I have an old thermostat that I'm ready to replace. The old thermostat has A/C, Heat pump with 1 stage auxiliary (electric forced air) heat.

I am replacing the old one with a Honeywell RTH6360D. I'm now following the mapping of the wires to insure I connect the proper wires in the proper places for the new thermostat.

Here's what my current wiring looks like:

original thermostat wiring

What I'm Mapping Those Wires To

Here's the new wiring mapping (from RTH6360D thermostat docs)

RTH63060D wiring mapping

The Main Challenge

The main issue is that the original wiring has 2 wires jammed into the W2 terminal block. I don't know how to map this to the new thermostat.

Aux / E: What Does That Mean?

I notice that at the bottom the label is marked Aux/E

Does that mean that one of those wires is AUX and other other is E?

How can I know which one to put into AUX and which one (black or tan) I put into E?

Does it matter which one?

  • 1
    Your plan to test the behavior of the system before and after the replacement is excellent. Note, turn off all circuit breakers supplying the entire system. The disable/kill switches near the equipment do not always turn off the control circuits.
    – jay613
    Nov 7, 2023 at 16:53
  • @jay613 I plan to 1. flip circuit switches on the furnace - there are two 2. pull fuse from compressor I notice that when I flip the two circuit switches on the furnace that the thermostat loses power (when no batteries are present in the thermostat). That should confirm that there is no power at the thermostat, right? And thank you for your comments and help
    – raddevus
    Nov 7, 2023 at 20:02
  • You need to find the other ends of those wires, and document where the two wires are connected. What equipment, and what terminals of that equipment. And preferably confirm where all the other wires are connected too, by checking the other end rather than relying on the old thermostat's labeling.
    – jay613
    Nov 7, 2023 at 20:38

1 Answer 1


Through some research and looking a bit closer I've discovered what the two wires are which are connected to the same terminal block (black & tan on the far left, labeled W2 in my picture).

The document at this link (Identify all the thermostat labels) helped me identify the meaning of all of the labels (C, G, E, etc)

That helped me understand that the two items (black & tan wires on the far left) which are connected to the same terminal block are:

  1. W2 - also know as Aux which is the Auxiliary (backup) heat
  2. E - Emergency heat - can be turned on when heat pump fails.

I believe this to be true also because I can identify all of the other wires in my system:

  1. G - Blower fan
  2. W - Heating (communicates that heat should be turned on) in my case the heat pump
  3. C - Common wire provides power to thermostat
  4. Y - Air Conditioning (communicates should turn on A/C)
  5. R - Power for entire system

With this information I know have all the data I need to replace the old thermostat.

Still Need To Identify Which Is Which

However, since the new thermostat requires that the E wire be placed in the E terminal and the W2 (Aux) wire be placed in its own terminal I need to differentiate which is which.

Here's My Plan

I'm now going to

  1. remove power from thermostat / system (for safety) - shut down furnace completely
  2. remove one wire from the terminal (example I'll remove the black one)
  3. reconnect the old thermostat
  4. reconnect power
  5. Attempt to turn on Auxiliary heat (if it powers up, then the tan wire is Aux, otherwise it is Emergency)
  6. If Auxiliary doesn't turn on, I will attempt to turn on Emergency heat to double-test my theory.

With those tests I should be able to know which wire is which and my wiring problem is solved.

  • This is not an answer to your Question. These are good tests, you should do them and then update the Question with whatever remains unknown. You should not rely entirely on this testibng, see my comment above, find the other ends and what they are connected to !
    – jay613
    Nov 7, 2023 at 16:54
  • 1
    @jay613 I will update the answer after I complete the removal (of old) and installation of new thermostat and it is working.
    – raddevus
    Nov 7, 2023 at 20:03
  • Asking a question about peculiar wiring in your own home, providing not enough information for others to answer it, and then answering yourself, is not a good use of this forum. The question and answer can never be useful to anyone else because it mainly pertains to the unique connection of two wires to one terminal at your thermostat. If you can figure that out on your own, you should delete this question entirely. If you cannot, your best bet is to figure out where they are connected at the other end and add that info to the question.
    – jay613
    Nov 7, 2023 at 20:36

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