Thermostat wiring for Y wire

I have looked through the archives and do not see this answered,
My furnace unit is in the attic, When I looked on the control board, all the wires seem to be correct except the Y.
These are the wires that are right.

However, the yellow cable goes to a wire nut, and is connected to a wire that goes down to my outside fan, and then comes back on another red wire that goes to C.
I attached a photo showing the wires (ignore the Y wire label on the top red wire, I did that to help me distinguish it from the other red wire.)

I tried attached the upper red wire to the Y on the C-wire adapter, and everything seemed to work, but the AC never worked. It just blew room temperature air.
I put the old thermostat back on until I can get this figured out.
I don’t know what the red/white wire that comes from the outdoor fan unit is for. Any HVAC people on here that can assist?
This is a Trane furnace. Gas heat, electric air.
Thank you.furnace

The red and white wire come from the outside unit’s contactor and they usually are connected to the C and Y terminals on the furnace. Usually from there, the Y terminal is also connected to the Y terminal on the thermostat, which seems to not have been the case in your attic.

Do you have before and after pictures of both the thermostat and the furnace terminals?

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Here are before photos from both the thermostat and furnace control board. In the furnace photo, the red wire on the C-post is the same one indicated in the original photo that goes to the outdoor unit.
I have restored the old thermostat to get air working so don’t have an “after” photo.
Tomorrow when it is daylight I can go to the outside unit and figure out where the red/white wire goes in the unit.

I think it’s common for the Y wire to sometimes go straight to outside AC condenser bypassing the control board. The red wire from outside needs to stay on C at the control board.

It looks like you can use the unused blue wire to use as the C wire. At the control board, you would add it to the C terminal with the red wire that goes outside. Using the blue wire for C you won’t need the adapter. Just tell the app you have a C wire during install.

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Do you know what the purpose of the wire from the outside condenser is? Is it some type of safety override or a communication wire?
Also, are you saying that I should keep the red wire on the c terminal and then add the blue wire to the c terminal as well, leaving the yellow post empty?

I’m not a HVAC tech, but from my understanding, when AC is called for, a signal is sent to outside condenser to turn on.

Yes, I believe the condenser will not turn on without that red wire on C. It’s common to have multiple wires on C. Typically the Y wire from the thermostat does connect to the control board along with a wire from the condenser but I’ve seen several other people have their Y go straight to the condenser.


I just went outside and looked in the outside unit. The red and white wire both connect to opposite sides of the condenser contactor.
I will post photos and a diagram shortly

Yep, that’s normal

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He actually has a unused blue wire he can use for C


Thank you everyone. You are all a tremendous help. I mapped out the wiring between my furnace, thermostat and outdoor unit. Sorry for using the wrong terminology.
Based on my existing wiring, would I use the diagram Speadie posted, or would I need to use the blue wire for one of the terminals?

As long as the blue wire is available in the furnace, and is a continuous run, you can just use the blue wire, connected to C on both ends like Ken S said.


For my knowledge in knowing what the purpose of the different wires are, what do I gain by adding the blue wire, or what do I lose by not using it? It seems like without the blue wire, the Wyze thermostat communicates across the green wire. Does the system run more efficiently, or do I lose certain features by not using blue?
One of the reasons I ask is that when doing the setup process in the Wyze app, it wants to walk you through reviewing your existing setup and then connecting cables based on which terminals have wires connected. Does the app use this information to program the thermostat to use specific terminals to perform specific tasks? If not, I can wire it up based on the diagram above, and then just skip through the wiring preparation section in the app.

You don’t lose features either way, but not using the C adapter is the preferred way of connecting, if you have enough wires to run a direct common. The app does configure the thermostat inputs and outputs, without telling it that you have a C wire, it will not work correctly. Therefore, when you connect the blue wire to C, you need to tell the wyze that you have the following wires on your “old thermostat”:
R, C, W, Y, G

Also, if you have a gas furnace, make sure to tell it that you have a “radiator” attached to W1 when it asks you what that is, otherwise it will short cycle your fan.

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Thank you. Everything worked. I wired it all up and tested it and it appears to be working. Thank you for all your help. The key was telling it that my old thermostat had the C wire, as you suggested,
I had not seen your comment about telling the system I had a radiator before setting it up, so it is configured as gas, furnace/forced air. Is it worth going through the setup again, or should I go into the advanced setup and change the Fan Cycle to a different setting other than the default 20min/hr?

If you have it set up as a “furnace” then it will act differently than your old thermostat, basically forcing the fan to run as soon as the thermostat asks for the furnace to turn on, instead of waiting for the burner to light and the heat exchanger to warm up. It’s a nuisance, because it will blow cold air around your house in the winter, and could cause long term reliability issues with your furnace. Unfortunately, the only way to rectify it is to reset your thermostat by holding down the button until it asks you to reset it and remove and re-add it to the app, going through the setup again.

Thank you for all your help. System is setup and running great!

Well, Wyze has made improvements to the app and firmware. You can now set the fan to come on with the furnace triggering it or set a time delay, 15 -60 secs. I initially removed the fan wire from the control board to allow the furnace to control when the fan came on, but since Wyze added this feature, I have it reconnected.

That’s better, but what happens if the burner fails to light? Does the fan still kick in and blow cold air from the basement or the attic all around your house in the winter?

Also, remember that a lot of furnace control boards run the fan at a specific speed for “fan” or cool and a different speed for heat. So that might mean that your fan will be running at a normal heating speed for a while and then wyze will boost it up to the faster “cooling” speed, thus reducing the temperature of the air that is coming out of the vents.

When I timed it, 60 Seconds in my burner had barely fired up, so that is still way too low. That delay should be at least around 120 seconds in my case.

And then, as @speadie mentions, there is the risk of the burner failing and then the fan still running. Many years ago, during A/C season, my furnace fan got out of balance and started banging around. It happened with such force that it seems to have disconnected the ignition cable, which I did not find out for many months, when I turned on the heater again. So this really is a possible scenario.

@WyzeChuan indicated some time ago that they were looking into implementing a setting to have the furnace control its own fan, and I hope that that is a feature that will be added soon.