How to replace a bathroom exhaust fan
Short answer
Replace a bath fan like-for-like: same opening, same switch, same duct. Difficulty 3 of 5; for a first-timer, plan on about half a day. Read the “Stop here” box below before step 1.
Draft guide written from standard practice, not yet reviewed by a licensed pro. Follow product labels where they differ.
Read the old fan's label and measure its housing, find where the duct ends outside, switch the breaker off and prove every wire dead, swap the fan or use a retrofit kit, clamp and tape the duct, wire it the way it was, then do the tissue test at the grille.
Difficulty 3 of 5 · about half a day · careful
Stop here
Photo of your room, the look you pick, then this guide with the materials counted for your room.
What you need
- Bathroom exhaust fan or retrofit kit that fits the old housing
- UL 181 foil tape
- Duct clamp sized to the fan's outlet
- Insulated duct sized to the fan's outlet
- Wire connectors (assorted)
Tools
- Non-contact voltage tester
- Two-lead tester or multimeter
- Step ladder
- Drill/driver
- Wire stripper
- Headlamp — The breaker is off, and so is the bathroom light.
Step by step
Step 1 of 10
Read the old fan's label and measure its housing
Take the grille off (squeeze its spring clips) and find the label inside the housing: the airflow (CFM), the duct size, and whether it is rated for use over a tub or a shower. Measure the housing opening. Buy a retrofit kit made for that housing, or a fan with the same housing size and duct size — never one that needs a bigger hole. The fan's box says what airflow suits your room's size; when in doubt, do not go below the old fan.
Step 2 of 10
Find where the duct ends
From the attic, or from outside, find the end of the duct: a cap on the roof or on an outside wall, with a flap that opens when the fan runs. Run the old fan and watch the flap. A duct that ends in the attic, a soffit, or between the joists, or one that is torn or crushed flat, is a stop — see the stop-gate.
Step 3 of 10
Breaker off, then prove every wire dead
Switch off the breaker for the bathroom fan and tape it. A fan with a light can be on two switches, sometimes two breakers. Prove the non-contact tester on a live outlet, then test every wire at the fan; confirm with a two-lead tester. Silence on a tester you have just proved is what counts.
Safe to start?
- The duct is traced and ends outside, at a cap with a flap — not in the attic
- The new fan or retrofit kit fits the old housing or opening — no bigger hole
- The new fan is rated for its spot (over a tub or a shower, if it is there)
- The breaker is off and taped, and every wire at the fan tested dead
- A steady ladder, eye protection, and a second person nearby
Step 4 of 10
Take the old motor out
Most fans have the motor plugged into a socket inside the housing: unplug it and unscrew the motor plate. A hardwired fan has a small wiring box in the housing: photograph the wires, then undo the connectors. Keep the photo for step 8.
Step 5 of 10
Keep the housing for a retrofit kit, or take it out
A retrofit kit goes into the old housing: follow its instructions and skip to step 7. For a full replacement, free the duct from the housing, release the cable clamp, and take the housing off the framing the way it is fastened, from the attic if you can reach it safely. Cut any caulk at the ceiling first so the drywall paper does not tear.
Step 6 of 10
Fasten the new housing to the framing
Hang it the way its instructions say — brackets or hanger bars into the joists — with the housing flush with the underside of the ceiling. It hangs on the framing, never on the drywall alone.
Step 7 of 10
Connect the duct: damper, clamp, foil tape
Push the duct over the fan's outlet past the damper flap, clamp it, and seal the joint with UL 181 foil tape. Keep the run as short and straight as you can, with no kinks or crushed spots, and the flap free to swing. In an attic that gets cold, use insulated duct so it does not drip condensation back into the fan — the fan's manual says what it wants.
Step 8 of 10
Wire it the way it was
In the fan's wiring box: ground to the green screw or the bare wire, white to white, black to black, with wire connectors, and the cable clamped where it enters the box. A fan with a light has its own wiring diagram — follow it and your photo. Wires that do not match the diagram, or a third wire you cannot explain, are an electrician's call.
Step 9 of 10
Motor in, grille on, gap sealed
Mount or plug in the motor and fit the grille. Seal the gap between the housing and the ceiling with the gasket or the caulk the instructions name, so the damp air goes into the duct and not into the ceiling.
Step 10 of 10
Power on and the tissue test
Breaker on. Run the fan: a single square of toilet tissue held against the grille should stay there on its own, and the flap at the outside cap should open. Run it for half an hour and listen for rattles; check the breaker has not tripped and nothing smells warm.
Quality check
- The fan runs on its switch, and its light too if it has one
- A tissue held to the grille stays there
- The flap at the outside cap opens when the fan runs
- The housing is fastened to the framing, flush with the ceiling, the gap sealed
- The duct is clamped and foil-taped at the fan, with no kinks or crushed spots
- GFCI protection is in place where the fan's label asks for it
- No warm smell and no tripped breaker after half an hour
Common mistakes
- Buying a fan that needs a bigger hole. A bigger housing means cutting the ceiling and maybe a joist, and that is not a swap any more. Read the old label and measure first; a retrofit kit fits the old housing.
- Venting into the attic. A duct that ends in the attic, a soffit, or between the joists fills them with warm, wet air: mold and rot where you cannot see them. The duct has to end outside, at a cap with a flap.
- Hanging the housing on the drywall. The housing hangs on the framing with its own brackets or bars. Fastened to the drywall alone, it sags, rattles, and pulls the ceiling down.
- Cloth duct tape on the duct. It dries out and lets go in the attic heat. Clamp the duct and seal it with UL 181 foil tape.
Questions
How do I replace a bathroom exhaust fan?
Read the old fan's label for the airflow, duct size and whether it is rated over a tub or shower, and measure the housing. Switch off the breaker (a fan with a light can be on two switches, sometimes two breakers) and prove every wire dead. Take the old motor out; a retrofit kit goes into the old housing. Hang a new housing on the framing, never on the drywall alone, push the duct over the outlet, clamp it, seal it with UL 181 foil tape, and wire it the way it was.
How do I know the new bathroom fan works?
Run the fan: a single square of toilet tissue held against the grille should stay there on its own, and the flap at the outside cap should open. Run it for half an hour and listen for rattles, and check the breaker has not tripped.
Where should a bathroom fan duct go?
Outside, at a cap with a flap. A duct that ends in the attic, a soffit or between the joists fills them with warm, wet air: mold and rot where you cannot see them. Clamp the duct and seal it with UL 181 foil tape; cloth duct tape dries out and lets go in the attic heat.
When should I stop and call a pro?
Like-for-like only: the new fan goes into the old fan's opening, on the same switch and the same wires, and into a duct that already runs outside. Stop and call a licensed electrician or an HVAC contractor if: there is no fan today, or no duct, or the duct ends in the attic, a soffit, or between the joists instead of outside; the new fan needs a bigger hole, a cut joist, a new switch, or a new circuit; you cannot reach the attic side safely; you find aluminum wiring, cloth or crumbling insulation, scorch marks, or no ground wire; or there is mold on the ceiling around the fan. A fan over a tub or a shower must be rated for that spot, and its label may ask for GFCI protection — follow the label, and ask an electrician if you are unsure. In a building from before 1980, a textured or popcorn ceiling may contain asbestos: do not cut or scrape it until it has been tested.