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Wiring for separate heating and ac systems with c-wire from heating

Mike35
Community Member

Hi, having some wiring issues that I am hoping for help with after purchasing a Nest Learning Thermostat.

My setup: Heating from oil furnace, with baseboard heat that also runs hot water heater, provides Rh and W wires plus a recently added C wire to thermostat to ensure power for Nest Thermostat. Cooling comes from completely separate system in attic running through forced air vents, provides Rc, G, Y to thermostat…I usually turn off power in winter months to this cooling system.

Install issue: connected all six wires listed above, received e79 error about no power to common wire. I confirmed with multi-meter that new common wire was working. Tried turning cooling system power on to see if that was needed but same error message. After some troubleshooting, I decided to disconnect all three cooling wires and the Nest began to work fine and heat the house.

Question: I’ve seen a lot of posts about determining the right combination of Rh, Rc, and C wire and how the internal Nest jumper works…but I haven’t been able to see something that matches my setup. What I mean is I have Rh and C coming from heating system with Rc coming from completely separate cooling system. I would like to wire this once and not swap around wires each season. Is there a configuration of the Rh, Rc, and C wire that will work for my setup?

I would be extremely grateful for a response, thank you!

5 REPLIES 5

eveluna
Bronze
Bronze

I believe for your concern, you need to hire a technician to do this for you. For more information about contacting technician, refer to this link: Have a professional install products or service your HVAC system

Mike35
Community Member

Thank you for sharing then,ink, I’ll check it out. I had a HVAC professional install the new C-wire before adding the Nest thermostat. That led to the error message I listed in my post. I’ve been trying every combination of wires. I know that if I use the five standard wires (Rh, W, Rc, G, Y) and leave the common disconnected both the heat and AC work. I’ve tried combinations of Rh/C and no Rc (get N72 error about less power but heat and AC work) and Rc/C and no Rh (get E73 error and system won’t work). The only one I can’t try without more professional wiring work is common coming from cooling power instead of from heat power. That is the key info I was hoping someone would be able to answer from this forum and if that is a nest requirement for C power wire. Thanks

Jeran
Community Specialist
Community Specialist

Thank you for helping out on this thread, eveluna!

Hey there Mike35,

I'm just checking up on this thread, do you have any more questions or need any additional help?

Best regards,
Jeran

Mike35
Community Member

My issue is resolved. I found this outstanding video on YouTube: https://m.youtube.com/watch?v=-c1zlfz1IeY. Directly matched my setup and multiple solutions. Confirmed that Nest expects c-wire power to come from cooling system. Used the G fan wire to provide power, all seems to be working well at this point. This post can be closed.

AbigailF
Community Specialist
Community Specialist

Hi Mike35,

Thanks for the update. As we got our resolution here, I'm going to mark this thread as resolved. I'll be locking this thread if we won't hear back from you in 24 hrs. Should that happen, feel free to create a new one if you have more questions or have other concerns in the future.

Cheers,
Abi