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Mark Henley
Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience:  UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
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I am installing a RTH7400 thermostat, and have one wire labeled

Customer Question

I am installing a RTH7400 thermostat, and have one wire labeled "T", with no corresponding label; where is it to go?
Submitted: 8 years ago.
Category: HVAC
Expert:  Jim Eastwood replied 8 years ago.
I need to ask you a couple of ? to make sure I can in a few minutes give you the right answer no charge for these ?'s
How many wire toat were tied to your original t'stat and is this a gas furnace it control with a standard A/C let me know on these ?'s and I can let you know what I need you to do next
Customer: replied 8 years ago.
Nine wires originally, a "Trane" thermo; this is an electric heat pump with aux. heat
Expert:  Jim Eastwood replied 8 years ago.
The best way to know for sure where this wire should go is to look at that same wire where it goes into the furnace it very likely ties in to C which stand for common if it does then the wire that was tied to t will be tied to c on the new thermostat Let me know if you need more help. Not every thermostat has a c terminal some are battery operated and have only 4 wires if you have a t'stat that doesn't have a c or x you will have to rewire at furnace or get new t'stat.
Expert:  Jim Eastwood replied 8 years ago.
we were apparently typing at the same time forget about the info on no common every heat pump t'stat uses a common there is usually a chart with the installation instruction that may help but if all the other wire were a direct match and you put the wire that was on t on c you should be good to go it was a pleasure to help if thius answers your ? please hit accept
Customer: replied 8 years ago.
Oops, counted the original wires again, only eight wires on original t'stat.
Expert:  Jim Eastwood replied 8 years ago.
Are u there please stop overposting I have this question handled
Expert:  Jim Eastwood replied 8 years ago.
That's o k no problem answer is still correct just remember you are going by wire location and in most cases it will also be a wire that's color matches the 1st letter of the terminal r -red g -green etc
Customer: replied 8 years ago.
The "C" terminal has wire already attached; according to the installation instructions which came with the t'stat, "if the old t'stat had separate O & B wires, attach the B wire to the C terminal". This I did, so the C terminal now has a B wire attached to it.
Customer: replied 8 years ago.

I am still confused, the "C" terminal now has a wire connected to it; am I to also connect the "T" wire? the new t'stat does have a "C" terminal and an "X" terminal. Still am at a loss.


Expert:  Jim Eastwood replied 8 years ago.
Stand by let me look at your answers to my ? sorry i lost connection for a minute standby
Expert:  Jim Eastwood replied 8 years ago.
there an x terminal on the new t'stat
Expert:  Jim Eastwood replied 8 years ago.
Put the t wire there now this is 95 chance correct the only way to know for sure is to check to see where each wire is tied in a the fan coil. It is already very possible that either the o or b does not tie in at all at the fan coil the only way to know for sure is to look and see where each wire goes at the fan coil
Expert:  Jim Eastwood replied 8 years ago.
If you need more help let me know if I have answered your ? to your satisfaction if so hit accept if not let me know and we can continue
Customer: replied 8 years ago.
Question not answered. I will look to see where the wires are tied in to the fan.
Expert:  coolman1973 replied 8 years ago.
the old trane heat pump tstat used 8 wires the new one u are putting on u really only need 5 or 6 if it takes batteries u do not even need a common.
Expert:  Mark Henley replied 8 years ago.



T is for an anticipator , you won't use it on a digital stat.


Trane and sometimes York used B as common,


Trane used X2 for aux heat.


Any other questions please reply back.


Thank you,