I've spent quite a bit of time trying to sort this by looking at existing solutions but still not following. Can you tell me where I can tap in to pull a "c" wire from to the thermostat?

I'm trying to hook up a google nest smart thermostat to an old furnace (25+ years old). Old thermostat was manual, only used red/white but there are unused wires. On the furnace control board, nothing specifically labeled C that I could find. I see the transformer(model cr453xt1ae1ep). I assume the thicker white and black wire are line voltage. The other side of the transformer, I assume are the low voltage. I see a red wire on the low voltage side and then 3 wires coming off the transformer, one seems to be grounded to the chassis. I tried to test these 3 wires (no luck, probably user error). I tried splicing onto one of them but didn't seem to solve the problem. How can I get a common on this old board? how do I check the transformer is working correctly? I don't have the furnace schematic. Thanks in advance for your help. enter image description here

extra photos in case it helps: https://drive.google.com/drive/folders/1uWmK44h9mqF6BQMAUz6m0mlK0DAGxA6g?usp=share_link

  • 1
    What model is the furnace? I didn't dig through your photos, so my apologies if it was there.
    – KMJ
    Nov 1, 2022 at 20:29

1 Answer 1


On the left side of the 24 Volt transformer.

You have a group of 3 wires and stand alone wire.

The stand alone is 24 Volt Hot.

In the group of 3 wires the top one is the "C"

C wire

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    Thanks. I tried connecting a "c" thermostat wire to one of the 3 wires coming off what you labeled C. The thermostat still report no power. It looks like to test voltage, I would just measure those two points on the transformer. so if I measure one contact on C and the other 24v (from your picture), it should measure 24v (once I press the safety switch)? I'll do some checking tomorrow. Maybe the transformer is going bad or the thermostat wire I chose was defective.
    – mv916
    Nov 1, 2022 at 21:43
  • I assume you activated the safety switch, that should give 24 volt on the transformer. Or even 120 Volt on the other side of the transformer on those isolate pins. Or 120 Volt anywhere measuring against the ground. Attach one test wire to ground, then check around with the other test instrument wire for 120 Volt, with the safety switch engaged to know if you have power.
    – Traveler
    Nov 1, 2022 at 22:20
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    This worked perfect. In my rush, I used the wire that went to G (fan) instead of the blue/silver wire that came from the point you labeled for C. After making the change you outlined, everything worked perfect. It feels like such a dumb question in hind sight. I appreciate your help!
    – mv916
    Nov 4, 2022 at 5:51

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