Installed an Aprilaire 400 yesterday and could not get the solenoid to kick in when hooked up to the humidistat. I had my 24VAC on the control. When I hooked up the solenoid directly to the 24VAC coming out of the furnace it worked fine so I have eliminated the solenoid. I also have my 24VAC on the terminals for the humidifier until the solenoid is hooked up then it drops to zero. I had another humidistat on the truck and that did the same thing.

Any suggestions. Thanks.