Where to Connect "Unused" C Wire

Hi everyone, I received the Wyze Thermostat yesterday and I’ve been working to set it up. After researching the heck out of “C” wires, I believe I actually have one that runs up to my AC unit (I don’t have a furnace since I’m in a warm location year round) but that was never connected.

See the attached picture where the blue C wire is and also comes out to my thermostat. So, my thought is, this should be easy to just connect my C wire to the correct terminal. However, I don’t have a control board on this old AC unit so I’m not sure where I should connect the “C” wire. Thoughts?

Welcome back @awross12!

That wire could definitely be used as a C wire if it goes to your thermostat. Unfortunately, I’m not sure where your C terminal is. Do you have a wiring diagram or model number for the unit that could be used to figure it out?

With the power off, dust off the top of that transformer in the upper left of the image. I think that the 24v side is the side with the yellow and pink(red) wires coming out of it closest to the thermostat wires, but I’d like to make sure before I make any recommendations.

If I’m right, then the thick yellow wire coming out of that transformer should be your common. It appears to go over to a common terminal with a lot of other wires on the fan relay right below the box with the thermostat wires in it. You could use another wire with a piggyback spade connector crimped to it in order to connect if you don’t have an open spot on the common fan relay.

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Ah, okay, I’ll go up on the roof to check that tomorrow! Thanks and I’ll report back here!

Alright, I’ve identified that terminal as 24v after dusting off that label, would it go on the only post left on that box?

@speadie Definitely appreciate the help in pointing me in the right direction on this!

That is a high voltage terminal for the fan.

If there are no free terminals directly attached to the yellow wire, you will need to connect it to the yellow wire, either by using a T tap or by cutting the yellow wire in half and adding a wirenut to allow the common to be sent out to your thermostat as well.

example T tap kit:

@speadie So, I did that as you can see in the picture, but it doesn’t seem to be giving me any power down at the thermostat. I’ll take my tester up in a bit to make sure there’s power flowing from there first. Otherwise, I’m wondering if I got too big of a tap connector. Anyway, I’ll do some more testing and get back.

Edit: So, I actually am getting power. Tested each wire down at the thermostat and it is testing as it should at 25v between the blue (C) and Red (Rc). Is it possible I have a dud? @Brlepage

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Well, I was paranoid so I studied up on what each wire at the thermostat should test at with corresponding wires. It does appear that the C wire does now test at 24v correctly when tested again the Rc wire. However, now that I’ve verified I’m getting power correctly, the thermostat isn’t actually turning on strangely. Thoughts?

The thermostat isn’t showing the Wyze logo when the power is on?
Is there any light from the screen? Try holding down the dial for 10 seconds to reset the thermostat.

@Brlepage No light or Wyze logo. I tried the reset and also didn’t see anything.

You may want to contact Wyze Customer Support about this issue! it is possible your unit is defective and may need to be replaced. Support can also help troubleshoot the wiring further if needed.
By phone: (206) 339-9646 Available Monday - Friday 5 am - 6 pm PT and Saturday 8 am - 4 pm PT
Or online: Wyze.com/support
phone support is typically faster however this isn’t always true due to fluctuation in the amount of calls.

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Maybe a photo of your Wyze Stat wiring could provide a clue?