I have a newer HVAC unit that was installed using only 2 wires to control the heating only on the unit. The unit is capable of having AC connected and I would like to wire it so that it will work with my new Honeywell Wifi Thermostat AND be capable of having the AC hooked up at a later date.

I think i know how to get the G, W, R and W wire hooked up, but i dont know where to put the blue wire? can anyone help?

This is the existing thermostat how its currently connected: enter image description here

This is the existing wiring diagram that came with the Heating Unit: enter image description here

This is how I believe it needs to be hooked up when i install the new 5 wire HVAC Cable, I am wondering how to install the 5th wire to the system to work with the wifi theremostat?: enter image description here


Land the blue wire on the "COM 24V" terminal

The typical use for a blue wire is to provide a C wire so that the thermostat can have constant 24VAC power available to it (saves money on batteries); as a result, simply land your blue wire on the terminal marked "COM 24V" on the control board.

As to the rest of the wires you depict? The green wire indeed goes to G, the yellow wire to Y/Y2, and the red goes to R just as you show, but the white wire needs to go to W/W1 not W2 for your single stage system. Your revised diagram, though, is correct, and so is your thermostat wiring, assuming it is configured for a conventional system (not a heat pump).

| improve this answer | |
  • Thanks! is where I have the other G, Y, R and W wires hooked up per my last diagram look correct? – schwein Sep 10 '18 at 23:53
  • Thank you for the update, I changed up my wiring diagram and updated it with an image of the thermostat backing, does this look correct? I greatly appreciate the help! – schwein Sep 11 '18 at 1:44
  • Thank you! will be hooking it up this weekend, hoping for an easy install! will update with confirmation! – schwein Sep 11 '18 at 1:50

Do note that the color of the wires is used to identify that specific conductor on both ends of the cable. So with that in mind, write down what terminal each color wire connects to so that you can ensure to connect to the same terminal at the thermostat. Since you didn't give a model number of the furnace, it looks like that furnace is capable of 2 stages of heat(w1 and w2 terminals ). The furnace is set by the manufacturer to kick in the 2nd stage after about 5 minutes if it doesn't get to temperature in that time. If your thermostat is capable of 2 stage heat then you will need to also extend the w2 terminal to the thermostat as well as find the proper dip switch setting in the furnace to disable the time delayed 2nd stage.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.