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How to identify the C wire on my Honeywell thermostat?

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I want to put a Honeywell RTH8580 thermostat on my Trane WCH042C100AA heat pump. I do not have a C wire on my existing Braeburn 3000 thermostat. Where do I find a C wire in the unit? I have extra wires to the thermostat.


On a Trane unit the 24vac common connection in the unit is terminal "B". This would go to terminal C on the thermostat. Heat pumps can be difficult to wire so Is there any other question about the thermostat wiring that I can help you with?

For your convenience here is a manual for your package heat pump unit. It shows a typical thermostat wiring. On your new thermostat W on the heat pump should go to E/Aux on your thermostat and O should go to O/B. There is no connection to T on your heat pump.

Customer: replied 4 years ago.


Thanks for the information and the pdf. I put the Blue on C and all of the other wires to their corresponding terminals, Red, Yellow, and Green.

However, I am unclear where the White wire will go. The terminal block has two sets of labels, Conventional and Heat Pump. I have a Heat Pump. There is no terminal label for W on the Heat Pump side. Looking at the pdf, it shows the White wire moving from W to X2. I have no label for X2. I have O/B E/Aux, L and K. Any ideas?

I don't have an O wire. I have Red, White, Blue, Yellow, and Green. I will put the White on E/Aux.

Hello again,
Here is how all wires should be attached with the first letter indicating your thermostat and the second letter indicating your package unit terminal strip:
R to R (normally red wire)
G to G (normally green wire)
Y to Y (normally yellow wire)
O/B to O (normally orange wire)
C to B (normally blue wire)
E/Aux to W (normally white wire)
You would have to open your heat pump unit's control panel cover to confirm what wires were connected to what terminals.
Also in the thermostat setup you will need to select your type of system and set the system up to energize the O terminal on the thermostat when the system is in cool.

Customer: replied 4 years ago.

I did not hook up the Orange wire because I do not have one. I powered up the unit on Cool and I got Heat from the air duct.

If you only have 5 thermostat wires you will need to open the unit control panel and see how it is wired. There NEEDS to be a connection to O both at the thermostat and the unit, otherwise the unit will not work in cool but will run in the heat mode when set to cool. If this is a Wi-Fi thermostat you will need 6 wires to operate the thermostat and heat pump correctly. Can you please provide me the entire thermostat model number? There should be more numbers/letters after the RTH8580. I will be able to give you a more definite answer on whether or not the thermostat will work with 5 wires when I get the complete model number.

Customer: replied 4 years ago.

It is N RTH8580WF1007

On the roof top unit (RTU)I have the following connections:

RTU /Wire # / to Thermostat Wire /Thermostat Terminal

Orange/ 40a/ Blue/ C

Red/ 35a/ Red /R
Yellow/ 44a/ Yellow and White/ Y=Y & W=E/Aux
Black/ 47a/ Green/ G
Unused wires at A/C RTU
Brown/ 49a
Brown/ 41a
Blue/ 36b

Thank you for the complete model number. As I suspected your thermostat is a Wi-Fi model and as such does need a 24Vac common connection (c) to operate as noted on page 17 of your thermostat's user guide. So the connections necessary for proper operation are:
Thermostat to Unit (color at unit) designator at unit
R to R (red) 35a
C to B (blue) 36b,
G to G (black) 47a,
Y to Y (yellow) 44a,
O to O (orange) 40a
E/Aux to W (brown) 41a
So you do need 6 wires to operate the unit and thermostat properly. You could temporarily use the wire you would normally connect E/Aux to W and use it to connect O to O (until a cable with more conductors could be run) but this would not allow for operation of your backup electric heat. All other functions would operate normally.
Thank you

Customer: replied 4 years ago.

I made all of the changes and matched all of the wire numbers, but I still get heat only, no cooling at all. I'm going back to the old thermostat. It' over a 100 degrees in here now.

Yikes! The one thing that might be causing that problem is the initial setup of the thermostat. Some heat pumps energize their reversing valves in heat while others like your Trane unit energize the reversing valve in cool. That is what the O/B terminal is for and why it has two designations. The initial setup of the thermostat asks you to set this parameter to the correct setting for your system. This is done in the settings and options menu. Parameter 170 should be set to 7 for a heat pump system with auxiliary heat and Parameter 180 should be set to 0 for a heat pump cooling changeover valve. If you wired the thermostat back up the way I last suggested so you were using a wire for the O terminals in place of the wire for E/Aux to W, I believe your system would work properly with the exception of backup electric heat being unavailable.

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Customer: replied 4 years ago.

Yes I have the back up heat for this system and I set all of the parameters as you say. Nothing but heat. I think it blew out something in my unit though because I went back to the old wiring and the old thermostat and now nothing but heat. I took pictures of the original wiring and made notes, so I am sure of the proper termination. I hope I didn't blow up the controller.

Since the unit does run, I doubt seriously that something you did could have caused this problem. When you hooked up the blue wire to the O/B terminal and the thermostat powered up it would have gotten it's common through the reversing valve coil but I really doubt that it would have damaged the coil. The reversing valve is what switches the refrigeration function of the unit between heat mode and cool mode. Do you have a volt meter? I could help you diagnose the problem that way.

Customer: replied 4 years ago.

Yes but not here. I just found out I get heat in the Cool position and cool in the Heat position. So I got something wired wrong despite my precautions. I'm thinking since the AC Orange wire goes to the Blue thermostat wire the Blue Thermostat wire should go to the O terminal on the thermostat.

Ok well that's great news! I would double check parameter 180 in the thermostat and make sure it is set to zero first. If heat and cool are reversed that indicates that your wiring may indeed be correct. Out of the 5 wires you should have connections at the thermostat to R,G,Y,O/B, and C since you don't have enough wires to use the backup heater. If the compressor and fan run in both heat and cool that means R,G,Y and C are correct so pay attention to the O/B terminal and the O wire on the unit (40a).

I'm sorry I was assuming that you had put the new thermostat back on. If you are using the old thermostat, both the unit and thermostat wiring would have to be as they were which I am unfamiliar with since we have only been working with the new one.

Customer: replied 4 years ago.

Ok, thanks. I moved the blue wire over to the O terminal and all seems to be back to normal. I could really use a recommendation for a WiFi enabled thermostat for this Trane WHC042C100AA unit. If there is an easier one to hook up and use I would go that route.

Thanks for your help.

The Honeywell thermostat is an excellent thermostat. I have sold and installed them for years. It's unfortunate you don't have enough wires to accommodate it. I would recommend that one first if you can get the extra wire needed to the roof or replace the existing cable with one with enough conductors. I will do some research for you to see if there maybe another brand but I would be willing to bet any WiFi enabled thermostat will need the same 6 conductors to work. A heat pump with backup heat needs at least 5 wires and I can't imagine a WiFi enabled thermostat that wouldn't need the 6th wire for the common side of the low voltage so it has enough power to run the WiFi. I will look though.

Customer: replied 4 years ago.

Thank you. I was starting to look at the Ecobee and the Nest, but I haven't checked out the wiring requirements yet.

Hello again,
After searching around the web I found, as I'm sure you have, there are now many thermostats that are web and/or wifi enabled. The challenge you will have again is the number of conductors needed at the thermostat. The good news is that there are also many wireless or communicating thermostats that would require far less conductors between your heat pump and the thermostat. The challenge with those is they will all have some type of interface module that would have to be installed at your heat pump on the roof. This shouldn't be a big issue as it appears you are an adept do-it-yourselfer. If it were me I think I would go with a brand that has a tried and true record in HVAC controls as opposed to an electronics or network equipment manufacturer that added thermostats to their unrelated product line. I know you had some issues with the Honeywell brand but they have been in HVAC control for many decades and were one of the first to offer wireless (not Wifi but wireless) solutions for installations where hard wiring was not an option.
On another note, there is good news about the line of Honeywell thermostat you were using. There is a device you can add at your heat pump that will effectively add a wire to your system and it operates with your model Honeywell thermostat. It is called the Wiresaver. Honeywell model number THP9045a and sells for around $11. If you are interested in using this device with your RTH8580 I am confident that I can help you wire it up and program it successfully. Let me know if I can help you further.

Customer: replied 4 years ago.

On your advice from yesterday, I took the in-operative Honeywell back and got another of the same model. I have all of the wires hooked up as you suggested previously, but I still have heat only and no cool in any selected mode.

I set 0170 to 7, but I don't have a menu selection of 180. I do have a 0190 which is set to 0 as I have the O (40a) wire hooked to O/B and the B (36a) wire to C.

I have a total of 8 wires in my bundle, so no need to run any more. I have all six wires you listed hooked up as outlined, but so far only heat.

Ok well I certainly misunderstood. I was under the impression you only had a five wire cable run between thermostat and unit. That is very strange.

I am at a bit of a loss. If the unit and thermostat are wired the way I last posted, it should work fine. You have all settings and wiring correct. It appears that before you made changes at the unit wiring that the original wire on terminal 40a was blue and was hooked up to O (or some other designator for the reversing valve terminal on the thermostat) on the old thermostat. If I understand correctly you have changed the unit wiring to the scheme that I posted and the system isn't energizing the reversing valve (O, 40a) I have to wonder if there is a break in that orange wire somewhere and the original installer used blue instead as orange is the industry standard color for a reversing valve that needs energized in cool as your unit does. If you brought your volt meter home we could diagnose this and get to the root of it or you could pick an unused wire in the cable and hook it up to 40a and O at the thermostat and try that.

Customer: replied 4 years ago.

Thank you for your input. I figured the same thing about the wire being broken. I reinstalled (again) the old thermostat and it didn't work using the orange wire. So, I used the blue wire again to bridge the orange. And all is working now. I now have an OHM meter and will check for continuity on the spares. I also had to recombine the white and yellow wires to the yellow wire at the unit in order to get the old thermostat to work. So, I am planning 2 things:

1) find a good spare and use that for the orange wire; and

2) leave the white and yellow tied to the yellow (at least for the first run).

What do you think?

That sounds like a good plan or with an OHM meter you could disconnect all wires at the unit and tie all of the thermostat cable wires together then OHM them all at the thermostat location to see if there is a break in one or more wires. Hopefully you will have six good wires. It's odd that you have to combine white and yellow together which seems to indicate one of them may be broken as well. Do you live in a multi-story building? (just curious, in my younger years I worked for a company that installed and/or took care of many high rises and we had to deal with a lot of wire problems).

Customer: replied 4 years ago.

Oh, I like your plan a lot. I will tie them all together and check them all at once. I don't know why I didn't consider another broken wire. I guess it's because the heater side seems to work so well. I'll let you know what happens in the morning.

Ok great, I look forward to the result. Have a good night.
Customer: replied 4 years ago.

Success! I found where the wire bundle had been spliced. The three spares were never joined. I made the connections, rewired the unit to conform to the correct color codes, but left the white and the yellow wires tied to the yellow at the unit. It seemed to require that even with the original thermostat. Everything seems to work. At least it produces both heat and cool in the correct switch positions.

I do have a couple of quirks I'll need to work out with Honeywell such as I cannot turn on the fan remotely, but I can switch from Fan On to Auto remotely. But it is connected to the internet and I can adjust schedules, temps, and On/Off. So that is really enough for the moment.

Thanks for your help.

That is awesome! I am so glad it all worked out. You could now get a position as a service tech :) If you have any other issues in the future please look me up!