. Separately, documented fire fuses cut power above 182°F to keep a running fan from contributing to fire spread. Never bypass one.
When the obvious fixes don’t help, the realistic answer is that the fan’s capacity or the attic’s venting is the limit — a thermostat dial won’t fix either. Start with the factory setting, confirm your inlet venting, and only then experiment.
FAQs
Is 110°F Too High For An Attic Fan Thermostat?
Not necessarily. Some guidance recommends 110°F for the best energy savings, because the fan only runs during the hottest part of the day. The trade-off is less ventilation during moderately warm weather. Check your model’s manual first — 110°F may sit outside a given thermostat’s adjustment range.
Why Does My Attic Fan Run At Night?
A thermostat set too low can trigger nighttime runtime if the attic stays warm after sunset, since stored heat lingers in roof decking and insulation. Raising the setting toward the manufacturer’s recommendation usually stops it. Persistent night running can also point to a stuck thermostat or a miswired control.
Can I Use A Regular Home Thermostat On An Attic Fan?
Usually no. Attic fan thermostats are typically 120 V AC, 10 A, SPST devices built for a cool-only, close-on-rise application. A household HVAC thermostat is designed for low-voltage switching and a different wiring scheme, so it won’t operate the fan correctly and may be unsafe.
References & Sources
- Broan-NuTone. “Model 345 Installation Guide.” Supports the 95°F factory setting, the 10°F shutoff differential, and the motor-life warning.
- Pro Attic Fan (AFR PRO-1.3 / Pro Attic Gable Fan). “Installation and Operating Instructions.” Supports the 80°F–100°F summer and 60°F winter recommendations, the 60°F–120°F adjustment range, the 182°F fire fuse, the 2–4 sq ft inlet requirement, and the screwdriver adjustment procedure.