Thermostat Wiring For Lennox Furnace

[Florentina Holcombe]

Confusedby the wiring, hope someone can help me out. At the furnace there's only four wires in the cable (R, B, G, and W), but at the other end, a yellow wire appears. It's not reflected in the picture of the thermostat wiring, but the blue wire is there, behind the plate. I figured it was the C wire and connected it to the Ecobee, but that didn't work. Opening up the furnace, I can see why, but I'm not sure how it should be wired. I understand that the diagram attached makes it pretty easy, but the previous thermostat wiring doesn't make any sense with the wiring at the furnace. For instance, the green wire isn't attached to anything at all! The blue wire is attached to the furnace, but unless it's tied to another color wire on the way to the thermostat, it's also not connected to anything. I have zero idea what the yellow wire is attached to. Do I just need to get a multimeter and test each wire??

The issue was that the C and Y wires apparently went nowhere. I traced them as best I could, but couldn't figure it out. Fortunately, there was a deprecated humidifier control box right next to the thermostat with two wires. It was cut close enough to the furnace that I could remove some staples, stretch it, and get it to the control panel. I used those wires as the C and Y wires and everything is fine.

I am trying to install my Ecobee3 now. After finishing the connections, Ecobee did not power up. I did some reading and I believe the C-wire, though it is available is not connected to the power at the furnace-end (see the picture where you can find the blue standalone wire). Can you please suggest where/how I connect the C-wire in my furnace (model # lennox g12q4-110) to supply power to the Thermostat? Thank you very much for your help in advance.

I have this exact same furnace (30+ years old) and very similar messy nest of wiring. As user76730 noted the yellow on the 24V side of the transformer is the key. In your photos, this is the left most wire nut that is pointing up with 2 yellow wires and a green attached. I had a spare blue thermostat wire from my old Honeywell mercury blob setup that was completely unused and just wrapped back in the wall. Hooking that up to the yellow bundle and then the C terminal on the thermostat is working like a charm. I have a Nest E with just the R and W, and it was power stealing badly. Now it runs very smoothly.

The pictures are a little difficult to follow but according to the electrical diagram there is a blue and a yellow wire coming off the low voltage side of the transformer. Connect the C wire from the thermostat to the yellow wire from the transformer

I also have the Lenox G12Q4-110 furnace (heating and cooling) system with a four conductor thermostat wire. I found the instructions posted here of tremendous value in confirming that I could update the thermostat to an ecobee3 lite using their five wire to four wire adapter that was included in the box. I was able to make all of the wiring connections, have the ecobee installed and the system up and running in less than 30 minutes. FYI, the Lenox internal wiring to the thermostat that I found is:

A few days ago Nest 3rd gen thermostat in my home to replace my failed Lennox thermostat. I used the online tool and mapped my old lennox connections to those suggested by the Nest tool, but my HVAC system is not fully functioning. AC using thermopump and secondary or emergency heat work, but heat using thermopump and fan only (for circulation does not, although Nest thermo shows fan as working)

My local HVAC suggested the pic below with the handwritten instructions. In speaking to others it seems I can also connect the white to "W2/AUX". In addition to the low temp deep freeze cut off, my thermopump also has a manual adjustment within the thermopump to adjust low temp cut over (can I bypass the "Dual Fuel" within the Nest, just concerned having both my pose a conflict?)

3. My furnace connections (note I have a air exchanger and humidifier as well so it could look strange to some), but you can see its basic. The Green Wire (FAN) may look odd, but it loops to my air exchanger (grouped within box) and yes installer should have used a Green wire to terminate, but they figure nobody will ever see it.

6. Pic with handwritten note suggested wiring from my HVAC guy who only really installs Lennox thermostats (dont want to go against what Nest install app advised me to do and damage the Nest or worse my eqpt)

I'm so sorry for the delay in our response, but I wanted to hop in here to make sure we addressed your concern. I can definitely understand the confusion when it comes to thermostat issues as it can be very odd to figure out sometimes, but I'm happy to look into this for you.

First off, I would suggest using the Compatibility Checker to see if your system is compatible. Then, if it is, I would suggest contacting a pro to come out to help you with this since I know that the Nest thermostat is not compatible with stranded wires, every wire has to be connected to the Nest Thermostat system.

Thanks GarrettDS for the suggestion, but I have already used the compatibility checker and everything checked out (as well the wires terminating at the thermostat are the standard set up) . I have installed many thermostats over the years without issue until the Nest and I have no intention of spending more to bring in a nest pro. Luckily for me I am still within my return window so I will likely be returning and going with another non nest model. Thanks again for your help.

I love the Nest Thermostat, but actually like my Lennox iComfort Wi-Fi thermostat better. It shows me the 5 day weather forecast, etc. and I like that I can adjust the display brightness. I only did this as an experiment. I will be using the Nest in an apartment that has a much simpler heat only application.

Awesome! I think my iComfort gave out and I'm trying to replace it with a Nest. See photos; my iComfort has an Interface unit between it and the Lennox AC and the old non-Lennox (non-dual-stage) furnace/fan. The Interface has 4 wires coming in from the thermostat and these same 4 go out to the oudoor AC unit (R, i+, i-, C). To connect the Nest, I think I want to entirely bypass the Interface. I'll connect the 3 R's (from the AC, the furnace, and the thermostat). Similarly I'll connect the 3 C's and the 3 Y1's. And connect the W from the furnace to the W from the thermostat. All without the Interface board. That should work, right? And hope the fan comes on when the AC comes on and when the furnace comes on (I only have 4 wires to the thermostat, not 5, so I don't think I can turn on a continuous fan, unless I run a new 5-wire strand to the thermostat. Although possibly I don't need the C at the thermostat and can instead use that wire to connect the G at the thermostat to the G at the furnace.) Thanks! iComfort InterfaceAC unit with iComfort

OK, the "O" must reportedly swithces it to heat pump (which I don't have), the B switches it back to AC, the L is for some kind of auxiliary (unused in my case?), the W is for "who knows what" :). So I'll connect the 3 R's (from the AC, the furnace, and the thermostat). Similarly I'll connect the 3 C's and the 3 Y's. And connect the W from the furnace to the W from the thermostat. All without the Interface board. That should work, right? And hope the fan comes on when the AC comes on and when the furnace comes on (I only have 4 wires to the thermostat, not 5, so I don't think I can turn on a continuous fan, unless I run a new 5-wire strand to the thermostat. Although possibly I don't need the C at the thermostat and can instead use that wire to connect the G at the thermostat to the G at the furnace.)

Hello Seton,

Thank you for the clear graph and it really helps a lot. I have the exact furnace + AC combo as you do. Couple question regards to your graph and set up,

-From the SLP98V manual, for a 2 stage Heat/ 2 stage cool set up, the graph is missing the G wire from the control board to the Nest. Do you leave it out on purpose ?If so how did it go without the fan control?

-I know you need to set the board to non-communicating mode, may I know how exactly you do it? Which DIP switch you need turn on/off, and anything else you did on the board for it to work?

I want to thank you all. I have a Lennox ML180UHE furnace, with an AC condenser outside. After reading these posts I got brave and went up to the attic. My furnace has your standard W-C-G-Y-R plugs, and they had wired those 5 wires out to another box, which then turned the signal into the C-R-I+,I-, requiring the communicating thermostats.

The issue I did have, I must not have been too careful with the power, because I took off the thermostat after it was working, hung it on the wall, and plugged it back in, and the entire system was dead. After an hour of searching and cursing, reinstalling the old setup to no avail, I figured out it was the fuse, and popped in a new fuse and it was good to go.

My iComfort thermostat shuts off at night. I want to replace it with a Nest 3rd generation thermostat. I checked the connection of the indoor and outdoor units. The wiring ports look the same as what was posted by @Seton_Carmichael. I have some questions:

I have no experience in HRV, just some simple DIY weekend stuff. Any advice on how to tie in the 3 wired HRV to all of this, and the functionality difference between the 2 wiring diagrams?

@bade011 any feedback?

My icomfort wifi display just died. I also have a single blower system. Are new wires necessary? Could you install a new Nest thermostat with the existing 4 wires by disconnecting those wires from the Lennox control box and rerouting them to the furnace board directly?

So, I have the thermostat installed, it powers on, and during the test, phase 2 of the heating does not come on. Originally there was a 6 wire thermostat with no C wire connected. It was only Y,Y2,W,W2,G and Rc. So I tried the C conversion device, and everything worked, except stage 2. So I ran an 8 conductor, and hooked up the C wire as well. As it stands, this is how the thermostat is hooked up:

Thermostat Wiring30244032 1.86 MB

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