Just moved to a 1985 built house in Ontario, Canada that has a Honeywell thermostat that I'm trying to replace with a Nest. I am unable to understand the labeling of the current thermostat as the wiring diagram is partially torn so I'm not sure how to connect the Nest's corresponding wires.

Old thermostat model is Honeywell T7512A - photo attached. New thermostat is a Nest - photo attached


  1. How do I mix and match? I am unable to find this information in the thermostats manual. Are the torn labels B for Blue and Rh by any chance?
  2. In the bottom row, that yellow/golden connector is a conductor - is that what they call the common wire or a jumper wire? I don't think this has any use in the Nest, right?
  3. Nest says its not compatible with a 110v or 120v system, where can I check my units voltage? I saw a junction box in the furnace room with wire nuts that has 220v hand written. Is there any other clue to be sure?

old thermostat wiring and the face of the Nest


I found where the cable is spliced under the flooring. Here is the sequence of colored wiring coming from the thermostat to the board:

  1. Red wire spliced and connected to new Red wire secured to the R slot.
  2. White wire spliced and connected to new Red wire secured to the W1 slot.
  3. Green wire spliced and connected to new Green wire secured to the G slot.
  4. Blue wire spliced and connected to new Black wire secured to the Y/Y2 slot.

Here is the furnace wiring diagram:

furnace wiring diagram

Here are the splices:

Spliced wires

  • Are the Red & White wires from the thermostat spliced to the same Red wire which is then connected to 2 places on the furnace, or are they connected to two different Red wires, each connected to its own place on the furnace? If the latter, I'd strongly recommend you do yourself and anyone who follows you a favor and put some white electrical tape around the furnace end of the Red wire that's spliced to the white one - that'll make it much easier to keep track of which is which.
    – FreeMan
    Feb 10, 2022 at 13:30
  • @FreeMan Yes, I have just retraced the spliced wires. From thermostat to furnance its Red to Red, Green to Green, White to White and Blue to Black. Therefore the color scheme is followed end to end except from Blue which has changed to Black. Black is connected to Y/Y2 on the furnance. Here they are traced: imgur.com/a/elrlXTl
    – eszed
    Feb 10, 2022 at 15:34
  • Do you have A/C as well as heat, and does your old thermostat contain a battery?
    – jay613
    Feb 10, 2022 at 17:59
  • @jay613, yes to both. I have just responded to your posted answer.
    – eszed
    Feb 10, 2022 at 19:29

2 Answers 2


I have never done this before but with my life knowledge I'm assuming the following:

  1. The colors of the cables follow a standard for which signal they carry. The torn diagram just shows what color goes where (W for white, G for green, etc), same thing with your nest (Y = yellow, G = Green, Rc = red) ... the manual should explain what to do.

  2. After a quick google search, it looks like nest can automatically determine to use the jumper, so connecting the red wire to either the Rc or Rh would work.

  3. 120V doesn't mean the voltage of your actual heater, it means the voltage for controlling it. Since these are tiny wires they are likely only ~24 volts (so you're safe to install)

  • 3
    You are likely correct in your assumption about wire colors, but it's not guaranteed. Check at the furnace end to see what they're connected to to be 100% sure. If someone was having a sleepy day back in 1985, he may have used Red for common at both ends - electricity doesn't care about the color of the wire's insulation, but your electronics do care if they get connected to the wrong thing.
    – FreeMan
    Feb 9, 2022 at 19:29
  • 1
    @FreeMan I opened up the furnance panels and looked for corresponding labels to match the thermostat. Photo 1 (Top) does have corresponding labels but does not have the same colored 4 wires coming out of the cable connected to the thermostat. Photo 2 (bottom) was the only area on the board that was color coded and each colored wire is connected to the correctly labeled point on the board. White is too, just not visible in the pic. Are these the wires coming from the thermostat then? The unit was manufactured in 2009 as per the sticker. Here is the photo imgur.com/a/KR3xthd
    – eszed
    Feb 9, 2022 at 21:13
  • Your furnace photos are not the other end of the wire seen at the thermostat. Either they're spliced somewhere or there's somewhere else they attach to the furnace. Heyitsmyusername is likely correct (I have installed 4 Nests because we move a lot and "guessing" was right every time), but to be sure you really should check, as FreeMan pointed out. Feb 9, 2022 at 22:20
  • @eszed can you post the wiring diagram for your furnace please? Feb 10, 2022 at 2:42
  • @ThreePhaseEel I have added new information including the connection diagram in the original post for better formatting. Please let me know if this is what you're looking for.
    – eszed
    Feb 10, 2022 at 3:21

According to your notes, your { red, white, green, blue } wires are connected at your furnace to { R, W1, G, Y }.

According to the documentation for your thermostat, showing the labelling that is torn from your actual one, that is what is expected: enter image description here

I therefore conclude the following, pending confirmation from you that you have A/C and your old thermostat has a battery:

  1. You have A/C, and your old thermostat has a battery
  2. The red, white, green wires are used in the obvious and expected way. The blue wire is used as Y, the demand for cool. (Usually a yellow wire is used for that).
  3. You do not have a C wire
  4. You cannot easily use a Nest thermostat with this cable. You need 5 wires.
  5. The advice, and photo, you got from Nest support is wrong.

If it's not too difficult, the solution is to pull a new cable with more conductors. If the thermostat is on the ground floor and you have an unfinished basement ceiling it is the best approach. Otherwise you should look for other existing Questions describing options for people without a C wire. There are LOTS of questions and answers about that.

You should closely inspect the brown cable to see if there's another conductor hidden inside. Long shot but worth a look.

  • Thanks for the response. Yes, I have an A/C and my old stat has 2 batteries. The cable from the stat (on ground floor) has just 4 conductors, however at the point where its spliced, the cable leading to the furnace has a 5th. The 5th can be seen at both ends. Basement is finished so the spliced area is in a hard-to-reach spot. Pulling a new cable would be a difficult undertaking. A/C model is Carrier 38TKB030 331
    – eszed
    Feb 10, 2022 at 19:48
  • Well if you can get a new cable from the stat to the splice point you should. If you have access to the dropped ceiling at a point directly below the thermostat that would help a lot, then maybe you can use the existing cable to pull a new one. Otherwise there are other options. There's one where you put a widget at each end (at the controller and at the thermostat), to multiplex the existing wires. Another where you use a nearby wall socket to power the thermostat.
    – jay613
    Feb 10, 2022 at 19:56
  • I have never fished cable through different floors, only the same floor so I lack architectural understanding. Would the stat cable dangle freely between drywall or would it pass through a hole in the floor joist ? If I can pass through a cable and it freely and vertically suspends I could use steel wire fish tape and hook it back to me. Here is my POV imgur.com/a/T6aSUd6 I am hesitant to disconnect the existing stat cable and use it to pull new wire as its peak winter and I just moved into this house 2 weeks ago. I gently yanked the stat cable and observed the furnance end move.
    – eszed
    Feb 10, 2022 at 20:48
  • 1
    The battery is at 3.85v after 36 hours. As per Support anything more than 3.7v is considered good. In the absence of a C wire the stat charges through Y1 (A/C) or W1 through what they call 'power sharing' through terminals. In my previous home I had the heating and cooling off for 72+ hours and the Nest did not lose its power. Therefore I don't think power draw would only occur during heating or cooling demand.
    – eszed
    Feb 13, 2022 at 22:31
  • 1
    An update for everyones benefit, the Nest battery is still at 100% charge after 2 weeks of running it w/o a C wire. Therefore the concept of power sharing seems to be valid.
    – eszed
    Feb 26, 2022 at 19:36

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