How to Install a New Bath Vent Fan

Plan the installation so the vent duct is as short as possible and makes as few turns as possible. In addition to a vent fan, a bathroom unit may have a light, night-light, or heater. A heater uses more power than a light and fan, so it may need to be on its own circuit.

If possible run the duct straight out through a wall. Or go up into the attic, over, and out through the soffit. If there is no room above, the best solution may be a special vent fan that you can mount on the wall.

A fan or a unit with a light and fan that come on at the same time requires only two-cable wiring. Wiring becomes more complex with separate function controls and additional functions. To replace an existing fan, check the wiring; you may need to replace two-wire cable with three-wire cable or even two cables.

To replace an existing ceiling light with a fan/light, shut off power to the circuit. Remove the light and pry out the ceiling box. If you cannot work from above, cut carefully around the box before prying. You may need to cut through mounting nails.

Disassemble the new fixture and use the housing as a template to mark for the opening. The fan must be securely mounted; if there is no joist to attach it to, install blocking nailed to nearby joists.

If necessary, install blocking to keep the insulation away from the fan. Cut a hole with a drywall saw or reciprocating saw. If the ceiling is plaster, drill locator holes at the four corners and cut the opening from below.

If necessary run new cable from the switch to the box. (The fan shown has separate controls for fan and light and requires one three-wire cable.) Screw the fan to a framing member.

For the wall vent, drill a locator hole from the inside through the outside wall. Outside, cut a hole for the duct.

Measure from the outside to the fan. Attach a piece of solid duct to the wall cap so it is long enough to reach the fan or as close as possible. Fit the duct to the cap, drill pilot holes, and drive sheet metal screws to hold it in place. Cover the joint with professional-quality duct tape.

Apply a bead of caulk around the exterior hole. Slide the duct through the hole and fasten the cap to the wall with screws.

Fill any gap between duct and fan with another piece of solid duct or with flexible metal ducting. Clamp each joint and wrap with duct tape.

First, turn power off. If necessary run the correct cable or cables to the switch box. As shown above, power enters the switch box. If power enters the fan, see the manufacturer's instructions.

To wire a fan/light switch, connect the grounds and splice the white wires. Connect the red and black wires from the fan to the fan and light terminals. Connect the feed wire to the remaining terminal.

Connect the grounds and splice the white wires. Splice the black wire to the black fan lead and the red wire to the colored lead. Attach the junction box cover. In the bathroom, install the light and the fixture canopy.