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Hi I'm Brian and I'm here to help!
It appears to me the "old stat" letters you posted are referring to wire colors as opposed to the terminal letters of the old stat. If that's the case, can you tell me what terminal letters those wire colors went on the old stat please? There is no published or official wire color to terminal standard so it would be a best guess with only the info you've given so far.
Ok thanks so you are going from a Trane thermostat to a new thermostat. I will type up the cross reference and be back shortly. Most thermostats have installer set up parameters that have to be set, I can help with that if you'd like but I'll need the make and model number of the new thermostat.
This is how you will wire your new thermostat: It is in the form of Old thermostat terminal, wire color to New thermostat terminal.
R, Red to Rc with jumper to R left in place
W, White to Aux
O, Orange to O/B
Y, Yellow to Y
G, Green to G
B, Blue to C (trust me on this one!)
T, Brown to N/A tape up the end and tuck back into the wall, it is not needed on the new stat
X2, Black to N/A tape up the end and tuck back into the wall, it is not needed on the new stat
I'll be back shortly with instructions for the installer set up options to change on the Honeywell RTH6450/RTH6350 thermostat.
You're very welcome! There is only one installer setup change that needs to be made on your new thermostat. It is installer setup function number 1. This is the system type. Once you have the thermostat wired and powered up, to enter installer set up, press and hold both the Fan button and the temperature UP buttons simultaneously until the screen changes. The installer set up screen should be shown and it should be at function number 1 at this point. Use the up/down buttons to switch the parameter (right hand number) of this function from the default of 0 to the new value of 5 for a heat pump with auxiliary heat. Press done when finished. Be sure to set the date and time on the thermostat! That should be all you need to do!
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