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I was looking around at my furnace wiring to see if my C-wire was connected on the furnace side and noticed that the C-wire from the thermostat goes to the R-terminal on the furnace. This doesn't seem correct to me, but are there any issues with such a configuration?

Currently the C-terminal on the furnace is connected to A/C and a humidistat, so there are currently 2 wires going to the C-terminal.

Here's the thermostat wiring: thermostat wiring

And here's the furnace wiring: furnace wiring The 2 black wires currently connected to the C-terminal go to the A/C and a humidistat. White wires are currently going to the Y, W1 and W2 terminals.

So if the current configuration is ok, then just wanting to double check. If the current configuration is not correct, what are the possible issues of this setup? Is the thermostat only receiving power when the heat is on? (the thermostat also uses batteries)

Is it safe to re-wire the black C-wire from R-terminal to the C-terminal instead? That would then make 3 wires attached to the C-terminal. At that point, should I pigtail them all together so that only one wire goes to the terminal?

Overall, I'm wondering why the furnace is wired the way it is.

2 Answers 2

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If the black wire on the R terminal goes to the thermostat and only the thermostat, you can safely move it over to the C terminal. The AC and humidistat being hooked up to C is correct.

R and C together carry the 24v power that a smart thermostat (and some programmable models) use for power. Right now the thermostat isn't getting any power. This is low voltage so you shouldn't have to pigtail the wires, but if it makes the connection seem better to you, you can pigtail it. Its up to you.

Just a guess, but someone may have known that the C terminal "needs power", and they also knew that the R terminal supplied voltage, so they just assumed power is power? I can't think of any other reason R and C would both be connected to R at the air handler/furnace.

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  • So you say the thermostat currently isn't getting any power... and that's because the C-wire is connected to the R-terminal? So if I just move the C-wire from the R to C terminal, then the thermostat will start receiving power?
    – akaspick
    Commented Nov 12, 2021 at 16:25
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    Right. Sorry if this is basic, but it takes two wires to provide something with power. To oversimplify, voltage has to flow to the device and then back to the source, wether the source is a battery or a transformer or an outlet. Right now since black and red are connected to the same R terminal, they are the same one wire. Moving the black wire to the C terminal provides the "path back to the source" that the current needs. On the furnace side, the R and C terminal represent the two wires from the 24v low-voltage transformer that powers the circuits of the system.
    – JPhi1618
    Commented Nov 12, 2021 at 16:29
  • You can get a basic voltage meter for cheap and it will help troubleshoot these issues or just help you learn about electricity in general.
    – JPhi1618
    Commented Nov 12, 2021 at 16:29
  • So I used my multimeter to test R and C on the thermostat together and got 0 volts. I moved the black wire from the R to C terminal and retested R and C on the thermostat and now I get 27 volts. So it looks like it's working as it should now. Just seems odd that somebody wired it up "incorrectly" in the first place. Thanks.
    – akaspick
    Commented Nov 12, 2021 at 16:48
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I think this is a bad idea. Generally, as I understand it (I'm a physicist and not an HVAC specialist or electrician) the C is a common return that allows "smart" thermostats (like Nest or other WiFi thermostats) to have a return path for power that does not go to the W (heat) return wire (the +24V power coming from R). This latter path either throws a relay or otherwise activates the "call for heat" which may open Taco valves and/or start the circulating pump(s) for the appropriate zone. The Nest "learning" thermostat and others that support a 2 wire system (+24V between R and W) get their power by "stealing" a little current from the +24V between R and W but not enough to trigger the heat. This is generally sufficient but you may get into situations where the Nest battery runs down and goes offline because it doesn't have enough power to support WiFi. This seems to happen when there is a prolonged call for heat....which may defeat power stealing (maybe). Adding a C wire provides a return (common) for power that doesn't trip the heating circuit. So....if R (24V is coming from one side of the 24volt transformer) the only logical "return" is the other side of the 24V transformer. If your furnace doesn't have a C terminal already correctly wired - or if you have a very old t wire heat only system (like I do) then you must wire C to the other secondary terminal of the 24V transformer NOT THE ONE THAT R is wired to. This isn't about supplying power to the thermostat - its about a return path for the power (hence the designation as "common").

At least that is my understanding of all this having thought about it a fair bit. I'm about to wire this up as above. I'll report back if I'm wrong but wiring R and C together makes no sense to me. I tried it on another heating system with a Nest and it didn't work.

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