I am installing a humidity sensing fan (Delta GBR80H) into a bathroom where no fan existed before, only have a light switch. What is the best way to accomplish this?
You'll have to check the light fixture and the switch to see where the hot wire is. It can be in the ceiling, with a wire running to the switch to break the circuit, or at the switch and a wire running to the light to power it. If the hot is in the ceiling, you are in luck. You can tie into that for power. If the hot wire is at the switch, you'll have to run a wire down the wall and into that box. Not impossible, but not easy either, it will be much easier if its an interior wall. I am an amateur and have done it many times.
I would suggest cutting the power at the breaker, pull off the light fixture from the ceiling, and the switch from the wall. Then turn the power back on and with the light turned off, carefully probe the wires with a contactless voltage meter to see where the power is coming from.
I have installed a couple of these I think they were delta fans, I’m not sure but the one that I installed I did as follows.
The fan does not need to be wired for constant power. It should be wired with a normal switch.
When you flip the switch up once it activates the fan to run like a normal fan, if you flip the switch up once back down and then back up again quickly it puts the fan in humidity mode. In the first scenario you will see a little yellow light just inside the grill cover of the fan telling you that the fan is on, and in the second scenario you will see a little blue light which will tell you it is in humidity mode.
On the ones that I’ve installed there have been two switches, one for the light and one for the fan. But if I remember in the instructions there may be an option for one switch wiring.