I tried upgrading to a Nest thermostat, and that's how I discovered I don't have a C wire...

I'm thinking how to add it. I have some interesting situation:

  • single furnace, and 4 zones connected to it through motorized valves
  • some sort of big power relay to help control those valves
  • also 2 A/C units
  • 4 thermostats throughout the house: 2 for heat, 2 for heat+ac
  • it seems that a/c wiring is entirely independent of the furnace, and that the heat+ac thermostats connect to two different circuits, one going to furnace, one going to ac - is that possible? are there any grounding issues here?

How would I add a C wire into this situation to all 4 thermostats? One thing I have going on for me is that the cables connecting thermostats to the furnace have some unused strands in them (so do the A/C cables). So I can use them to make the C wire, then bring those 4 wires to the furnace room, and then... what would I do next? Connect them to the furnace circuit? The power relay? Some external power supply?

And what about the A/C circuits - is it ok to just connect C to the furnace, and keep A/C out of this? Again, I'm not sure how grounding works here.

Wiring diagram below. NB there are no C wires connected yet, but the cables have spare strands that I could repurpose for this. The relay is a TACO SR502.

I could connect the C wire to the A/C or to the furnace. I'm thinking about getting 3 Nests, 2 for A/C+furnace, one for furnace. The 4th zone I'll leave with regular thermostats.

wiring diagram

  • Can you post photos of the wiring at all equipment involved please? Sep 2, 2021 at 23:33
  • I added a wiring diagram, I can take some photos too.
    – Jasiu
    Sep 4, 2021 at 21:22
  • Are all 4 zones connected to zone relays for heat, or are some connected directly to the boiler control? Sep 4, 2021 at 21:40
  • it's weird like that: the furnace control board supports only 3 zones, so there is a relay to make room for the 4th zone. and then there are peculiarities: there could be 3 zones on the furnace itself, but there are only 2; tstats for A/Cs could be both paired with the relay, or both with the furnace, instead they are paired one each.
    – Jasiu
    Sep 5, 2021 at 2:24
  • What make and model is your furnace/boiler? Sep 5, 2021 at 3:34

2 Answers 2


The thing to consider is that each thermostat is getting power from somewhere, and where the C-wire comes from needs to match that. For the furnace thermostats, the C-wire needs to come from the same transformer that the red wire comes from. Its possible that multiple units share a single transformer, but that's not likely. I would assume that each unit has its own transfomer and therefore needs to supply its own C-wire.

For the thermostats that control AC and a furnace, the Nest (and almost all) thermostats will have a terminal for Rc and Rh (Red cooling, Red heating). Many smart thermostats need the C-Wire to come from the same transformer as the Rc wire, that is the transformer that controls the AC. Of course if the AC and furnace are combined, there will just be one R wire. Separate wires are used if the AC and furnace have separate transformers.

Some AC and Furnace control boards will have a screw or terminal to attach a C-wire, but if they do not, the C-wire is just "the other wire from the transformer". The 24v transformer has two wires - one ends up being the R(ed) control wire that goes to the thermostat and "the other wire" is the C or Common wire. There should be a 24v measurement between the R and C wire.

For the thermostat wires, blue is the most common color for the C wire, but color isn't important. Labels would be nice if you can't use blue.

  • Thanks for this info. Why does C wire have to come from the A/C? If I have C wire both from heater & A/C, I still have to use the one from A/C? Why is that? Also, is C wire kind of like ground? Can they be linked together to make a common ground? I saw people having trouble with Nest where it bleeds some voltage to th R/Y furnace loop, and makes the relay go crazy...
    – Jasiu
    Sep 4, 2021 at 20:29
  • The 24 transformer for the air conditioner is alternating current (AC) so there is no "ground", but conceptually, yes it is a ground. Also, you might be able to tie all the C wires together, but depending on irregularities with various transformers, it may not work the way you expect. They are all generating their own 24v sine wave, and they should be the same because they are using the same 60Hz AC current from the mains, but... its just not recommended. The C wire must come from the AC because thats what the thermostat expects. It assumes that Rc and C is a 24v supply it can used.
    – JPhi1618
    Sep 7, 2021 at 19:28
  • Is that specific to Nest that C wire must come from A/C? But I can also connect it to heat-only, in which case C would come from the furnace, right?
    – Jasiu
    Sep 8, 2021 at 17:03
  • It's possible that Nest has changed, so you'll have to give it a try or dig into the technical docs for your model (or give them a call). I know I looked this up years ago and it required 24v between Rc and C and I've seen that with other smart thermostats as well. I'm not sure what happens for a heat-only install, but may it requires a jumper or config change.
    – JPhi1618
    Sep 8, 2021 at 17:05

For the A/C zones -- Nests need the C wire from the cooling side

One downside of the Nest in a two-transformer application is that it requires that the C wire come from the same transformer that is powering the cooling side of the house (Rc, Y, G, O/B). Fortunately, this works for the two Nests you have that are controlling both heating and cooling, so you can handle that conventionally provided you can find the C terminal on your air handlers.

For the heat-only zone that's getting a Nest...

However, we have to think a bit more about the heat-only zone. Fortunately, current/newer Taco SR50x-4 relays supply a 24VAC/Common terminal block above the transformer, so you can tie your C wire to that if your heat-only Nest is running off the Taco. If it's running directly off the Navien, then matters are equally as simple as the Navien NFB-H control board provides corresponding C terminals for all its R/W pairs. Either way, the R wire on the heat-only thermostat needs to land on the Nest's R/Rc terminal, not Rh in this case, so that the Nest can pair it with the C wire.

However, some older Taco SR50x relays may not provide a 24VAC/Common terminal block; in that case, you may need to take a separate 24VAC transformer and wire it to Rc and C on the Nest, or simply replace the relay with a current production unit.

  • Thanks! Elsewhere I saw that Nest causes the following issue: when connected to Rc/C of the A/C, it causes chattering of the Taco relay. Apparently some little voltage is bleeding to the Rh/W circuit and causes the relay to chatter. Are you familiar with this issue? If I run into this, is there anything that I can do?
    – Jasiu
    Sep 10, 2021 at 18:31
  • @Jasiu -- I'm not terribly familiar with that issue, no Sep 10, 2021 at 23:39

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