It's troublesome to turn off the exhaust fan and feel a cold draft coming back through the exhaust system. This might be a problem in the bathroom, kitchen, attic or wherever exhaust fans are installed. The solution is to install an inline backdraught damper. This damper opens automatically when the exhaust fan is blowing air, and it closes by itself when the fan is switched off. It's a sure way to keep the cold air from backing up through the pipes.
Turn off the circuit breaker to the exhaust fan. Remove the exhaust fan cover. Most covers come off by prying the edge with a flathead screwdriver, although you may have to remove some screws. Remove the exhaust fan motor and fan unit by unscrewing the screws that hold it to the fan housing. Remove the exhaust fan housing (the metal box that holds the motor and fan) by unscrewing the screws that attach it to the wood framing.
Measure the exhaust pipe behind the fan unit. Purchase an inline backdraught damper in the same diameter as the exhaust pipe.
Use the saw to cut open the wall or ceiling, if necessary, to give yourself enough room to work on the exhaust pipe. Using a hacksaw, cut off enough exhaust pipe to fit the inline backdraught damper in place of the pipe you removed.
Attach the inline damper to the existing pipe with two sheet metal screws and duct tape, with the airflow arrow on the damper pointing away from the exhaust fan. Attach the exhaust fan housing to the inline damper with two sheet metal screws and duct tape.
Repair the wall or ceiling with drywall and plaster, if you cut it open. Replace the exhaust fan, motor and cover. Turn on the circuit breaker and test the fan.
It's important to use an inline damper, since the exterior shape and dimensions of the damper are the same as the pipe. This will make it possible to fit the damper in tight places.