Figured I’d pay this forward like always, I ended up figuring this out. A broken solenoid valve feeding my humidifier prompted me to revisit this. It’s amazing how bad the air quality got in my house because of the stopper valve.
I had a C4-THERM lying around and replaced my legacy C4 thermostat with it. After replacing the valve I wanted to know what the relative humidity was in the house which the C4-THERM supplies. I lacked enough wires in the thermostat wire to hard wire the humidity control, so originally I thought about getting a Z2IO. But for some reason I completely ignored the HC800 10 feet away from the valve in the same closet.
I ran the 24vac connection for the humidifier valve from the control board on my HVAC to red lead going to NO. The COM goes back via the white wire connected to the valve. The other valve wire goes back to the COM on the HVAC control board.
Programming looks for changes in variable HUMIDITY_STATE on the C4-THERM. If string is “Off”, open relay. If string is “Humidifying”, close relay.
So now I get a Humidity read on my mobile app, along with the valve status from showing the relay in my equipment room. Next to setup reminds to change the humidifier pad (which will also remind me to check the valve for build up). I should have done this long ago. Hope this helps anyone else dealing with an analog humidistat on their return with no read out.