Very rookie DIY guy here...I have installed an Aprilaire 600M but can't figure out how to wire it. My thermostat currently is using the U1 port for a normally-closed damper on a fresh air intake.

I wanted to connect the humidifier so that it would use the humidity control of the thermostat but (I don't understand how that would tell the humidifier what RH% to max out at.)

At this point I'm just looking for the simplest way to get this humidifier working because our house is at 14% humidity.

The Orange U1 wire at stat connects to the red wire of this fresh air damper . The white wire of damper connects to a 20VA transformer in the furnace (yellow wire). The black wire at STAT U1 connects to the thicker blue thermostat wire (2nd pic)

Aprilaire 600M came with it's own 24 volt transformer if I need to use it.

Thanks in advance for any help these two solenoid wires are all that's keeping us from comfort!!

thermostat furnace damper 600M waterline

enter image description here

  • 1
    I’m voting to close this question because it's been abandoned.
    – FreeMan
    Jan 2, 2021 at 19:58

1 Answer 1


I skimmed through the thermostat manual in the link but it didn't seem to address your model very well. For example, I didn't see anything about the capabilities of the U1 terminals.

The signal from the thermostat to the humidifier to quit humidifying is simple: the thermostat's output controls the water valve. When the thermostat turns off its signal the water valve closes and the humidifier dries up.

A simple solution is to get the Aprilaire humidistat and let it control the humidity rather than having the thermostat do the job.

It's a little nicer to have the thermostat control the humidifier valve, but to get there it looks as if you'll have to find some other way of controlling the damper. An upgrade of the thermostat to one that supports two accessories could work. There might be another way, though. It depends on when the damper should be operated. The furnace control might already have support for operating an accessory, for example. Or, if the damper should operate only when the thermostat calls for heat, you could add a relay to make it so.

  • Thanks for trying to help here Greg![P] [Page 86 here] (customer.resideo.com/resources/techlit/TechLitDocuments/…*ply9ei*_gaMjUwNTc3MjE3LjE2MDYzMTg5MzM._ga_YHMJCTKRN2*MTYwNjk1NTg0NC4zLjEuMTYwNjk1NjY2Mi41Mw..) shows the function of U1 ports and how to use with humidifiers. The actual model is TH8321WF1001
    – T Prince
    Dec 3, 2020 at 0:57
  • Greg I added a pic from the manaul to my original post with I believe shows the correct wiring diagram.
    – T Prince
    Dec 3, 2020 at 1:11
  • Does it mean that one of the 600M solenoid wires connects to the U1 wire and the other connects to BOTH 24V transformer wires ??
    – T Prince
    Dec 3, 2020 at 1:14
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    @TPrince Yes Figure 227 is the right way to do it. A jumper between R/Rc and U1, a wire from the other U1 down to one of the 600M solenoid wires. The second solenoid wire connects to the C terminal in the furnace. The thermostat has blue wire connected to C, so if you open up the furnace you should be able to find where that blue wire lands and connect the second solenoid wire to that same place.
    – Greg Hill
    Dec 3, 2020 at 1:44

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