BunchInBasementCeilingBetweenTstat&Board) FurnaceBoard Did some more findings at furnace level (see furnace board image). The furnace board Y, W, G, and R terminals, ALL have a pair (2) of wires connected to them, instead of a single wire, making it a total of 8 wires leading up towards the Tstat. The Y terminal has an additional 3rd wire, which is part of a 2 wire cable going outside to the A/C unit, the other wire being connected to the “C” terminal. All 8 cables, when traced, they go up to the finished ceiling (I was able to climb a ladder to get a picture through the drywall box), and are all bundled into a bunch with some wire caps seen, coupling cables. Somewhere after this bunch of wires, it seems each pair is converted into a single wire going to the Tstat, thereby the Tstat has 3 single wires, each connected to the G, R, and W terminals (which makes 2 pairs or 6 wires coming from the furnace) , and for some reason, the Y terminal on the Tstat is left with the 2 wires connected to it, thereby catering for all the 8 wires coming from the furnace board. One thing I discovered from this exercise is that there is no C wire between the Tstat and the furnace board, so it seems I will need to pull a new one between the C of the new WiFi Tstat and the C of the furnace board: (1) Can I connect another wire to the C terminal of the furnace board, along with the other one going to the A/C unit, thereby making it 2 wires on the C terminal of the furnace board ? (2) The W R and G wires are obvious on the existing Tstat, so can I connect them to the new thermostat ? (3) Can I also connect the 2 wires on the Y terminal of the old Tstat to the Y terminal of the new WiFi Tstat, along with the newly pulled C wire? OR (4) Do I need to go check how the wires transit at the closed basement level? It’s a tedious task as I will need to cut open the ceiling drywall at the exact location, check the wires and then conclude, or it is obvious given the fact that my existing Tstat is working fine, except for the original question of the 2 cables on the Y terminal of the old Tstat and the C wire question (which concluded to be newly pulled).

Existing Thermo Wireing Here is the existing thermostat wiring diagram for my Heat and Cool, 4 Wire, single transformer system. ANYONE HAS ANY CLUE WHAT / WHY THE OTHER BLACK WIRE IS CONNECTED TO THE "Y" TERMINAL (along with the yellow wire) on my existing thermostat. I am desperate to find out.

I cannot trace the wires as the control board wiring on the furnace are totally different colored (see Image of the furnace board). It almost seems like the wires on the thermostat go down the basement, and are converted into different wires that come to the furnace. FYI, the furnace board’s C terminal has a 2 wire cable, both wires being white and black, going out to the air conditioner (maybe this will give a clue). All I want to know is, when I switch to WiFi thermostat, should I be connecting both, yellow and black wires to the Y terminal of the new thermostat, OR just connect the yellow wire to the Y terminal and tape down the black wire. In any case, I still am without the C wire. And I cannot use the COMMENT section as it does not allow me to fully explain, and ran out of characters.

Can anyone atleast tell me why my thermostat has 2 wires connected to the Y terminal? One is yellow (obvious) and the other is black. They both come from the same strand of a single cable with 4 wires in it (yellow, black, red, and green), of which the green and red are not connected anywhere.

Need to install a new WiFi Thermostat, and looking for solutions for the infamous C (common) wire. My existing thermostat does not have a C wire connection. My furnace has a C wire connection but it lloks like it is going out to the air conditioner. But the puzzling thing is, the color coded wires at the furnace level do not match the thermostat wire colors (e.g.the W wire at the thermostat is white, while the W wire at the furnace is Red / Blue ? also, there are two wires for W, Y, R and G at the furnace wiring and only one white wire connected to the C terminal (which goes to the air conditioner). How do I co-relate the cables at the thermostat going down to the furnace as the colors are different. All I am trying to do is to add a C wire from thermostat to the furnace, so I can install a new WiFi thermostat. 2 pictures attached.Furnace WiringThermostat wiring

  • It sounds like there's a junction between your thermostat and your furnace -- can you track it down and report back to us? Commented May 2, 2019 at 22:59
  • You'll need to find that junction and report back to us if we ever want to get to the bottom of this mystery.... Commented May 12, 2019 at 3:36
  • Basement finished. Looked for it to no avail. Is there anyway I can test the 2 cables using multimeter and clue.
    – Moe
    Commented May 12, 2019 at 20:31
  • Can you show more of what's going on at the furnace? Commented May 13, 2019 at 3:31
  • ZOINKS! Can you get us more photos of that ratsnest? Commented May 18, 2019 at 0:40


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