Has anyone installed a humidistat on the ceiling of a bathroom? A consultant wants to put in new fans, with a humidistat mounted on the ceiling, and a digital timer so that when it gets too humid, it will turn on whether the owners are at home or not..but why the ceiling, why not on the wall? is there a difference? thanx

My guess would be because steam rises, and you'd have a higher humidity up at the ceiling.

If they were not home, where would the steam come from?
I agree the humidistat should be down low if you are looking for ambient humidity. Wet air is heavier than dry air.

What about something like the Nutone/Broan QTXEN110S fans that have the humidistat and a 5 to 60 minute timer built in. You could also install a wall switch for manual fan operation.
You can look over the wiring options in the install manual:
Nutone/Broan

I don't think we fully understasnd what they're trying to accomplish.

It seems to me that they want the fan to operate automatically. This means two things:

First, the fan will run for a period after the bather has left the room; and,

Second, the designer believes that the room won't be dry until the fan is drawing in dry air. Hence, placing the sensor closer to the fan.

Sure, most folks would simply use a timer. Trouble is, a timer measures time, not moisture. Plus, there is something creepy about flipping the switch 'off' and having the fan continue running.

Now, if only the fan had a 'foul odor' sensor!

I live in the mildew capital of the world. It always seems to start down in the corners at the bottom. That is where the high humidity is. (for a number of scientific reasons) I suppose we put the fan up top because the duct work is easier to run from there.

I don’t know if it is still applicable, but a few years back on new construction, either builders or HO’s in the area were apparently getting some sort of energy tax credit for installing a timer on one of the bath fans, so it could cycle on and off during the evening to remove humidity and stale air. We would put the electronic timer switch in a remote location, but also install as wall switch in the bath to override the timer for manual fan use.

The crux is to put the fan where it will best force the damp air in the bathroom to circulate, mix and draw fresh, drier air in, from say the corridor or bedroom adjacent, while ejecting humid air to outside. The speed that gas and water vapor molecules move is incredible, [hundreds of mph], so the air in a room will become homogenous eventually. Where you put the humidistat is thus pedantic, IMHO, considering the use-ratio, but perhaps it's easier to wire in the ceiling? Cold spots on walls can create dewpoint conditions creating condensation that mold can live on, and these spots are generally low down or in corners out of drafts. But surely the main job for a fan is to get rid of nasty niffs from the khazi? A delay-off timer on the light circuit works great.

I have one bathroom fan with a humidistat built-in - works just fine but 'her indoors' can't leave the danged thing alone and keeps fiddling with the adjuster knob, either because it didn't go on when she ran a bath or it was already on when she entered the room. Keeps her busy I suppose!

[wiping condensation off the mirror for a shave, my pet hate! Why can't we have heated mirrors? ]

Use your wife's hair dryer to clear off the mirror

has your designer seen this

http://www.bcelectrician.com/furnace_fan_timer.html