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Install a Bathroom Exhaust Fan

Moisture can cause paint to peel, doors to warp and the accumulation of mold spores. You can greatly reduce or eliminate the many problems created by excess moisture in your bathroom by adding a simple exhaust fan. The details of installing your exhaust fan may differ by model. This how-to describes the process in general. Specific installation instructions are included by fan manufacturers with their product.


Exhaust Fan Options

From the simplest fans, to more elaborate models with built in lights, heaters and timer switches, there are many types of bathroom exhaust fans available.  Some models are designed to be mounted on the ceiling, while others are mounted to the walls.



Wiring Your Exhaust Fan

A double switch is convenient for wiring the fan and light separately.

Wiring at the Switch
Wiring at the switch should be performed after the unit has been installed. However, since your choice of switching for the installation determines the type of cable that must be run from the switch to the fan unit, the information is included here first.

Wiring Options

  • Installing a fan/light combination as a replacement for an existing light fixture is an easy project. The wiring is already in place, so the project requires no special knowledge of electrical wiring. For this installation, the light switch will operate both the fan and the light at the same time.
  • If you wish to operate the light and fan independently by separate switches, it will be necessary to run a 3-wire cable between the unit and the existing light switch. Then install a double switch at the electrical box. The switches should be wired as indicated in the drawing.
  • Exhaust fans with lights/heaters/and or timers may require additional cable from the existing switch box to the fan. Consult the manufacturer's instructions. These more sophisticated exhaust fans often include their own timers and switches which are designed to fit the existing switch box.

When working with electricity always:

  1. Turn off the power.
  2. Test the wires to ensure the power is off.
  3. Lock the panel box to ensure no one accidentally turns the power back on while working
  4. Check with local authorities to see if a permit is required.
  5. Have your work checked by an inspector.
  6. Wear eye goggles and a dust mask.
  7. To avoid overloading the circuit, consult a licensed electrician.


Installing Your Exhaust Fan

Bathroom Exhaust Fan

Step 1

Shut off the power to the circuit before you begin any work. Lock out the circuit or the panel so no one can turn the circuit on while you're working. From the attic, remove any insulation from above the area of the ceiling in which you plan to mount the fan. If you're replacing a light fixture with the fan unit, remove the existing fixture and disconnect all electrical connections.


Step 2

If possible, place the housing in a position which will allow you to fasten it directly to a ceiling joist. If this isn't possible because of the location of the original light fixture, install a wooden braces between joists to provide a solid attachment point for the fan housing. Hold the housing in place against the ceiling and mark around it to define the perimeter of your cut. Drill pilot holes in the corners, and cut along the lines with a drywall saw or jigsaw.


Step 3

Secure the fan housing in place.


Step 4

Run a length of cable from the existing light switch to the exhaust fan unit. If you wish to have separate switches operating the fan and light functions, use 3-wire cable. If your unit includes a heater and/or timer, additional cable may be required. Consult the manufacturer's instructions. This step isn't necessary if you're replacing an existing fixture and don't require independent switch operation for the fan and light.


Step 5

Make the necessary electrical connections at the fan unit using wire connectors. Follow the wiring diagram provided by the manufacturer. Simple fan installations may only require wiring bare copper to bare copper, white to white and black to black. More elaborate units with lights and/or fans will require more connections. Attach the ground wire to the grounding clip provided on the housing. Push the wires into the wiring box area of the housing and install the cover.


Step 6

Connect the duct to the fan housing.


Step 7

Follow the instructions of the unit's manufacturer regarding how far away from the fan the insulation should be kept. Some units allow insulation to be right next to the housing. Others, particularly those with lights or heaters, may require that you add dams between the joists to keep the insulation a minimum distance away from the unit.


Step 8

From inside of the bathroom, install the unit cover.


Step 9

Wire the cable into the existing electrical circuit at the wall switch.



Venting Your Exhaust Fan

Always vent the fan to your home's exterior.

This is an important part of fan installation. Improper venting of the fan can cause moisture problems which arise as a result of condensation forming within the vent. This condensation can run back down the duct, leaking around the fan and creating stains and problems in the surrounding building materials. Don't vent the moisture directly into the attic. After all, your goal is to avoid moisture problems, not transfer them to a different area of the house.

When possible, vent the fan through the nearest soffit. This allows you to make the vent installation under the overhang of the roof, preventing the need for a roof vent.

The fan unit can be connected to the soffit vent using flexible plastic duct material. The duct material is easy to cut and it's expandable, which helps make fitting an easy task. Just clamp one end of the duct to the vent shroud on the fan housing and the other end to the vent. Installing the duct horizontally across the attic reduces the possibility of condensation trickling back down around the fan. Insulating the duct with insulation wrap also reduces the formation of condensation within it.

Some fans are designed to be installed in the wall instead of the ceiling. These fans are mounted on exterior walls and are ducted directly outside. For these installations, hoods with gravity-hinged dampers are installed against the home's exterior siding immediately behind the fan. These hoods are similar to the type commonly used for venting clothes dryers.

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