3

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

Questions:

  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

EDIT:

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

FreeMan
  • 43,073
  • 21
  • 81
  • 173
eszed
  • 499
  • 3
  • 7
  • 18
  • 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 '22 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: https://imgur.com/a/elrlXTl – eszed Feb 10 '22 at 15:34
  • Do you have A/C as well as heat, and does your old thermostat contain a battery? – jay613 Feb 10 '22 at 17:59
  • @jay613, yes to both. I have just responded to your posted answer. – eszed Feb 10 '22 at 19:29

2 Answers2

3

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)

heyitsmyusername
  • 579
  • 2
  • 7
  • 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 09 '22 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 https://imgur.com/a/KR3xthd – eszed Feb 09 '22 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. – Fredric Shope Feb 09 '22 at 22:20
  • @eszed can you post the wiring diagram for your furnace please? – ThreePhaseEel Feb 10 '22 at 02:42
  • @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 '22 at 03:21
  • Just try: BLUE (should be) neutral: needed to power the stat, RED should be CALL-FOR-HEAT (connected with the white wire it should turn on the burner), WHITE is "incoming power", green is FAN (connected with white wire should start the fan). Just try the connection and see if you get the expected effect. Just don't splice white with blue. From the wiring the connection is standard. – DDS Feb 10 '22 at 11:28
  • I chatted with Nest Support and they asked for my Thermostat model as well as my furnance model. After which they sent me these wiring instructions: https://imgur.com/a/TyDlVXz I could follow it but after reading and exploring I'm in an analysis paralysis state because I've read too much for my own good =/ – eszed Feb 10 '22 at 15:04
0

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.

jay613
  • 34,254
  • 2
  • 43
  • 136
  • 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 '22 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 '22 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 '22 at 20:48
  • Nest support just got back to me. I gave them the model number of both my Furnace and A/C this time. The previous diagram was based on only Furnace. They updated the pic with this: https://imgur.com/a/WmJEK7I I inquired about not needed a C wire and said I'll only need is if my Nest has power drain issues. They maintain that this Nest Learning is compatible with my home. In my previous home I operated the same Nest without a C wire. – eszed Feb 10 '22 at 21:34
  • That's cool. Apparently the nest can leech power off the signal wires without affecting signaling. At least that's what I've read. Let us know if it works or not. – jay613 Feb 10 '22 at 22:10
  • Hooked it up today based on Nest Suppport's latest diagram and so far so good. Fan and heating work. Unable to check cooling as the Nest detected its too cold to run the in-built test. At the end of it, it was simpler than thought. However, the learning has been invaluable and I'd do it all over again. – eszed Feb 12 '22 at 16:14
  • Great! The test is if the battery is still at 100% after two or three weeks. – jay613 Feb 12 '22 at 17:00
  • Good point. The battery is currently at 3.73v after 2 hrs of reconnecting the Nest. If it falls below 3.6v that could indicate a power issue. 3.8v or higher indicates normal battery level. So long as we use the heating and cooling regularly the battery should remain charged. As per the specs, it can go 10 hrs without a charge and should last 5-10 years. – eszed Feb 12 '22 at 17:25
  • So it only does its power leeching when there is demand for heat or cool? If that's the case, 10 hours is a problem! As you obviously have both winters and summers (Wherever you are) there is a period in between when you presumably go a few weeks, or at least more than 10 hours, without either heating or cooling. – jay613 Feb 13 '22 at 21:43
  • 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 '22 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 '22 at 19:36
  • 18 month update: battery is still holding steady at 3.83v. – eszed Jul 27 '23 at 02:48
  • You should add your own answer, and accept it, to indicate that a nest thermostat or at least some of them, can function without a C wire, apparently by recharging an internal battery by leaching power from the signal wires and that for you this has proven effective over an 18 month period. – jay613 Jul 30 '23 at 07:49