Looking to switch from an old Honeywell CT87A thermostat to a smart thermostat, probably something requiring a common wire. There are 2 thermostats (one on each floor) in a 2 zone forced hot water oil burner system, heat only.

I want to make sure I have the right wires before buying a new thermostat, and was worried with an old system like this I'd have just 2 wires. But looking at the existing wiring, there are 4: red, black green and yellow.

  • Red and Black wires are on the thermostat's R terminal,
  • Green and Yellow on the W terminal
  • nothing on the Y terminal

Looking up thermostat wire color charts, I am now very confused as I have no White wire which is pretty much always shown as being for heat.

Can I assume either my Green or my Yellow is actually for heat, since they are both connected to the W terminal?

Assuming my Red is for power (since that's consistent with 99% of color guides, and it is connected to the R terminal), is my black wire a common?

Can the wire roles be deduced in this situation just from the terminals they're connected to? Is there a way to find out for sure?

Thermostat on 1st floor: thermostat1

Thermostat on 2nd floor: thermostat2

On the boiler end, there's only one thermostat-gauged cable coming from the junction box (red arrow):

junction box

And that one cable goes from the junction box to the following taco box, and has just black and white wires:


  • What is on the other end of that cable? There are no mandatory color codes for thermostats, merely suggestions in the line of R, W, G and Y. So in your case you have to look at what it is actually connecting to. May 7, 2022 at 3:02
  • Tracing the wire through the wall, through the conduit and to the boiler is not really a feasible option here...
    – dmaftei
    May 7, 2022 at 3:24
  • I didn't ask about the route. I asked what the other end is connected to. We need to know how it is wired at the furnace end(s) or no one will be able to do anything more than wild-guess. And you don't need us for that lol. May 7, 2022 at 3:33
  • 1
    Added pictures of the wiring on the 2 taco units and the aquastat. Does that help? I have no idea how to figure out which out of all the wires involved on the boiler end go back to those 4 thermostat wires; trying to match colors as you can see is no help as there are several wires of the same color in multiple places.
    – dmaftei
    May 7, 2022 at 3:50
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    They used “telephone” wire not a big deal you need to examine how it’s hooked up by finding the splice where it changes to red white. If you can’t trace it remove both ends and check continuity both open and shorted to identify the wire,
    – Ed Beal
    May 7, 2022 at 7:21

1 Answer 1


Someone used telephone wire (2-pair red/green & black/yellow) for the thermostat wire here.

So, ignore anything you may think applies about "thermostat wire colors" and just pay attention to what is connected to what.

In this case, they have joined the pairs, so it's essentially acting like red/white "thermostat" wiring. Red/Black is "Red" and Green/Yellow is "White" as seen on your first device.

Telephone wiring is normally smaller gauge than thermostat wiring, so it may prove inadequate for powering your "smart" thermostat if the pairs are separated into 4 individual wires. Sounds like that won't matter anyway as you only have actual red/white from the boiler, so you'd need to upgrade all the thermostat wiring.

Or manage to deprogram yourself from the marketing mayhem that convinces people they "need" a "smart" thermostat, but evidently that's some major ju-ju of diabolical marketing genius.

  • I don't think wanting to upgrade to a thermostat that can be controlled remotely is the result of falling prey to diabolical marketing genius ju-ju, it's really just a matter of convenience. Sure the old ones work, but they could be more convenient.
    – dmaftei
    May 7, 2022 at 15:43

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