Fan runs, but mirror stays fogged: placement or makeup air?

Fan location matters for capturing steam, but I wouldn’t rank it above airflow capacity and the room’s air path. A properly sized fan close to the shower can still perform poorly if the door undercut is tiny or the bathroom is effectively sealed when the door is closed.

Cracking the door or window is a useful diagnostic: if the mirror clears noticeably faster, the fan may be starved for makeup air rather than failing. Compare that with a tissue test at the grille, then check the exterior termination while the fan runs. A weak pull at the grille points toward the fan, duct, or termination; strong grille pull with poor moisture removal points more toward steam capture, duct discharge, or insufficient replacement air. Be cautious with tissue tests around a grille, since they show airflow direction and rough strength, not actual CFM.

I’d be cautious about interpreting strong pull at the grille as evidence that the fan, duct, and termination are moving air effectively. A crushed, disconnected, or severely restricted duct can still produce noticeable suction at the grille while delivering little useful exhaust outdoors.

A better check is to compare the grille-side observation with the exterior termination while the fan runs. The damper should open and there should be a clear, sustained discharge. If possible, use a tissue strip or light ribbon at the outlet, then inspect the duct for kinks, compression, loose joints, or an unsealed connection. An airflow hood or vane anemometer at the grille is more informative than a tissue test, but even that measures what enters the fan, not necessarily what exits the building.

So if cracking the door improves clearing, makeup air may be part of the problem. But if the grille feels strong and the exterior discharge is weak or absent, I’d investigate the duct path before blaming the room’s air supply or replacing the fan.

If the exterior discharge proves weak, a smoke-pencil test at the closed bathroom door can help show where replacement air is entering. Compare the door undercut with the jamb, trim, and other gaps while the fan runs, using a non-combustion smoke source well away from the fan motor, open flames, and combustion appliances.

If smoke enters mostly around the frame and little moves through the undercut, the undercut may not be providing much of the replacement-air path. Repeat the test with the door cracked slightly and compare the mirror-clearing time, smoke pattern, and exterior damper movement. A large improvement with the door cracked points toward limited makeup air; little change, especially with weak exterior discharge, points more toward the duct or termination.

A damper that flutters can make the outlet look intermittently active without providing a reliable discharge path, which can muddy the makeup-air diagnosis.

For anyone tracing a similar problem, watch the exterior damper as the fan starts and while it continues running, not only after it has begun. If it barely opens, chatters, or closes with the fan on, inspect the flap, screen, and termination for debris, paint, insect buildup, or wind effects. Then repeat the closed-door versus cracked-door comparison. Confirm a clear, sustained outlet discharge before treating weak makeup air as the leading explanation.