Aprilaire 600: No power coming out of humidistat

Question

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.

Any ideas?  And thank you.

Jamie M · Answer

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.

Aprilaire 600: No power coming out of humidistat

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