The most common annoyances in bathrooms are moisture and odours. A properly working exhaust fan will eliminate fogged windows, mirrors and sweating walls caused by hot shower water, as well as mildewy or bodily smells. Often, the exhaust fan is paired with a light fixture to save space. The fan and light can be connected to the same switch or separate ones.
Shut off the power to the bathroom. Turn the light switch on and off to make sure the light does not come on.
Remove the existing fan or light fixture and clean ducts. If this is a new installation, make sure the duct runs to the outside of the home, not exhausting into a crawl space or the attic.
Remove the switch and detach the wires from it.
Securely attach the new Romex to the existing wire with electrical tape. Pull the wires that were attached to the switch through until the new wire end appears in the switch box.
Install the fan housing. Use wood screws to attach directly to a ceiling joist if possible. If not, install a 2-by-4 between two joists and attach to that. The vibration from the fan makes this a necessity.
Attach the duct to the fan housing. Run the wires into the wiring box portion of the fan housing.
Make the electrical connection to the light and fan fixture according to the manufacturer's instructions. Connect the bare ground wire to the grounding clip on the fan housing. Connect the other wires to their corresponding wires on the fixture using wire nuts.
Push wires into the wiring box in the housing and attach the cover. Install the fixture in the fan housing.
Connect a small pigtail of bare copper wire to a grounding screw in the back of the switch box.
Attach the ground wire coming from the fan, from the electrical box, and another small pigtail to the pigtail from the ground screw together with a wire nut.
Attach the free pigtail to the switch's grounding screw.
Attach the remaining wires to the switch. Attach the white wire to the silver screw, and the remaining two wires to the brass screws on the double switch.
Install the switch in the box and replace the cover. Turn on the power and test.
Purchase a fan that is the right size for your bathroom. To find the size, multiply your bathroom's square footage by 1.1. This number is rating in CFMs (cubic feet per minute) your fan needs.