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I am trying to trouble shoot my Aprilaire 600. It was already installed when we purchased the house 3 years ago but I don't believe we ever turned it on after purchasing the house in the summer.

The humidifier is power from the furnace and it is getting 24 V from the furnace to the humidistat. Out of the humidistat, I get no voltage going to the solenoid. I have replaced the humidistat and the solenoid, and still: nothing.

Should there be wires going directly from the furnace to the humidifier? Currently, I only have one wire going to the humidistat and one coming out of the humidistat to the humidifier.enter image description here

Any ideas? And thank you.

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    Hello, and welcome to Home Improvement. What make/model is the humidistat, and would you give us a picture of the wiring? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Nov 14 '19 at 18:36
  • I failed to mention that I have a nest thermostat in the home which was not there when we purchased the house. I am not sure if that matters for the wiring. I will take a picture and upload when I get home. I also am not sure if it is connected to a relay or not. Does the fact that I have 24 V coming out of the furnace indicate there is a relay? – Andrew Nov 14 '19 at 21:08
  • Sorry for stating the obvious, but you've turned the humidistat up higher than the ambient humidity....right? It won't operate the valve unless it 'sees' a demand for humidity. On mine I can turn the knob back and forth to toggle the valve open and close. – Steve Wellens Aug 12 '20 at 4:29
  • Did you ever get this resolved? If so, please give a check-mark to the answer or write up your own answer explaining what you did to get it fixed and give yourself a check mark. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer. – FreeMan Aug 12 '20 at 18:35
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I had a similar issue with my Apirlaire humidifier last winter. Turns out the humidstat was bad, not the solenoid like I originally thought.

In mine, I have a red wire from the transfomer to the humidstat. Then a white wire from the humidistat to the solenoid, and white again coming out of the solenoid. This white goes back to the furnace to a door switch (furnace won't run with the door off). The other side of the switch is a yellow wire to the relay, yellow wire back out of the relay to the starting transformer again to complete the loop.

The main points are that you should have a relay (as asked in your comment)- this makes it so the humidifier won't run unless the furnace is also running. While checking all this, set your furnace to run (i just cranked the heat after installing) otherwise it will appear your humidifier isn't working.

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