An attic fan thermostat is typically supplied with the fan unit. If the thermostat fails, replacement units are available online and from local electrical supply dealers. Selection is limited, because an attic fan thermostat is a relatively simple device, and the best one to choose is one that fits within your budget and does what it is designed to do. When attic temperatures reach a certain level, the device turns the fan on to remove hot air from the attic. The more expensive units are more accurate and dependable but not necessarily your best choice.
Failure of an attic fan thermostat is unusual, so before replacing the thermostat, check the fan motor and fan blades. If the unit is functioning properly, without excessive vibration or noise, then the attic fan thermostat is the problem. A limited selection of these units are available, and the prices vary. An inexpensive or mid-priced unit should provide years of service, outlasting the motor. Expensive units might be more accurate with better components.
Precision accuracy is not necessary for the removal of hot air from an attic. The best attic fan thermostat is the one that does the job. Before replacing the thermostat, though, be sure to check the other components.
To check whether the fan blades and motor are working, first turn off the power supply to the fan unit at the breaker box. The fan blades should spin freely when turned by hand. If the fan blades do not spin freely, then the entire unit will need to be replaced. Roof-mounted units and gable end-mounted units are available from the nearest home improvement supplier.
If the fan blades are spinning freely, the electric motor should be tested. Use an electrical testing device to be sure that the power supply to the fan is off. Remove the cover from the thermostat, and undo the wiring. Rewire the fan, bypassing the thermostat. Turn the power supply on at the breaker box.
After this procedure has been done, the fan unit should be working. If it is not working, the motor and blade unit will have to be replaced. It is possible to replace the motor and blade unit in the existing roof or gable vent housing. This procedure would eliminate unnecessary exterior work. A new thermostat should be supplied with this assembly.
You will have to measure the existing housing to find a replacement motor and blades that will fit. If the same brand is available, it might be cost effective to buy the entire unit and just replace the motor, blades and thermostat. This can be done indoors without working on the roof, as long as the existing housing is in good shape. Replacement motor assemblies are available online and from local electric suppliers.