For the humidity function to work the fan would need to be hot wired or switch left on. If it is on the same switch as the light it would go off as soon as you turned the light off. Looking at the Broan website the humidity level on some models is preset. My guess would be you can adjust the CFM level on this fan based on need (room size and presence of a shower for example) and run time after it turns on for a raised humidity level. In a powder room without a source of extra humidity I doubt the fan would ever operate based on humidity. I believe if it is on a switch it will come on when you turn the switch on. The timer feature may automatically shut it off after the time set on the timer. I can't tell from the angle of the picture but it looks like it is set to around 20 minutes. In a powder room this would be a handy feature if that's how it works. You could set a relatively low CFM and have it shut off 20-30 minutes after you leave the room. It wouldn't turn on automatically when you enter the room though, so you'd need to toggle the switch to turn it on if you wanted the fan on.It could be set up to come on with the light and then remain on for 1-60 minutes after the light is shut off. Might be more for smells than humidity.
I’m far from an expert on fans, but it would be possible if the fan had a timer relay and the power originated at the fan to have a leg run to the switch. When the switch was closed it would operate the light and latch the timer relay on until its preset time was reached when it would then turn the fan off.For the humidity function to work the fan would need to be hot wired or switch left on. If it is on the same switch as the light it would go off as soon as you turned the light off. Looking at the Broan website the humidity level on some models is preset. My guess would be you can adjust the CFM level on this fan based on need (room size and presence of a shower for example) and run time after it turns on for a raised humidity level. In a powder room without a source of extra humidity I doubt the fan would ever operate based on humidity. I believe if it is on a switch it will come on when you turn the switch on. The timer feature may automatically shut it off after the time set on the timer. I can't tell from the angle of the picture but it looks like it is set to around 20 minutes. In a powder room this would be a handy feature if that's how it works. You could set a relatively low CFM and have it shut off 20-30 minutes after you leave the room. It wouldn't turn on automatically when you enter the room though, so you'd need to toggle the switch to turn it on if you wanted the fan on.
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