Why Do I Blow a Fuse When I Adjust the Thermostat on My Attic Vent Exhaust Fan?

About Attic Fans

• Attic temperatures can reach over 150 degrees Fahrenheit on a sunny day. Passive ventilation from screens in the soffit and shielded vents on the peak of a roof or in attic eaves continually circulate cooler air throughout the attic. Motorized attic vent fans improve this air flow, expelling up to 1,500 cubic feet of hot air per minute, effectively cooling the attic further. Thermostats start and stop attic roof vent fans at temperatures set by the manufacturer. Attic vent fans powered by solar cells on the roof cut electrical costs, but lack the power and dependability of fans powered by an AC house current.

Whole House Fans

• An attic fan run by a programmable downstairs thermostat cools the house and ventilates the attic. Whole house fans installed in the ceiling of the living area activate at a temperature selected by the owner and pull outside air through the house and into the attic. Whole house fans turn off when temperatures climb uncomfortably high. Homeowners close windows, shutter the fan and turn on the air conditioning system in hotter weather. Whole house fans can push up to 12,000 cubic feet of air through the attic every minute and may require a dedicated power circuit.

Blown Fuses

• When fuses blow on the circuit powering newly installed attic fans, check for faulty wiring. Poor connections allow wires to slip after assembly. Incorrect wiring can also cause shorts. If other appliances run off the same electrical circuit, adding a powerful attic fan can overload the wiring. Home appliance circuits typically blow fuses at 20 amps, but run safely at 18 amps and below. Fan motors draw the most power when starting, and can cause a temporary surge that blows the fuse. The safe current load in older homes might fall to 15 amps or below because of aging wiring.

Other Problems

• Attic vent fans often include thermal overload protectors that cut power when the fan motor overheats. If loose insulation blocks the fan intake, the motor temperature spikes and the fan shuts down. If the intake vents in the attic can't emit the same volume of air the fan expels, the extra resistance can cause thermal overload. A roof vent fan pushing 1,500 cubic feet per minute needs a total intake vent area of 5 square feet, according to Air Vent Inc. Whole house attic fans might malfunction if the attic vents can't handle the outflow. Insufficient attic venting can burn out fan motors and cause blown fuses.