No C Wire on Thermostat but Two C Wires on Furnace

I am installing my thermostat but having wiring challenges.

For background, I replaced the furnace (and thermostat) last winter, ie, both are new. There is also an inline humidifier which turns on when the heater is running. It’s dry in Colorado so humidification is needed.

Wiring at the Thermostat: R (red), W (white), W2 (light blue), Y (yellow), G (green). There is no C.

Wiring at the furnace is difficult to see, but I believe the same wires are attached as they are on the thermostat end.

This is where it gets tricky. At the furnace end, there are two wires (both white) going into C connection/slot. I believe one of the C wires goes to the humidifier, not sure where the other white/C wire goes.

The big question: can I still install the C wire adapter as described in the instructions? I would be stuffing a third wire in the C slot.

Let me know what other relevant info I can provide.

Thanks in advance for any help.



The other wire on C is probably going to the AC condenser. Is there also 2 wires on Y? Adding a 3rd wire to C shouldn’t be a issue, it’s just the “common” wire.

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Need better pic at furnce. and pic the C wires where do they go.

Need to undo exiting Tstat (correction not furnace) and pull wire bundle out from hole.

Thanks all. I will get a better shot of the wires and update the post.

Being a very simple/obtuse person, what other considerations are there if I can simply add another C wire?

Should be ok if easy to pull wire to Tstat.

The C wire is the Common wire that supplies a constant 24VAC. If you have a free wire on both ends on your cable, great, you can use that free wire and plug it into C for both ends. If your C connection point is already being used for something else, you should be able to add another wire into that green fitting. Should be enough room. Loosen the top screw to unclamp the wire in it and add the new wire into it also and tighten back down.

I basically did the same thing on mine though it wasn’t being used, it was just empty but I had a couple spare wires on each end that were just twisted around the other wires out of the way. If you don’t have any extra wires, if you can run a new thermostat cable which will have a bunch of wires in a single sleeve, great. I could have done that also, but wasn’t needed in my case.

I did a friends system also on their 2 story house and it was a little more complex, but basically ended up doing the same thing. Lucked out there also. The smart thing is to wrap the extra wires by folding them back and around the outer casing. If the installer didn’t leave a little extra in the wall to pull out enough to gain access. Sometimes they just cut the extra wires off. There’s still there, but hopefully you have enough extra wire to pull out of the wall to peel back the outer casing to gain access to the cut off wires to use.

If you don’t have any extra wire and you can easily pull new wire from the furnace to your thermostat, great. You can buy new wire from like Home Depot. Thermostat Wire like 18/5 or 18/7, or 18/8 depending on how many wires you need. Having extra wires won’t hurt anything. Who knows what the future holds. Just like this whole C wire thing.

Thanks for the info. The easiest solution, based on the majority of responses to my question, is to use the C wire adapter because I don’t have any extra wire at the T-stat and don’t want to run more wire from the furnace to the T-stat. It sounds like it’s OK to have three C wires at the furnace end (2 existing wires and the one from the C wire adapter.

Pls let me know if the plan above has any issues.

Thanks again

It should be fine. Inside the furnace, the common wire is connected to every single 24v switch and component. As long as the load on the transformer stays under the fuse rating, you wont have an issue. If it goes over, and you start popping fuses randomly, it’d be a good idea to upgrade to a higher power transformer, but it’s fairly unlikely the wyze will cause that issue.

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Yes, go with the simple thing and use the adapter. Don’t worry about any wires in C currently. It’s common, almost like ground, but not.

Normally that common wire is black on the furnace wiring. But if all you had was a extra blue wire or something you could use, why not. Wouldn’t hurt anything so long as it was common C on both ends.

Again, if you can’t replace your wiring, it’s not the end of the world or a big deal. Use that Adapter by all means. That’s why it’s there. I hope with newer homes they aren’t so lazy, or cheap with the thermostat wiring?!?! Why they just haven’t been connecting it all these years?!?! Save a few cents and just run on batteries seems to be what they do.

My office at work has it’s own. I needed to replace it. I put a cheap programmable type in. But didn’t like only batteries. Hard to see the screen. So I was able to wire up a C wire for it. Now the screen is lit up all the time and I don’t have to keep replacing batteries. I just don’t get why they do this crap. Have enough wires to also make the C connection. Still put it a cheapo Thermostat if they want.