Design procedure, products and reference for electrical designers
Exhaust fans are usually treated as a line item: a box on the plan and a switch by the door. They deserve more attention than that, because a fan that is undersized or never left running long enough is the most common reason a finished room develops moisture problems within a few years.
The usual starting point for a bathroom is roughly one cubic foot per minute of airflow per square foot of floor area, with a minimum of about 50 CFM, and more where ceilings are high or a large shower or tub is present. Kitchen range hoods are sized to the appliance instead. Rated airflow assumes a short, straight duct; long runs, flexible duct and tight elbows cut real airflow sharply, so specify the fan against the actual duct design.
Most residential exhaust fans draw well under an amp and can share a lighting circuit, but check the listing for any fan with an integral heater, which usually needs its own circuit. Fans installed over a tub or shower must be listed for that location, and the manufacturer may require GFCI protection.
A fan on a plain switch runs for the length of a shower and stops when the light goes off, which is exactly when the room is most humid. Designers called in to replace that kind of fan regularly find staining on the ceiling, soft drywall around the grille and spotting on the framing above. In the older housing stock of western Pennsylvania, where many bathrooms were never ventilated to the outside at all, owners often have a Pittsburgh mold mitigation firm clear the affected materials before the new fan and duct go in, so the replacement starts from a clean, dry structure.
Better options, in rough order of effectiveness: - a countdown timer or a run-on timer that keeps the fan going for twenty minutes after the light is switched off; - a humidity-sensing control that starts the fan above a set point and stops it once the room recovers; - a continuous low-speed fan with a boost setting, which suits tight modern houses.
Duct to the outside through a wall or roof cap with a backdraft damper, never into an attic or soffit vent. Leave a gap under the door or provide a transfer grille so replacement air can reach the room; a fan with nowhere to draw air from moves very little of it.
Measure the delivered airflow at handover with a flow hood or a simple bag test, set the timer or humidity control, and record both on the closeout documents. A fan that is never checked is often found years later moving a fraction of its rated air.