How do I use the green wire as a C wire as suggested in most installation instructions for the newer wi-fi thermostats?
They all reference terminal strips at the furnace, all I have is the 3 wire nuts on a older Lennox ES5 series.
Thank you.
Adding a C wire to older 3 wire
hvacthermostatthermostat-c-wire
Related Solutions
There should be no problem connecting the C wire from your thermostat to the C terminal in the furnace. Take a look at this diagram, which is a rough approximation of your system.
Notice the cable going to the condensing unit has a red wire connected to the Y terminal in the furnace, and a white wire attached to the C terminal. When the thermostat calls for COOL, it does so by energizing the Y terminal (connecting R to Y). This provides power to the coil of the contactor, which allows it to close the contacts and turn the condensing unit on. The other side of the contactor coil is connected to the C terminal, as to provide a complete circuit.
You'll notice in the first image that two relays in the furnace, the thermostat, and the contactor coil all connect to the same leg of the transformer. This is because the thermostat basically works like a switch, connecting the R wire to the C wire through control circuitry. Anything that needs 24VAC power, will be connected to both legs of the transformer.
Note: the solution here is the querent's -- I'm simply writing it up in a CW answer to get this off our plate as the querent has not returned to migrate it from his question-edits to an answer himself.
SOLVED (01/04/15): I decided that the T terminal was exactly what I needed so gave it a try and everything is working perfectly. I also pulled in some 18-5 thermostat wire. Here is my setup, in case it might help someone else:
Furnace: Lennox G14Q3-60-13
Thermostat: Honeywell RTH6500WF
I've attached pictures of my connections.
Best Answer
It would appear from the pictures that the blue and orange wires (wire nutted to the red thermostat wire) should go back to the transformer for one of the two 24 volt wires. The other 24 volt wire on the transformer is the common to hook up to the thermostat. There should be 2 other wires that are line voltage coming out of the other end of the transformer. Don’t mess with those. Depending on the transformer there may be multiple taps, without seeing the transformer it would be difficult to tell which ones to use unless the transformer is marked.
Great background on thermostat wiring is at: How can I add a "C" wire to my thermostat?