Thanks everyone. I will have some thinking to do and see what route I take.
I had same problem before and one of my expert friend solved problem . Firstly I had dual transformer system, and he didnât add another transformer . He set C wire and we are glad and we can use our thermostat effectively .
I have the ecobee. Have the same setup with the dual transformer system. The c wire is no good and my system is too old to replace the motherboard. So I bought a 24v plug in transformer. I already have something plugged into Rh and rc. What should I do?
I thought I didnât have a C wire.
Lo and behold, I did⊠there was actually the C wire terminal on my system, and a wire on the run to my thermostat that was not being used. 
Hi John, I think I have the same C wire issue as you did with your nest. I have a two transformer system with 3 zones. 3 heat circulators off the boiler and two AC fan units. My one nest that is furthest from the boiler has constant power issues. Nest support said to run the common wire to the âCâ terminal from the AC unit. I was going to tie a common wire into the fan unit for the problem thermostat and tie it into the C jack on the nest. Is that what you did to fix your proble.?
I did exactly that. The nest has all the power it wants now for its WiFi radio and keeps charge great. The problem I then had was the nest want to send 24volts to your boiler when you call for heat. A boiler does not want or need 24vots. Your boiler just wants itâs two wires to latch together, just like an old non WiFi thermostat would do. To solve this problem I put a relay in between the thermostat and the boiler. So the 2 wires coming from the nest connect to one side of the normally open relay. On the other side of the relay is the 2 wires to the boiler. So now the nest sends 24volts to the relay, the relay latches shut connecting your 2 wires from your boiler together and the boiler fires up as expected.
Thanks so much for your response John. I was worried about something like that happening. I was going to get this item:
https://www.supplyhouse.com/sh/control/product/~product_id=IR882
Is that the same type of product you used? I was going to just hook my 3 heating zones through it. Did you put a common wire on all of your zones? Iâve only had trouble on my living room I think because of how far the wire is from the boiler, but I was thinking I should just put a common on all three zones.
My only issue is my basement just has heat which nest said I could then use the boiler for the common wire but Wels McClain advised against that so I dont know if I should just leave that one alone.
Do you have a similar zone setup with 3 heating and two cooling?
I just have one zone on my boiler and a separate ac system for cooling. My common comes from the ac unit. The relay I use I got from my local hvac supplier. Only cost about 10 dollars or course Iâm only running one zone. I was told these relays are sometimes referred to as peanut relays. The system you are running is much larger and a bit over my head as I do not do hvac for a living. I wish I could be more help.
You have been alot of help. Would you maybe be able to post a picture of your relay setup so I can show my HVAC guy (who is my Uncle - lucky me)? Since I only have one nest giving me a problem I bet my set up would be pretty similar to yours assuming I have the same issue on the boiler once the common wire is installed.
I realize this is an old post, but Iâd add a vote for getting a separate 24VAC adapter. You can get one for ten or twenty dollars. They even make them with extra long cords, especially for C wire additions. Wiring couldnât be easier. You connect one wire to Rc (or Rh), and the other wire to C, then plug the adapter into an AC outlet. Maybe a little unsightly, but you can get creative as to how you hide the wire, even putting it in the wall and drilling a small hole near the AC outlet. If Iâm not mistaken, this is the ONLY solution that would work if there is no access to the heating or cooling units, or the air handlers.
Despite comments to the contrary above, you absolutely can add a third transformer this way. The AC adapter is just powering the thermostat. It doesnât interact with the rest of the system at all. In fact, while it may be counter-intuitive, it doesnât even matter what polarity you make the connection with. Itâs floating.
Iâd like to propose another solution (please correct me if this wonât work). If you could get to all the units, and the layout was amenable, couldnât you ditch one of the existing transformers (letâs say Rc) and short Rh and Rc everywhere else? Then use the unused Rc wire to the thermostat as the new C wire? In my case, both Rc and Rh are available at the air handlers, So couldnât I just remove the Cooling transformer? Shorting Rh to Rc at the air handler would supply all the the cooling systemâs relays from Rh, freeing up the Thermostatâs Rc wire to be used as the C wire. Youâd short Rh and Rc at the thermostat as well. comments?
No, thereâs no problem with adding a third transformer. It wouldnât affect Rc or Rh at all. The system would be unaffected. The third transformer would only be supplying 24VAC to the thermostat itself. Technically, you donât even have to pay attention to the polarity. If the third transformer is wired in phase, then the thermostatâs C connection will always be at the same ground as the rest of the system. If you wire it out of phase, then the thermostatâs C connection will be at 48VAC, but the rest of the system will still work properly. The thermostat and relays will only see 24VAC. The thermostat will not be tied to ground, but it really doesnât have to be. It will still work fine. Do be careful though, if you for some reason added an actual C wire in addition to the transformer while it was out of phase, then youâd have a short.