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Humidifier problem stumping me!

I have a Honeywell HE225 whole-home humidifier connected to an Amanda 80 SV HVAC system that uses a White Rodgers 50A55-288 control board. Water is not flowing to the humidifier pad. So I confirmed that I have good water pressure to the solenoid and the tubing from the solenoid to the humidifier panel is not obstructed. I replaced the solenoid and transformer. When energized, the transformer produces 27 VAC. I confirmed that it was connected to the proper terminals on the control board. The solenoid coil resistance is 21 ohms, so it’s not short circuited. I confirmed that the humidistat is working by measuring the voltage across the leads and noting it changing from 27 to 0 volts when it activated.

Here is what’s stumping me. When the furnace is running and the solenoid is disconnected, the leads from the transformer measure 27 volts, as expected. Immediately when I connect the solenoid, the voltage across the leads drops to 0.00 volts (obviously the solenoid does not activate). This was the same behavior as the previous solenoid.

I can’t figure this out. The only other idea is that it may be some issue with the control board, I just don’t know what that could be. Any thoughts as to what the issue may be would be much appreciated. This problem is driving me crazy!



  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Are there more than 1 transformers on this system? Maybe you have
    Be the wrong common c on extend to the humidifier coil.
  • ratio
    ratio Member Posts: 3,626
    Sounds like an SCR controls the humidifier. They are similar to relays, but not exactly. Instead of opening & closing the circuit, they go from high resistance to low resistance. The implication is that they have a minimum load necessary to "open"; that is, a multimeter will show voltage present until a suitable load is connected.
  • Jamie Hall
    Jamie Hall Member Posts: 23,295
    The simplest possibility is a really bad connection in there somewhere. As @ratio implied, if you have a bad connection you will read voltage until a load is applied -- then all the voltage drop will be in the bad connection.

    Now which connection is bad...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England