Aprilaire 550 Solenoid not engaging

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Old 01-26-15, 09:03 AM
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Unhappy Aprilaire 550 Solenoid not engaging

Hi all, I have an Aprilaire 550 that has stopped working. I can't say I know much about HVAC topics in general, so please bear with me.

The symptoms are that the solenoid will not engage. I checked the water supply and the itty bitty filter inside the solenoid and they were good. I disconnected the solenoid and tested the voltage at the red and white wires with a multimeter and verified I'm getting 24 volts at the wires. This will go on and off when I turn the humidistat up and down.

I replaced the solenoid but no change. The weird thing is that with the solenoid wired in, there's no voltage at the wires, and when I disconnect the solenoid, the voltage goes to zero.

I let the furnace go thru a whole cycle, even hearing a "click" inside the furnace which I assume is to engage the humidifier (the voltage at the wires goes on at that time) but the solenoid does not click or make noise, and no water flows through. When I test the wires, there's no voltage.

As I said, I replaced the solenoid but this didn't help. Open to any ideas and input. Thanks so much everyone for your help.

Dave
 
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Old 01-26-15, 09:37 AM
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Welcome to the forums.

You need check for voltage at the solenoid with it connected.

You said when the solenoid was disconnected you measured power and when connected you measured nothing. Your next step is to check the humidistat. If you have the basic two wire h'stat try shorting those two wires and see if that returns power.
 
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Old 01-26-15, 10:33 AM
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Great, thank you PJmax. I'll give that a try.

If shorting the wires makes the solenoid open, does that imply the humidistat needs replacing?

I just re-read what I typed before. When I connect the solenoid and measure the voltage at the solenoid, I get nothing. When I disconnect the solenoid and measure the voltage, I get 24 volts. Sorry for that.

IN any case, I'll try shorting the humidistat wires and report back. Thank you again.
 
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Old 01-26-15, 09:17 PM
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If you short the humidistat wires together and the humidifier starts.. you'll need to replace the h'stat.
 
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Old 02-01-15, 10:12 AM
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That was it. Thank you so much for your help!
 
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