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.
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.
About 7 hours to install ducting, a fan, and a switch (not including cutting a pathway for the cable and patching walls)
Voltage tester, pry bar, drill, drywall saw, jigsaw, hammer, ladder, fish tape, screwdriver, strippers, long-nose pliers, lineman's pliers
Cutting through siding or roofing; stripping, splicing, and connecting wires; installing boxes; running cable
Find the shortest path for the ductwork. Find the power source and make sure the new fan does not overload the circuit. Spread a drop cloth on the floor below.
Vent fan, switch, ductwork, duct tape, sheet metal screws, cable, clamps, switch box, wire nuts, electrician's tape
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.
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.