Connecting Generalaire humidifier to Nest

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Old 01-13-15, 10:52 AM
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Question Connecting Generalaire humidifier to Nest

Hello,

I'm trying to connect my Generalaire 1042 whole house humidifier to a Nest thermostat. First some background. Just finished building a house and I knew I wanted a Nest. My installer is somewhat "old school". He didn't want any part of installing the nest and planned to install a separate humidistat near the thermostat. He agreed to put in a temporary thermostat during construction with the understanding I would install the Nest myself. The furnace is working as expected and the humidifier is installed, but has never been turned on yet.

I have this humidifier:
Generalaire 1042
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and this humidistat:
Generialaire MHX
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I started my research at Nest's support page. This document says there are 4 potential ways to hook up a humidifier. Standalone humidifier 1-wire, Standalone 2-wire, Builtin 1-wire, and Builtin 2-wire. Based on the manual for the humidifier, I think I have a bypass or standalone unit. I don't understand if I have a 1-wire or 2-wire though. Both examples show 2 wires coming out of the humidifier. This picture shows two different ways to wire and speaks about a bypass humidifier (rather than stand alone) and a Powered humidifier (rather than built in). Is that right? Are they just using different words to describe the same thing?

Here's a picture of the solenoid connection. Note, I've already disconnected the wires that were there from the install.
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The 1-wire/2-wire question determines if I need a relay, right? I went ahead and purchased the recommended White Rogers 90-290Q, but now I'm not sure if I need it. Here's a picture of the furnace board and the nest.
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Here's my questions:
  • What do I need to do to make the humidifier work with the Nest?
  • Do I have a 1-wire or 2-wire system, and do I need the relay or not? Can I just get by with 1 wire from the humidifier to the * on the nest, 1 wire from the humidifier to the C on the furnace, and 1 wire from the C on the furnace to the nest?
  • The installer didn't run the optional common wire to the Nest. It would be easy for me to do this now (unfinished basement). Should I do that?
  • Did I wire the R from the furnace board to the Rh on the nest correctly?

One more thing. I'm not sure the humidistat would have worked as expected anyway. The manual says that, "When using this product as a standard humidstat terminals C and NO must be used" This picture was taken after I cut the wire but the terminals are still connected. It clearly shows the NC and C terminals being used. Does that seem right?
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  #2  
Old 01-13-15, 07:24 PM
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Welcome to the forums.

All these nice pictures!

I'm not a fan of running thermostats on power stealing. Always use a Common connection when you can.

I see the four wires connecting your nest to your furnace. Those are correct.
It looks like you have a second two wire cable from the nest to the furnace. (old humidistat)
Mark the red and white wires with something like a marker to keep them from getting confused with the four wire cable.
Connect the white to C on the nest and C on the furnace board.
Connect the red wire to the * terminal on the nest. Connect the other end to one of the solenoid power wires. Now take a spare piece of wire and connect the other solenoid wire to the C connection on the furnace. You will now have three white wires on the C terminal.

Be sure to turn off furnace before changing wiring.

You should program your nest to humidify when furnace is heating. I don't recommend having the nest turn the blower and humidifier on. Cold air doesn't carry enough humidity to make it a cost effective setting.
 
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Old 01-14-15, 11:24 AM
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So like this?
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I'm confused about 3 wires coming from C on the furnace. One goes to C on the nest, one goes to solenoid on the humidifier, where's the third one? Also, you're saying the only thing connected to the transformer will be the red wire going to the AC when I'm done, right? Where will the white wire from the AC go? So I can get away with not using the relay too?

Thanks for the help.
 
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Old 01-15-15, 10:33 AM
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In your furnace closeup diagram.... the four wires circled in orange are your thermostat wires and the two circled in red are the wires that go to the A/C condensor outside.

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Old 01-17-15, 06:25 AM
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Ah, I understand now. I had the AC and the humidifier mixed up in my furnace picture. I hooked everything up as you suggested and it's working as expected (I think. How long should a 1800sqft house take to change from 30% to 45% humidity? I've never had a whole house humidifier before). I removed the transformer from the mix and didn't need the relay. Thanks for all the help!
 
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