Some of the fans are simple on/off fans controlled by a switch. But the fans do have a constant CFM feature. ie, if your wall damper is stuck partially open, the fan will run at a higher RPM to move the same amount of air if the vent had no obstruction at all.

The next level up of fans can run full time at a low baseline CFM to constantly move a small volume of air. That baseline can also be set to "zero", which essentially turns the fan OFF. When the switch is actuated ON, the fan then ramps up from the baseline setting to a higher moisture removal CFM rating. It stays at that higher rating while the switch is in the ON position. When the switch is moved to the OFF position, it continues to run on the high CFM setting for a specified amount of time, then the fan ramps back down to the lower CFM setting. Or the fan turns OFF is the lower constant run setting is set to ZERO.

I actually prefer the shower fan and shower light to be on the same switch. When the shower light is turned on both the light and the fan turn on. When the shower light is turned off, the light turns off but the fan continues to run for a set amount of time.

Hope that makes sense. That is my understanding of the fans.

Thanks mongoct. Based on your explaination it sounds like it works the way I thought it did.

We weren't originally planning on having a separate light switch for the shower light but I guess that is something we should think about. It would avoid the complicated situation where the fan switch could be up or down/ on or off at any point in time....and someone having to hit two switches versus one.

The only issue is if someone wants to use the toilet and turn on the fan, then they are also turning on the shower light. I guess that isn't a huge deal since it is a guest bathroom.