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A properly working bathroom exhaust fan replaces contaminated air in one room with fresh air from a neighboring room or hallway. A bathroom without proper ventilation suffers with foggy mirrors and unpleasant odors. An open window helps air out the room. This works fine unless the next person needs to use the bathroom in a hurry. Fortunately, many homeowners find it possible to install a bathroom exhaust fan in their home.
Before starting this project, acquire the appropriate permits and verify the local building codes and your homeowner's insurance do not require a licensed contractor for this type of task, particularly the electrical portion. Use the building codes to clarify certain requirements, such as the acceptability of a soffit discharge vent. An experienced DIY homeowner who uses common sense, works safely and refers to the manufacturer's instructions when questions arise often completes this project within a single day without any problems at all.
When shopping for a new bathroom exhaust fan consider the common options available, such as light, nightlight or heater. Often the benefits of these features outweigh their expense. However, keep in mind that many of these options require additional wiring, or an electrical circuit upgrade. Most bathroom ventilation fans use a 15-amp circuit breaker and 14-gauge wiring. Due to the increased power drawn, fans with heaters usually require a 20-amp circuit breaker and 12-gauge wiring. Use the manufacturer's electrical specifications to confirm this.
Pay particular attention to the fan's cfm and sones ratings. The cfm, cubic-feet-per-minute rating measures the amount of air a fan moves and the sone rating identifies the unit's noise level. For an exhaust fan to reach these ratings, the unit must connect to a properly sized duct system. For instance, connecting a unit to a 3-inch duct when its specifications call for a 4-inch duct increases air restriction and vibration. A ventilation fan pulls air out of the room; however, a ductless fan simply circulates the air back into the room.
Completely laying out the system helps the installer identify any issues that may arise later. Installers who blindly cut a hole in the bathroom ceiling without laying out the system first sometimes run into trouble when they discover an unseen obstruction, such as a plumbing vent or air conditioning duct, in the way. A quick trip into the attic will locate these items beforehand.
Turn off the circuit breaker controlling the fan's power source. Keep the circuit breaker off for the remainder of this project. Follow all electrical safety rules and when in doubt, stop and call a licensed contractor.
Cut a 6-inch square hole in the drywall about halfway between the ceiling and the switch box. Reach a hand into the hole and feel around for restrictions, such as a fire block. Feed the fan unit's wire set from the attic down through the wall and into the switch box, using the hole in the drywall for access. Push about six inches of wire into the switch box. Run the wire set toward the hole in the ceiling for the fan housing. Cut the wire set about one foot longer than needed. The extra wire makes it easier to install the housing.
If using two or more switches to control the unit, attach a four-inch long section of insulated wire to each switch wire terminal, including the ground terminals. Combine all white wires in the switch box and cover the ends with a wire nut. Press this group of wires to the back of the switch box. Group the bare copper wires and the wire leads connected to the switch's green-colored ground terminals together and cover the ends with a wire nut. Push the ground wires against the white wire set in the switch box, leaving room in the box for the remaining black wire sets and the switches. Take one wire lead from each switch and combine it with the black wire from the circuit breaker. Twist a wire nut onto the ends of this group and force it into the switch box. Pair a black wire from the exhaust fan unit with the remaining wire lead on a switch. Twist the ends together and protect it with a wire nut. Repeat this with the remaining switches and fan function wires. Wrap a piece of electrical tape around each switch, covering the wire terminals. Mount each switch to the switch box and install the switch cover plate.
Because some local building codes allow a soffit discharge, while others only permit roof and/or gable penetrations, the planning of the duct system starts at the discharge vent. Ideally the discharge vent lets the duct run in a straight line with a slight uphill slope from the fan to the louver. Each bend in the duct increases air flow resistance and reduces fan efficiency.
Northern climates should consider ice-dam possibilities before using a roof-mounted vent. Ice dams sometimes occur when warm air moving through the ventilation system melts snow that collected above a roof-mounted discharge vent. When the bathroom exhaust fan turns off, sub-freezing exterior air freezes the melted snow. The resulting ice gathers under the shingles nearest the vent, eventually causing an ice dam.
Many technicians prefer to use flexible aluminum duct on a bathroom ventilation system. This lightweight type of duct bends easily and cuts with a non-serrated knife. Choose the duct size, usually 3- or 4-inches wide, that matches the fan's discharge port.
© 2017 Bert Holopaw