Nest Humidifier Install with GeneralAire Humidifier


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Old 01-11-20, 03:22 PM
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Nest Humidifier Install with GeneralAire Humidifier

I know there are lots of forum posts for this already but I don't seem to have an answer for my specific situation.

I'm trying to install my Nest Thermostat with my whole home humidifer (bypass humidifer). My humidistat is a generalaire MHX3 manual control. I've traced the wires down to the furnace and it looks like the humidifier is running on its own 24VAC transformer instead of using the Common 24VAC power. Is there a reason for this? How much power does the humidifier solenoid take?

I've seen some solutions that involve using a separate relay on the C line but can the common power not just power the solenoid on its own? I can confirm that the solenoid takes 24-60VAC for a max of 3.5W. I've attached a wiring image of my current humidifier setup.
 
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Old 01-11-20, 05:29 PM
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Welcome to the forums.

I can see your diagram but can only read the transformer.
It looks like the other two are humidistat and solenoid.

You can wire your humidifier directly to the nest. There are many threads here showing that.
One solenoid wire goes to C on the furnace board and the other solenoid wires goes to the * terminal. That 's it. Nothing else in the line.
 
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Old 01-11-20, 06:12 PM
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Oops I must've compressed it too much. But yes you are right in that it's humidistat and solenoid.

I guess I'm more curious why there's a transformer at all. Why wouldn't they just hook it up to the C for 24VAC on install?
 
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Old 01-12-20, 11:34 AM
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The humidifier needs to know when the furnace is heating to know when to operate. The HUM connections becomes live when the burner is running.

Many furnace control boards have 120v HUM (humidifier) connections. That would require a transformer to reduce the 120v to 24v for the humidifier to use. That's what your diagram is illustrating.

Some furnaces have a 24v HUM connection and wouldn't require a transformer.
 
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Old 01-13-20, 06:43 PM
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Ahhh ok that makes sense. I assumed it was there to power a separate electronic humidistat or something.

As an update for the forum I removed the transformer and wired it as you suggested and everything seems to be working!

Thanks for your help!
 
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Old 01-14-20, 09:00 PM
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Good job. Name:  thumb.jpg
Views: 124
Size:  1.6 KB Thanks for letting us know how you made out.
 
 

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