Installing Nest on older furnace.


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Old 01-05-18, 11:50 AM
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Installing Nest on older furnace.

Hello all,

I hope its ok that I'm piggybacking on this thread. I've been doing a lot of research on how to make my own millivolt system work with the Nest and it seems like the folks with the know-how are here. I'll try to explain everything as clearly as I can, but please let me know if I can clarify anything. Thanks in advance for your help!

I have an old forced air natural gas furnace under the house, controlled by a two-wire thermostat installed on the wall.

The thermostat looks like this under the cover:
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The two wires, red and white, from the thermostat connect to two boxes that are attached to the furnace under the house. The white wire connects directly to the upper box (thermopile?) while the red wire connects to the lower box (gas valve?). A black wire connects the lower box to the upper:
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Another angle:
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I've purchased the Honeywell Universal Relay (https://www.amazon.com/dp/B000LDBP9Q...d384196bccc511) based on reading this: https://medium.com/@chrisvale/contro...t-b9493bbc59da. I understand that the Honeywell relay gets wired into power so as to supply the Nest, and moderates the signal between the new Nest thermostat and the old furnace hardware.

My question is how exactly should I wire in the Honeywell relay. Do I simply take the white and red wires that are currently running from the existing thermostat to the two boxes on the furnace and wire them into the Relay instead?
 

Last edited by PJmax; 01-05-18 at 12:02 PM. Reason: reoriented pictures
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Old 01-05-18, 12:05 PM
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Welcome to the forums.

I will probably make this a thread on its own.
That is not a millivolt system. The thermostat is in series with the 24vac transformer.

The gas valve is down low. The silver box with two terminals is a 24vac transformer and the black wires going into the side of the plenum are for the high heat safety limit.

Try connecting the two wires to Rh and W1 and see if it works. It should.
 
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Old 01-05-18, 12:42 PM
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Hi PJmax,

Thanks very much for welcome and the quick reply. Please feel free to move this post to wherever it is most appropriate (or let me know if thats something I need to do!).

To clarify, do you mean that the red and white wires running to the existing thermostat should already be supplying 24V, and that I should try to connect those two wires into the Rh and W1 terminals of the Nest backplate as-is?

In case it helps, here are a few more pictures I took while I was under the house this morning:
This Honeywell box connects to the corrugated metal pipe protruding from the left side of the 24vac transformer ("top box" in my layman's parlance). Not sure what it does:
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And this switch is connected to the corrugated metal pipe protruding from the top of the 24vac transformer. I assume this is the on/off switch for supplying power to the transformer from the mains?
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Old 01-05-18, 02:03 PM
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Yes.... take the two wires off the current stat and attach them to the nest.

Allow the nest a few minutes to charge and then it should come to life.
If not....let me know.

The top picture is called a fan/limit control. It controls the blower and it shuts the burner down on excessive overheating. The second picture is the 120vac service switch.
 
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Old 01-05-18, 02:23 PM
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Pete,

You were correct. I remembered I had a multimeter in the garage and tested the two wires at ~24vac. Installed the Nest with no issues after that, no extra hardware necessary.

Thank you for cleaning that up for me!
 
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Old 01-05-18, 02:41 PM
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You are very welcome. The pictures were a BIG help.
 
 

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