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.