"The humidistat sensor switch is buried deep inside the fan casing. It is soldered directly onto the timer controller PCB which is hidden and sheltered from detection of bathroom air humidity by the internal PCB compartment cover and the main fan front cover. Although both covers have small narrow slots for air to enter the casing and PCB compartment. The slots are far apart and not directly connected with ducts to air flow and humidity of the room.
The design prevents the humidistat sensor from detecting room humidity to either, switch the fan on or off. If humidity sensitivity is adjusted to detect steam from showering it will not switch off and continues to run for hours or, if adjusted to prevent over running, it will never detect humidity when showering steam fogs the room up.
The design of front cover makes it difficult to access adjustment of timer and humidistat sensor control dials which has to be removed first."