Question about Aprilaire 600 Humidifier

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Low Voltage to Solenoid


I am having trouble with my AprilAire 600. I originally thought that my solenoid was failing but I took it apart and it seems fine. I then used a multimeter to test the voltages across the different connections.

I found that the power from the furnace was coming in at 24V but the voltage going out to the solenoid and sensor were around 6V. I'm pretty sure the solenoid needs 24V in order to open. Any thoughts on why my voltage is coming out to the solenoid so low? I touched the solenoid to the contacts coming into the panel and the solenoid opened right up no problem.


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  • short_thumb Jan 18, 2008

    I don't think my furnace is a multispeed and I'm pretty sure it is a downdraft furnace.

    The funny thing is that the humidifier worked for two seasons, then I didn't use it for two seasons because our winters were so mild, and then I tried to start it up this season and it didn't work. I have not changed the wiring at all so I'm assuming it is wired correctly.

    There are three pairs (black/red) of leads going into the controller--power from the furnace (which is only supplied when the furnace is running), a line going to the solenoid, and a line to the sensor.

    I expected to have the voltage to the solenoid go up to 24V when I turned the dial to "test" but it just stays at 6V. I turn up the furnace to ensure that it is running whenever I perform these tests.

    At this point, I'm inclined to think that the controller is bad.


  • short_thumb Jan 18, 2008

    Thanks for the help! I know the voltage with the solenoid disconnected is 6V and I'm pretty sure the voltage is the same with it connected. I'll test it tonight and will let you know.

  • Rob Robinson May 11, 2010

    Is your funace a multispeed? Is it a downdraft? The only thing between your furnace and humidifier should be the controller. Have you verified correct installation of wiring?



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The controller is faulty. The previous question was hopefully to help determine why it faulted. The switching for the solenoid is a solid state device that is allowing the 6v through but not getting turned on hard enough to supply the current to operate the solenoid. I am assuming that you checked the voltage with the solenoid connected. If you disconnect the solenoid and look at the voltage it will probably be 24v. If you are not comfortable troublshooting down to the component level a new controller will be your best bet. It may be that the transistor that drives the solenoid could just be replaced. You would have to look at the signal that drives it to determine if it is being turned on with adequate signal.

Posted on Jan 18, 2008


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