I have an Aprialaire 56 humidistat that is connected to an Aprialaire 560 humidifier. Currently the humidifier is not turning on and after testing the wiring with a multimeter, the wires going in to the humidistat are not getting a current.

This system was installed by the previous homeowner and was tested and working previously to my knowledge.

The humidistat is wired into the furnace control panel in the C and W2 terminals. From my research, W2 is "stage 2" of the furnace heating cycle. My thought is to move the wire from W2 to W1 in order to draw power to the humidistat.

Are there any other remedies to troubleshoot why the humidistat and in return the humidifier are not powering on?


humidistat wiring

  • 1
    I would say that you are on the right track. Perhaps they thought previously that the only time they wanted additional humidification is whenever the house temperature dropped significantly below the set point of the t-stat. These are the conditions when second stage in your system will be activated, not necessarily a bad strategy. If you want more humidity, I see no reason not to switch one of those from stage 2 to stage 1.
    – Paul Logan
    Nov 30, 2017 at 5:26
  • Did you test with furnace running? Not AC, not blower only, but burner on heat producing.
    – Tyson
    Nov 30, 2017 at 5:26
  • @PaulLogan thanks for the info, that makes a lot of sense. I didn't think I could mess anything up by switching it, I just wanted to double check. We got an Ecobee thermostat when we moved in, so that might be effecting when it hits stage 2 as well. I'll try switching to W1.
    – james
    Nov 30, 2017 at 13:41
  • @Tyson yes, I made sure to test when the furnace was fully running (blower + heat).
    – james
    Nov 30, 2017 at 13:42

1 Answer 1


Wired in this way, this should have never worked. W2 is the terminal that the thermostat uses to activate second stage heat. It is not energized by the furnace, but by the thermostat.

Also, it looks like the humidistat is not wired properly. The wires from the furnace should be in the R, and C terminals. It looks like they're in the I-, and R instead.

Aprilaire 56 humidistat Wiring diagram
Figure H from manufacturer's installation instructions.

  • Thanks for the info about the wiring. Would you agree that W1 is the correct place for the wire, as opposed to W2? I ask this because I tried tonight to switch it over to W1 and then my Ecobee3 could not call up the heat any longer. I ended up having to use my old thermostat to get heat to work again (the blower had been working all along). So i'm wondering if have the humidistat going onto W1 as well as the heat caused some "confusion" among the accessories. The Ecobee thought the heat was turned on, even though it was not.
    – james
    Dec 1, 2017 at 3:44
  • Typically, yes. Using W1 makes sense, however, without inspecting the wiring it might not be correct. A better option might be to use the HUM terminals on the control board. Though this will require an additional transformer, since HUM is at 120V.
    – Tester101
    Dec 1, 2017 at 17:13
  • @jamez14 did you fix the wiring at the humidistat? Moving the red wire from I- to C. Then putting the white wire to W1 at the thermostat should work.
    – Tester101
    Dec 1, 2017 at 17:16
  • yes, i moved the red from I- to C. I'll give it another try now to see if the old thermostat will play nice with the humidistat
    – james
    Dec 2, 2017 at 0:20
  • With the old thermostat installed and moving the humidistat to W1 i was able to get the humidifier working. Thank you so much! Now I just need to test out my Ecobee3 again and see if I can get it to work with the humidistat now connected to W1.
    – james
    Dec 2, 2017 at 0:40

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