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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8769
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Trying to install a Honeywell programmable thermostat to a

Customer Question

trying to install a Honeywell programmable thermostat to a Lenox system. There is no C wire on the thermostat. On the Lenox unit it looks like the C wire is the wire connected to Rc on the thermostat. Honeywell says I must have a C wire on the rhermostat.
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: specific to this thermostat
JA: Do you plan on doing the work yourself?
Customer: yes
JA: Anything else we should know to help you best?
Customer: no
Submitted: 1 year ago.
Category: HVAC
Customer: replied 1 year ago.
original thermostat wiring picture attached (WP_20161005_001.jpg), Lenox unit wiring picture attached (WP_20161005_007.jpg)
Expert:  Phil replied 1 year ago.


We can go from here as needed. Please locate the finish and rate button and follow the prompt when you have what you need, Thanks!

Expert:  Phil replied 1 year ago.

Ooops, I misread your question, give me the model number of the thermostat, it may be battery operated (Rc is not a C wire connection)

Customer: replied 1 year ago.
I have a Honeywell RTH9580 WiFi Series
Customer: replied 1 year ago.
it is not battery operated. All of their literature and videos say you must have a C wire. They suggest using the G as a c wire. Only problem is there is already the Red wire connected to the C on the Lenox unit
Expert:  Phil replied 1 year ago.

Connecting C to Rc would create a dead short and blow the 24v fuse in the printed circuit board or worse.

C is shown on page 9 if this is what you have

Let me know, you may have a different set of options on your thermostat.

Customer: replied 1 year ago.
There was no C wire on the thermostat. See attached WP_20161005_001.jpg
Expert:  Phil replied 1 year ago.

Hello, this is going to take some unraveling...

- Your wiring is on the *heat pump* terminal strip, that is what the B and O are all about.

Look for another terminal strip that says *conventional system*

- the other issue is the model number you gave does get me a manual that matches your thermostat... we need to double check those numbers.


Customer: replied 1 year ago.
There is no other terminal strip. Attached are pictures from the Lenox G40UH(x) series installation instructions.
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** don't need the Lennox manual, the photo of the thermostat in the lennox furnace that you sent tells me everything.. its a very very standard thermostat hook up.

The problem is that the thermostat you sent pictures of does not show the C wire connection, a connection that shows up clearly in the thermostat manual on page 9.... *this is what we need to reconcile.

Additionally and very importantly, the thermostat you sent pictures of showing the Rc/B Rh/O terminals is for a heat pump.... you do not have a heat pump, you have a conventional system... it is as though someone sold you a bogus thermostat in the latest model Honeywell box??

We need to focus on that. can you give me those details, how and where you bought it?

Expert:  Phil replied 1 year ago.

this Honeywell number Honeywell RTH9580 has 4 more digits to it that you are not showing here, they should be on the box... get those to me and I can dig a little deeper.

Customer: replied 1 year ago.
The picture I sent was the original connection to my old thermostat.
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** a bit old, I need a photo of the wire connections to the new thermostat, I am expecting them to match the manual.... the manual shows a C connection, and you have a C connection in the furnace.

I will research the 1005 variant

Expert:  Phil replied 1 year ago.

I am having no luck finding anything specific to the 1005 variant, but under the RTH9580 series I found this statement

"The C wire originates from your heating/cooling system and provides 24 volt power to your Lyric. "C" stands for common and there is no universal color used to designate this type of wire. If you do not have a C wire, your Lyric will instead draw power from your heating and cooling systems to power its battery. "

Send me photo's of the thermostat terminal strips in the new thermostat (many of them have 2 terminal strips, one for heat pumps and one for a conventional system such as you have.)

Customer: replied 1 year ago.
This is how I connected the Honeywell
Expert:  Phil replied 1 year ago.

Thanks but that was too far out of focus to read, can you send a new one in tight focus?

Customer: replied 1 year ago.
It will be a couple of hours before I can retake the picture.
Expert:  Phil replied 1 year ago.

I will be in for another 4 hours this evening,

Customer: replied 1 year ago.
I have two of these. The WP_20161005_22.jpg is a better picture without the wires. WP_20161005_19.jpg is the one on the wall with wires in White in W, Yellow in Y, Green (former G) in C, Red in Rc.
Expert:  Phil replied 1 year ago.

Hello again, lets focus for a moment on the red wire connected to the C terminal in the unit..... it most likely runs to the AC unit outside, along with another wire (probably white) doubled up on the Y terminal that also runs to the outside unit.

Connect your green wire to the C terminal along with the red that is already there.

Let me know how that goes.