I have recently purchased a honeywell digital thermostat model number RTH7500D to replace a regular mercury thermostat. I have a Trane XE1000 heatpump model number TWRO42C100B3.
The wires that I have labeled and unattached are 7 WIRES :

B
G
Y
O
R
W
T

I began to hook the wires up to the honeywell thermostat as the book said but It is somewhat confusing to me because my wires are labeled differently. The honeywell backplate where you connect the wires reads...


C=C
G=G
Y=Y
O/B=W
RC=RC
R=R ( RC & R HAVE A CONNECTOR WIRE ATTACHED)

W2=AUX
Y2=E
_=L



I don't see anything explaining the T wire... also, would I put the o,b and w wire in the same connector? If anyone can tell me what I need to do please help :(
cindy

By the way, I have an electric furnace also.

This is by far probably the worst tstat question that I've come across. I'm only guessing based on minimal info.

The "T" terminal is called the "Outdoor anticipator reset", but I can't get a handle on what it is. Info on the net suggests it may be a thermister. In some respects it makes sense, but your setup looks like it falls into a "dual-fuel" setup rather than a setup with emergency heat using heat strips. This thermister may actually tell the electric furnace when to turn on based on outdoor temperature. Heat strips are allowed to operate when the heat pump is running, but not allowed to operate with fossil fuel backup. A full blown electric furnace may count as Fossil fuel. So, the anticipator makes the system work more efficiently.

Some of the Honeywell vision pro stats support duel fuel and some support the ability for the furnace to determine when it switches to the alternate fuel.

One of the stats also has the ability to add an outdoor sensor, to select when it's not cost-effective to use the heat pump.

B G R Y O and W are standard designations except very few systems need B and O. B is the complement of O and it specifies how the reversing valve acts.

With some wiring trickery, B and O can be reduced to B or O. Usually you have to tell the stat whether B is on in heating or off in heating. O is the complement. That's why the new stat is labeled O/B.

Rc to R or Rh to Rc is common terminology. When jumped together, it becomes R. They are separated to allow two different transformers for heating and cooling.

AUX MAY mean Auzillary Heating or heat that is used without the heat pump being on.

E may mean emergency heating or heat that can be on with the heat pump. The terminal designations don't make a lot of sense.

L may just be a light.

I really believe you have a dual fuel system where the T terminal is telling the furnace when to use the electric furnace. It may be a thermister or it may be a contact. Just don't know.

The following info is unknown: How does the current system know when to use the electric furnace? How does the T terminal work?

(C) is an added terminal in modern stats because of the necessity to power the stat with a constant source of 24 VAC. Battery operation eliminates the need for a (C) terminal.

So, you can get a dual fuel thermostat, but it's unknown how to turn on the electric furnace.

r- red jumped to rc
g- green
y-yellow
Aux-white, jumper to E
o-orange.
c-blue
L is only for the fault code from the deforst board to flash inside so you will not need it. And t you can tape it off, what color was it?

Thank you for attempting to help me. "keepingitsimplestupid" I researched online before finding this site and also could not find anything. The thermostats are suppose to be simple to install but with the instructions I had it wasn't much help. Anyway, I got it working this morning... all I needed to do was to remove the T wire and place electrical tape around it and place a jumper between the aux and E. I also had a couple of codes to change on the honeywell thermostat. Thanks for answering.
"joshdta" thanks I wish I had your explanation last night. LOL
Anyway, I got it fixed that is all that matters. :)

Glad you got it going.