If your vent fan does not have a built-in switch to reverse the direction in which the fan's blades rotate, you can manually require the fan to run in a reverse polarity. Vent fans, such as those found in attics, run off 110-volt AC (alternating current), and you can reverse the wire connections on the motor's coils to achieve this goal. This will allow you to pull air into a room instead of pulling air out of a room.
Unplug the fan, or turn off the breaker that controls the fan.
Remove the fan blade shroud from the fan housing to expose the motor. In most cases, the shroud attaches to the housing with screws. This exposes the motor inside the housing. You may be required to remove additional components to access the fan motor in your particular vent fan. However, in most cases, you will need a simple screwdriver and/or wrench set to remove the components.
Locate the housing that covers the coils of wire. Remove the housing bolts from the cover, and then remove the cover from the coils. Generally, the housing is made of cast metal, and you will need a wrench to remove the nuts that hold the housing together. However, depending on your specific model, the housing could be plastic and may require a screwdriver to remove the screws that hold the housing together.
Locate the electrical wires that connect to each coil. The wires should have been tucked inside the housing cover you removed. Loosen the connectors that hold the wires onto the coil terminals, and then switch the wires to the opposite coils. This reverses polarity and forces the motor to run backward.
Reattach the housing to the coils, using the original fasteners and the wrench, and then reassemble the fan by placing the fan shroud back on the fan housing. Secure it with the original fasteners and the screwdriver.
Reconnect power to the vent fan vent by plugging it into its electrical outlet or by turning on the electrical breaker.