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I have separate heat and cooling systems with wiring to Y, W, G, Rh and Rc (see below). For the external transformer, where do I attach the two wires. C and ?

Current wiring

FreeMan
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Richard
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  • What do you mean by "external transformer"? Rh and Rc each come from a transformer associated with each of the two systems and you can use either one. Are you using a third transformer, over and above the two that the furnace and A/C already provide? Ideally you should just use the A/C's one for C. – jay613 Jan 23 '22 at 23:58
  • Thanks jay613. Yes an external transformer simply to get the c-wire to power the Nest. I can't run new wires from the AC (old house and I would need to open up the walls). – Richard Jan 24 '22 at 00:38
  • IDK if it's possible if you use both Rc and Rh to have C come from a third source. There would have to be a third R terminal on the thermostat. Might be a way. – jay613 Jan 24 '22 at 01:04
  • Thanks @jay613. There has been some posts indicating that it is possible if you check the phasing, but not exactly sure what that means. Keen to see if others have encountered this type of set up and can advise. – Richard Jan 24 '22 at 02:10
  • Can you post photos of the wiring at your furnace/boiler and air handler please? – ThreePhaseEel Jan 24 '22 at 02:46
  • Thanks @ThreePhaseEel. Unfortunately my system is in the attic in a small crawl space and I have not been able to get up there for a picture. Sorry – Richard Jan 24 '22 at 02:57
  • "Check the phasing" sounds like connecting one side of your third transformer to Rc along with the actual R from the A/C. Maybe if you're lucky that will work, and if you're even luckier it will work reliably. There is a more reliable way where you isolate the thermostat from both HVAC systems using relays, where the thermostat and the relays are all powered by your new transformer. https://imgur.com/a/CTkc3qG You need somewhere near the thermostat to put the relays. – jay613 Jan 24 '22 at 03:12
  • Thanks @jay613, I will try the phasing check. Also appreciate the relays suggestion. I will check it out. – Richard Jan 24 '22 at 12:38
  • If no pictures of the actual install are available, how about [edit]ing in the make/model of the furnace & A/C? Maybe even do the search yourself for the manuals in and include links to them. – FreeMan Jan 24 '22 at 19:25
  • Can you do this https://diy.stackexchange.com/questions/239320/adding-a-c-wire-using-a-24v-ac-transformer-to-a-2-wire-heating-system-for-nest with two relays (one for heat, one for cool)? – Fredric Shope Jan 24 '22 at 20:16
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    Are there any other wires in either of those thermostat cables? Probably easier and less perilous to source the 24V from the furnace or A/C unit's transformer. After all with 3 transformers in the system, if you phase them wrong you could have 72V across them. Your Nest might not like that. – Harper - Reinstate Monica Jan 24 '22 at 21:46

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