For a fan intended to remove warm, moist air, the best location is the ceiling, and the moderate-capacity fans that are sufficient for bathrooms are small enough to fit inside a standard ceiling space. Such a fan needs a duct to exhaust outdoors.
For an upstairs bathroom, the duct generally runs straight up through attic and roof. If the fan is in an interior room on the first floor of a two-story house, you simply need more ducting and an improvised method for getting it to the outside—inside a built-in cabinet, through a closet or if necessary along a corner where the duct can be concealed with a wallboard enclosure. If the bathroom, upstairs or down, has an exterior wall, you can eliminate ductwork by installing a through-the-wall kitchen-type fan using the instructions presented earlier.
Be certain to get a fan that's the correct size for your bathroom, using the formula (see above). If the exhaust duct will need an elbow, or bend, in it, buy a fan with the next higher CFM rating.
To estimate how much ducting you will need, measure the route from fan to roof, allow for bends and add 2 inches for the roof thickness and 2 inches to extend above the roof. You also will need a roof cap to fit the duct, roofing cement to install it, plus No. 12 electrical cable and fittings to wire the fan.
The fan is connected to the bathroom light so that it operates whenever the light is switched on. If the light fixture is in the ceiling and you can reach it from the attic, all you need do is connect the wires. If not, you will need cable fish tapes to pull the new wiring inside the walls from fan to light fixture.
1. Opening the attic floor. Drill a 1/8-inch test hole through the bathroom ceiling where you want the fan. If the bit does not go through quickly, it has hit a joist. Fill the hole with spackling compound and drill another test hole 2 inches away. If the attic is unfloored, proceed to Step 2. If the attic is floored, enlarge the hole with a 3 bit, then, using an extension and a 1/8-inch bit, drill through the attic floor. If the attic floor is insulated, run coat-hanger wire up through the test hole to mark the spot. The test hole marks the center of the fan. Use it to mark an opening in the floor that will extend between the two joists adjacent to the hole and be large enough to accommodate the fan housing and its duct connection. Cut the floor with a keyhole or saber saw and remove insulation if there is any.
2. Mounting the fan housing. Detach the housing from the grille and the fan-motor assembly. Working from the attic, center the housing over the locator hole and against the joist nearest the hole. Mark a line on the ceiling around the lip that will extend through the ceiling and cut a hole along the line. Remove a knockout from the fan outlet box and install a cable connector in the box. Lower the housing along the joist until the lip is flush with the ceiling, then screw it to the joist.
3. Framing the opening. To support the flooring you will replace after installing the fan, nail 2-by-4s along the joists at the opening. For additional support around the opening, toenail 2-by-4s around it. The tops of these pieces must be flush with the tops of the joists.
4. Installing the duct and roof cap. Hold a piece of duct against the roof underside at the point where the duct will go through. Angle the duct if necessary to avoid obstructions and to bring it to a convenient point on the roof. Mark around the duct end and drive a nail through the center of the marked area. Outside, on the roof, center a piece of duct over the protruding nail and use a linoleum knife to mark a line around the duct. Remove shingle, roofing felt and nails inside the circle as for a roof vent. Saw through the roof sheathing, following the circle. Slide the cap’s built-in flashing under the shingles around the hole, and cut away any portion of shingle that prevents the roof-cap hood from fitting over the hole. Remove the roof cap and set it aside.
Run the ducting from the fan housing through the hole in the roof. Seal connections in the duct with duct tape, and cut off the duct ½ inch above the roof surface at an angle that conforms to the roof. Thinly coat the underside of the roof- cap flashing with roofing cement and press it into place. Apply roofing cement to the under- sides of shingles that overlap the flashing.
5. Wiring and mounting the fan. Working from the attic, run No. 12 cable through the connector you installed in Step 2 and tighten the clamp on the cable. Working from the bathroom side, re move the fan’s outlet-box cover and attach the wires—black to black, white to white, and the bare copper wire to the green screw terminal. Replace the outlet-box cover and mount the motor-fan assembly in the housing, making sure to center the thumbscrew that will hold the grille in place.
Wiring the Fan to a Light Fixture
Connections for a middle-of-the-run switch. At the fuse box or circuit-breaker panel, turn off the power to the bathroom light. Remove the light fixture and disconnect its wiring. Check with a voltage tester to make sure the power is off—one prong of the tester against the metal of the box and the other against each wire in the box; the bulb of the tester should not light. The drawing above shows one wiring arrangement you are likely to make; the other common arrangement is at right. If only one cable—from the switch—enters the box, take out the box that holds the light fixture’s wiring and bring the new cable to the fixture, using fish tapes. Then follow the instructions in Step 9, to connect the new wires to those in the box. New wires are indicated by broken lines.
Connections for a switch loop. If you find two cables already entering the light fixture, the light is part of a switch loop. Both wires in the cable leading to and from the switch are hot—the white wire should be recoded black with black markings on its end. To be sure you are connecting the fan to the wire that brings power to the box, have a helper turn on the power while you care fully hold one prong of a voltage tester against a black wire and the other against the metal of the box—the bulb will light when the probe is touched to the wire carrying current. Turn off the power and then connect wires as indicated by the broken lines in the drawing above.