When does the humidifier fan start? Does the water flow trigger it? Will the fan nt start if I turn off the water supply?

That suggests a faulty sensor (or sensor connection) or controller board.

Are there no other symptoms, please?

As long as the humidifier start/stop is regulated by air flow there is no way to cycle it on/off with another device. If, however you disconnect that capability and, instead wire it to the heat on cool on switch it will only turn on when the stat calls for heating or cooling. You may need to install an intermediate relay off the stat wires to regulate the different(assuming that they are) voltage of the humidifier start/stop.What a fun project, thanks!

Is the saddle valve open? if it is check for sediment build up in it, if it;s clear you will need a new saddle valve. If the fan is not running than most likely you are not getting power to the humidifier, check for 24 volts to the solenoid valve. By the way do you have a current sensing relay or sail switch hooked up to this humidifier?

Humidifier receives voltage when the indoor blower motor comes on in the heating mode, and blower motor dose not come on right away.See if water flows when indoor fan is blowing heat, Good luck.

If you used the plastic tubing to the humidifier(I recomend copper) there should have been a little brass tube with a flare on it. This goes in the plastic tubing to grip the the sleeve better. See if this helps

Tinner
Sunsetmech

Valves do go bad, but I had the same issue and it was the inlet orifice that was plugged.
To fix: Turn off the water supply. Disconnect the water inlet line to the solenoid. The water inlet line connect to ~ 1" long piece that goes into the solenoid. Unscrew the water inlet piece and remove it from the solenoid. Check it for water flow - water should slowly drip out the small orifice at the soleniod side. There is a pin-hole sized orifice on the solenoid side and it doesn't take much to plug it. There is also a narrow cone-shaped filter inside the inlet piece. You can use tweezers to remove the filter and & use a safety pin or sewing needle to clear the orifice. Check the filter, clean it if necessary, and reinstall the filter. Now check for water flow thru the part before reinstalling. Reconnect the water inlet line and turn on the water supply. Turn your humidistat to trigger the humidifier. You should see water flowing through the drain tube.

Humidifiers usually are wired up to ONLY come on when the Heat is on and the fan running....anyother time you check it it will not be powered or wet...turn your heat on , wait till the fan comes on...then turn up your humidistat you should hear a click if its a flow through type and water should start flowing from the top...or if its tyhe drum style with the sponge...you should see it then turning...if it doesnt turn the pad/spindle may not be installed properly...if only fits one way

You may need to replace the valve that is wired to the furnace that controls the water flow

No. when you remove the front cover it will automatically disconnect the power supply. Try changing the water panel and cleaning out the orifice and filter screens located in the water lines connected to the solenoid valve.

Loosen the brass fitting where it connects to the soleniod valve. If water comes out your saddle valve is ok. Carefully retighten this fitting. With the humidifier turned on loosen the first fitting down stream from the valve see if water comes out. if it does turn the humidifier off and see if it stops, it should. That will tell you the the valve is ok then, just clean the filter and the oriface. if it does not turn on and off check the wiring. if ok replace the valve.
Good luck