A friend of mine wants to upgrade his thermostats to WiFi thermostats but the thermostat wires ran to the thermostats are missing the C wire. There are 2 thermostat cables running from 2 units to 2 different thermostats. We can run a new 18-5 cable from one of the furnaces to one of the thermostats. But we can't get the other unit open because of the way it was installed. Is it possible to split the common wire down at the thermostats and use it for both new thermostats?
Electrical – Can a C wire be used for two thermostats
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Related Solutions
On the schematic, the BL wire in the section labeled "Low Voltage Field Connection" near the bottom of the diagram. That's the C wire.
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It appears as though the "C" wire is bonded to the chassis, so you should be able to connect the C wire from the thermostat to the chassis as well.
In the last photo, if you follow the yellow wire from the thermostat cable. It connects to a yellow wire from another cable, which I would assume leads to your A/C system. The blue wire from the A/C cable is attached to another blue wire, that blue wire ties in with all the other "C" wires. So you could connect the C wire from the thermostat, in with those blue wires as well.
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I'm not sure exactly what's going on with the wiring, since in the one photo there's a giant ball of wires and cables, and in another there's extra wires hooked up. Here's what I see in the photos you've posted.
In this photo, you'll notice the cable with a red and white wire. That cable should go between the furnace and the A/C unit. The white wire from the cable is connected to Y, while the red wire is connected to C.
The other cable I've labeled should go to the thermostat. The wires from that cable should be connected as follows:
- White to
W - Green to
G - Yellow to
Y - Red to
R
It's difficult to tell if that's what's going on, due to the big mess of wires and cables in the middle of the photo.
Also take note of the blue and black wires, that are wrapped around the thermostat cable.
In this photo, the cable coming from the furnace looks completely normal. However, there seems to be an extra cable that comes from who-knows-where. It appears that the wires from this mysterious cable, are connected to the blue and black wires that at the furnace are wrapped around the thermostat cable. Which if that's the case, they're doing nothing.
It's possible that you might be able to attach the blue wire that is wrapped around the thermostat cable, to the C terminal at the furnace. Then attach the blue wire at the thermostat, to the C terminal on the thermostat. Though without knowing what the mysterious cable at the thermostat is, there's no way to be sure.
I wasn't able to decipher the diagram on the wall behind the thermostat, so I'm not sure what that's all about. I'm sure it meant something to whoever drew it, but it's not helpful to me.
I'm not sure if you have multiple A/C units, and the extra cable is used to control that. You'll have to investigate further, to determine what exactly is going on with that.
Best Answer
I think NO. The C-wire connects to the transformers on the individual units. It's not like a common neutral or ground wire. You'll need to get a C-wire to both furnaces as there is no real electrical connection between the two on the low voltage side. There might be other solutions that the "big three" might be able to come up with, but I think your stuck with getting a C wire to each furnace independently.