I live in Ottawa, Ontario. I bought a brand new house 12 years ago. It is equipped with a high-efficiency gas-fired furnace with a tubular heat exchanger. The wiring module is a Honeywell THP9045 and it has five terminals: R, C, G, Y and K. The original thermostat was a programmable Honeywell T8132A. There are four wires coming out of the wall; they were connected to terminals W, Y, G, and R/RC. About two years ago, as part of a free program, an Ottawa Hydro technician replaced that thermostat with a Honeywell PeakSaver model T2060F. The four wires are now connected to terminals C, W, K and R/RC. Everything works great. But here's my issue.

I want to buy the new Nest thermostat. I contacted the company and described my current system in detail. They were puzzled by that K terminal and because of that they cannot guarantee that the Nest unit will be compatible with my system. I search the web for an answer and found a post (of unconfirmed reliability) stating that the K wire can simply be ignored.

If that is any help, here's a quote from the THP9045 wiring module manual: "The K terminal on the thermostat can be used to operate both the fan and compressor on a single wire, and the module is designed to receive the signal from the K terminal, split that signal and reroute it to operate the compressor, and/or fan for normal operation."

Also, when I provided Nest with my wiring configuration, they replied: "It really depends on what that K wire does, if your system can operate without it maybe the Nest will work but we would need to find out what it does first."

Can someone out there shed some light on this? My field of expertise is light-years away from HVAC. Thanks in advance!

Daniel

Daniel,

I am in Markham ON and have the same problem! Did you find any solution? Or, I return nest? Thanks for help.
Ramzan

I called nest and they help me point to use Y1 connector of nest for K wire. It works for me, so far.

After a number of emails to Nest, the best I could get was a definite maybe. They basically said that it should work but could not guarantee it. I was about to give up on it when I thought of something. My old thermostat (the one that came with the house) had four wires connected to terminals W, Y, G, and R/RC. Based on that configuration, my system was compatible. Now with the new thermostat, the terminals were now C, W, K and R/RC, a configuration that didn't seem to be compatible with Nest. But I thought, hey, if the previous configuration made my system compatible with Nest, then my system should still be compatible, as they are the same wire. So I went for it and ordered the Nest. It took me longer to read the instructions that to actually install the device. And it worked great right off the bat. I love it and have absolutely no regret.

Hi folks.

Opus. In your original question you say your furnace control board has the terminals r,c,g,y and k. Is this correct? Or did you mean r,c,g,w, and k? What brand furnace do you have? Ive been in the industry for many years and have never seen a k terminal on a high effeceincy furnace. However, it appears the k is the same as the y on most brands.

In the original set-up, the four wires were connected to terminals W, Y, G, and R/RC. On the Honeywell PeakSaver thermostat that was installed two years ago, the wires were connected to terminals C, W, K, and R/RC. My furnace is a York Diamond 90 and it says on the cover of the manual that it is a "high efficiency gas fired furnace". I can't say for sure that there is indeed a K connector inside the furnace, as I have not looked at the wiring module inside, but all I can say is that one of the wires was connected to a K terminal on the PeakSaver thermostat. And my Nest works like a charm.

One interesting note. At one point, while I was on the phone with a Nest technician, I asked if I could damage the Nest thermostat or even my system is I were to hook it up to the wrong terminal. He said no, that the Nest was designed so that a miswiring would not damage it. That encouraged me to try it, safe in the knowledge that I could not burn my house down... Or maybe I just got lucky.

No worries. I misunderstood. I thought you were saying there was a k term in the furnace. There is not. The stat you had was just using the letter k instead of y for some reason. Same purpose, just a different letter. Miswiring the thermostat normally won't harm the stat, but it may blow a low voltage fuse in the furnace (easy to change). Since you got it all wired up and everything is working, you did everything correctly. Good job. Take care.