I am trying to update my old Lennox thermostat to a Sensi smart thermostat. I checked the compatibility on the Sensi website and it appears to be a match except for this cluster of wires on the left side of the photo. I do have a heat pump—is that what these are for? The Sensi installation instructions don’t cover how to install for these. I have no experience with any of this so any advice would be appreciated.enter image description here!

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    Can you post photos of the wiring at the air-handler end? – ThreePhaseEel Apr 26 at 22:32

It appears that you have a thermostat that is designed to control a standard split-system heat pump. However, the wiring color code does not follow what I would call a conventional design. The control wiring for these heat pumps are fairly sophisticated. You may want to consult a Lennox dealer for help. Or a standard HVAC dealer for help with the control wiring. Good Luck.


Posting the correct response to my own question in case any other novices are looking for the same info in the future:

The wires on the left side of the thermostat (ALL the way left, between the mercury bulbs) are not coming from the wall; they’re just part of the old thermostat and aren’t needed with the new one.

Also (for Paul, mainly), the wires connecting to the labeled terminals are not a problem at all as long as you take note of which wire is connected to which terminal. NEVER go by wire color alone, as it’s possible the installer did not follow the color-coding standard. I recommend (as did my new thermostat’s instructions) taking a photo of the old thermostat’s configuration with your phone to refer back to if needed. It’s really not that complicated if you’re able to read instructions. 😉

You can do this! And don’t let some “experts” trick you into thinking this is more complex than it is by playing dumb when you ask simple questions!


Thank you for the advice, Paul. For clarification, I’d really just like to know what the cluster of wires on the left side of the thermostat are (the ones between what appear to be levels?). I’ve looked high and low for a diagram explaining this thermostat, and have found plenty of pictures of the same configuration online, but all the information seems to center on the lettered wiring on the right, completely ignoring the left. What is the function of everything on the left?

  • I'm not sure what you mean? The items on the left (and far right) are the functional components of the thermostat. The coil of metal expands and contracts with the temperature. As it moves, it opens and closes the mercury contacts to indicate calls for heat or cool. The wires from these switches go behind the circuit board and are soldered into the circuit somewhere. The temperature adjustment is done by tilting the whole mechanism with the clear plastic lever. The labelled contacts are for the connections to the HVAC system. – DaveM May 1 at 20:31
  • Thanks, Dave. The items on the left are precisely what I meant. I figured it out on my own (that the wires on the left were just part of the old thermostat and would not be needed with the new thermostat) when I pulled the base off the wall and saw they weren’t connected. It seemed like a pretty straightforward question anyone with any amount of HVAC knowledge could have answered, but I guess that was expecting too much, lol. Anyway, my new thermostat works great, no thanks to the experts! 😉 – Jtoxx May 2 at 22:02

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