What is a dryer vent booster fan?

Look in the duct, not at the machine. The fan is a can-shaped housing spliced into the run, typically 15 feet or more downstream of the dryer and hung somewhere a person can reach: a basement ceiling, an attic near the termination, a laundry soffit. Next to it sits a small electrical box and one of two controls, either a pressure switch teed into the duct through a thin tube or a current sensor clamped around the dryer cord. If your laundry sits at the end of a long run and you have never seen either part, you do not have one, and that may be the entire reason drying takes two cycles.

Duct math drives the decision. Code allows 35 feet of developed dryer duct, then charges 5 feet for every 90-degree elbow and 2.5 feet for every 45, so a 30-foot path with three 90s is effectively 45 feet and well over the limit. Beyond that point the dryer cannot hold air velocity, lint drops out of the stream instead of riding it outdoors, and the duct silts up faster each month. A booster restores velocity: loads dry in one cycle again and the run stays cleaner between services. The catch is that a dead booster fails silently, and the first symptom is the same slow drying you installed it to cure, which is why better units include a flow switch or lint alarm.

Service the fan and the duct together once a year, or every 6 months in a heavy-use laundry. Kill power at the breaker, open the housing, and brush lint off the impeller blades, since a loaded wheel loses a real share of its rated flow and starts to hum or wobble. Blow out the pressure-switch tube while you are in there, because a plugged tube keeps the fan from starting at all. A booster depends on a clean duct rather than replacing one, so keep the whole run on the dryer vent maintenance checklist and verify the exterior flap still opens. The whole-house version of this schedule is the home maintenance checklist.