Hooking up Nest G3 with a Lennox furnace and using nest for humidistat

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Old 06-09-19, 10:36 PM
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Hooking up Nest G3 with a Lennox furnace and using nest for humidistat

So Iím fairly certain I have my nest wired correctly as I have tested all the functions and they are all working properly. My only concern is that nest isnít getting its power from the common terminal even though it is hooked up. For some reason Iím reading zero volts on my common terminal. Iíll attempt to attach a schematic for my furnace and the one I used to wire the nest.

 
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Last edited by Pauly77; 06-09-19 at 11:01 PM.
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Old 06-09-19, 11:15 PM
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The schematic isnít turning out very clear but the model is a Lennox ML195.
 
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Old 06-09-19, 11:57 PM
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Welcome to the forums.

The C terminal has no voltage. The C stands for common.
With many furnaces.... the C is actually the same as ground or the metal housing.

The nest puts out 24vAC on the * terminal on a call for humidity.
So at that point you should measure 24vAC from the * to C.
 
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Old 06-10-19, 10:36 AM
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I didnít realize the yellow/common side was grounded when I first looked at the print. If it wasnít grounded wouldnít both outputs of a single phase transformer have 24v, in a 120 step down? Iím just curious for future reference, I canít seem to find any information this.
 
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Old 06-10-19, 06:42 PM
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The transformers used for this purpose have two wires or terminals for the 24vAC output. The secondary is floating from ground. Either terminal can be designated as the Common. Some companies keep the C insulated from ground and other companies ground the C terminal thus grounding one side of the transformer winding.

As an analogy only.... the C can be thought of as negative while the R is the positive.
 
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