How to Wire a Thermostat to a Vent Fan

Wiring a thermostat to a vent fan is a matter of convenience for automatic operation of the fan. A typical use for this application would be in the case of an attic fan to remove unwanted hot air from the confined space. Consider the type of thermostat that is employed. Do not use regular furnace thermostats because the internal contacts are not capable of handling the line voltage. Only a line voltage thermostat made for cooling must be used for wiring a vent fan into the thermostat.

Things You'll Need

• Wire pliers
• Pocketknife
• Wire strippers
• Electrical box
• 1-inch long screws
• Wire nuts
• Screwdriver
• Line voltage thermostat for cooling

Instructions

1. Remove all electrical power from the vent fan. Perform this task by either shutting off the circuit breaker or pulling the fuse that supplies the power to the vent fan. Typically the vent fan is wired by a cable assembly that is covered with a plastic sheathing. Under the plastic sheathing are three wires. Two of the wires are covered by a black and white, one each, insulated covering. The third wire is bare uninsulated copper.

2. Cut the wire cable assembly in two with the wire pliers. Make this cut at the location you chose for the thermostat. Use the pocketknife, and remove approximately six inches of the outer plastic sheath from both ends of the cable assembly. Reveal approximately one inch of the bare copper wire from under both the white and black insulated wires with the wire strippers.

3. Mount the electrical box to a wood frame member using the 1-inch long screws and the screwdriver. Insert the two cable assemblies through the wire insert holes on the box. You will have to pull some extra cable so all of the separate wires will fit fully inside the box. There should be extra cable to pull enough of the wire into the box to make connections to the thermostat.

4. Use the wire pliers, and twist together the two bare ends of the white insulated wire. Cover the bare copper end with a wire nut. Twist the ends of the bare uninsulated copper wires together in the same manner. Cover the end with a wire nut.

5. Connect the thermostat in the following manner. On the rear of the thermostat will be two screws. One screw will be identified as "line" and the other as "load." Connect the black wire that leads to the vent fan to the screw terminal labeled "load." Attach the black wire coming from the electrical panel to the screw marked as "line."

6. Secure the thermostat to the electrical box using the manufacturer's supplied mounting screws. Reenergize the circuit breaker, or place the fuse back into the holder. Set the thermostat to the desired "on" temperature.

Tips & Warnings

• Follow all local electrical codes whenever servicing or installing any electrical devices.

References

• Wire a Line Voltage Thermostat (PDF)
• Line Voltage Thermostats

Resources

• Greenhouse Forum