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Thread: Nest 3.0 with Aprilaire 500M humidifier

  1. #1
    Junior Member

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    Nest 3.0 with Aprilaire 500M humidifier

    Hello,




    I've wired up my Nest 3.0 to my Aprilaire 500m humidifier as described in the earlier posts (diagram below). One difference is that I'm using the White Rogers 90-290Q relay suggested by Nest.

    Click image for larger version. 

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    I've verified that the transformer/relay/humidifier circuit is working as expected by checking voltages across all leads and also disconnecting * and instead applying Rc (24V) to the relay, causing my humidifer solenoid to activate (water starts flowing).

    On the Nest side, it detects the * wire, and I modify the default settings to use HUM+heat. Everything looks great on the Nest side, it shows water droplets on the display (meaning that it's trying to activate the humidifier), but the humidifier itself is never actually activate (water doesn't flow)! I next tried running the Test mode in Nest and it said something like "Your humidifier should now be running (G,*)", or something close to that. My humidifier still didn't activate. I thought that maybe my humidifier circuit (or something else along the wire) was loading it down and reducing the voltage, so I tried replacing the * wire (I had another spare wire that I could use between the Nest and my furnace). Using that wire yielded the same results. I noticed that the voltage across the * and C wires was 0.01VAC, so I tried unplugging the * wire from my relay and the voltage across these two leads remained 0.01VAC.

    In essence, it looks like my Nest isn't wanting to provide a "Turn on" signal (which I assume to be 24VAC) on the * wire. Did I miss something obvious here or am I simply dealing with a defective Nest unit? What voltage should one expect between * and C?

  2. #2
    Member

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    Sounds like broken Nest. Give them a call.

    Remote chance of issue but Nest uses Rh to drive the * so if you have separate Rh and Rc you need to use C from the furnace, not a/c. Otherwise try moving Rc wire to Rh.

  3. #3
    Junior Member

    User Info Menu

    Thanks very much for your reply!

    I have only one line on my furnace control board called "R". It is currently hooked into Rh on my Nest. I also tried connected my C line to the Nest, but when I do, the Vin/Voc voltages (shown on the Nest screen) both drop from 38V to 2.4V. This looked very bad to me, so I disconnected the C line.


    Quote Originally Posted by Stuff View Post
    Sounds like broken Nest. Give them a call.

    Remote chance of issue but Nest uses Rh to drive the * so if you have separate Rh and Rc you need to use C from the furnace, not a/c. Otherwise try moving Rc wire to Rh.

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