When I moved in to the house the first thing I changed was the old mercury style T-stat to a digital one, I went wire for wire, and it has works (knocks on wood) so far. Now that I have been reading questions about WiFi T-stat and started looking at all of the different kinds that are out there, man now I want one. From what I found am going to have to run a new 6 wire T-stat wire to add the constant 24V and I have to match the rest of the wires but that is where I am confused


I first I opened up the T-stat and from what I have read it doesn't look right. Next opened the indoor unit and found all of the wires from the T-stat and I am now really confused.

From the T-stat I have five wires.

  1. Green wire terminates to a Yellow with Red Dash wire, which goes to what I think is a contactor (Yellow/Red Dash wire on the right of Pic 2) = Fan motor
  2. Yellow wire terminates to a White wire, which goes to the outside unit = compresses
  3. Blue wire terminates to a Blue wire (It is hard to read but I think the wire marker says 2M= Heat or Cool contactor
  4. Red wire terminates to White wire that goes to one side of a low volt X-former= Control Voltage for T-stat
  5. White wire terminates to a Purple wire, which goes to what I think is a contactor= Heat or Cool contactor

Wire Terminations Lower Contactor Low Volt X-Former Upper Contactor

Now at the T-stat I have this... T-stat Wiring

So why is the blue and white wire together, what am I missing?

Update, Traced the Blue wire out to another contactor. Its the Blue wire on top. Blue Wire

  • What is the make and model of the furnace/air handler? There should be a schematic printed somewhere inside the unit, can you post a high quality photo of the schematic?
    – Tester101
    Sep 19, 2015 at 17:10
  • Schematic is of no use, I have tried to get a good pic but it is to faded. I have a Carrier 40VUC03320.
    – WarLoki
    Sep 19, 2015 at 17:26
  • Can't find any documentation on that unit. Do you have the installation guide for the unit?
    – Tester101
    Sep 19, 2015 at 17:43
  • No info at all, house was built in the 70's.
    – WarLoki
    Sep 19, 2015 at 17:46
  • Can you trace the blue wire, to see where it terminates?
    – Tester101
    Sep 19, 2015 at 17:54

2 Answers 2


My second stab at answering this:

Since this is not a heat pump, I now see more clearly you have a photo that shows your blue wire connected to the coil of a contactor in the unit that is separate from the contactor controlled by the white wire but also with heavy supply wires coming into it. Judging from the large size of the wires fed into this contactor, I believe this was originally meant to be control for stage 2 electric heat. However, given the small gauge white wire that is connected to the load side now, I cannot say what has been connected in place of stage 2. I can say that unless that small white wire coming off that contactor is separately fused, then it is probably a fire hazard. I also see in the same photo that you have a lot of bare wire exposed on that heavy conductor.

  • 1
    sorry not a Heat Pump.
    – WarLoki
    Sep 19, 2015 at 18:58
  • 1
    Well, if it is not a heat pump then it would be a simple electric furnace with an air conditioner. You might have 2 stages of electric heat in which case they are being switched on together. I can think of no other reason why someone would bother to tie Blue and White together. Rather than my guessing, perhaps you could tell me exactly what type of system you have?
    – user39367
    Sep 19, 2015 at 19:33
  • Chris it is a standard Heat/Cool unit by Carrier Model #40VUC003320
    – WarLoki
    Sep 19, 2015 at 19:53
  • Yeah, I saw you mentioned that model number which I presume is the air handler. Many such units have a heat pump. It all depends on the option selected for the condenser outside. Seeing as you are an electrician, I imagine you will have no difficulty rewiring the electric furnace with A/C however you see fit.
    – user39367
    Sep 19, 2015 at 19:57
  • Yeah. I see that now. I will fix this today. Thanks
    – WarLoki
    Sep 19, 2015 at 20:04

I see this often when someone uses a thermostat with only a single stage option. If I recall White is stage one and blue is stage two. Depending on the furnace this might just be high and low fans. If you upgrade follow the install instructions for the thermostat and you will likely become more efficient by using both stages.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.