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Thread: C wire question

  1. #1
    Junior Member
    Join Date
    Jul 2012
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    3

    C wire question

    I received my Nest thermostat yesterday. I have a basic single stage AC single stage heat funance / air handler. My current thermostat has Red, White, Yellow, Green, with a Blue wire not used. They are connected to the thermostat to the terminals as you would expect Red -> Rh, Y->yellow, etc.

    In my air handler, those wires are connected to their respective R, W, Y, G with the Blue unused. Also in the air handler, there is a 2 conductor wire going to the AC compressor outside. On wire to the Y, one wire to the C terminal.

    Its been my understanding that the Nest does not require the C wire, however, there seems to been some people that have problems without it. I assumed it would be simple to use the spare blue wire to connect to the C terminal on the Nest to the the C terminal on the board... problem solved. I metered the C terminal across the system board to the R terminal and it had 26v on it. I connected the blue wire to the C terminal (which was now sharing it with the AC), hooked up the nest Rh -> red, Y, G, W, and Blue to C. Threw the breakers back on and nothing. Nest thermostat was dead, would not turn on.



    I had read some posts online about the battery not being charged so I hooked the USB port up to the thermostat, it turned on, charged it and put it back on. It came up through the settings, connected to my wifi, then reported a wiring error on Rh, no power. Called nest support, they went through a series of troubleshooting steps and ultimatly determined I had a bad thermostat and they are over nighting me one.

    So, I went to hooked back up my old thermostat. System powered up fine.... then curiosity got the best of me and I started to wonder if my assumption about the C wire was wrong and that in hooking up the C wire, I fried my Nest. So, I hooked up my the blue C wire to the C terminal (which says Optional with batterys) to the thermostat and the other end back to the C terminal in the air handler. Powered the system up (I always shut the funace and AC off at the breaker when messing with wires), AC unit failed to start. I noticed on the thermostat that the "On Cool" was flashing. Worried I blew something on a 90+ degree day, I hurried up and threw the breakers, took off the C wire back off. Powered things back up and thankfully the AC unit turned on.

    Now that I'm puzzling over this, I googled what the flashing "Cool On" means and according to honeywell documentation, it simply means that it was in a delayed start because it detected that the AC unit was running less than 5 mintues ago. Which was certainly the case. So now, I'm back to scratching my head in that the C wire really wasn't to blame in the first place.

    Sorry for being so long, but I wanted to make sure I put out as much information as possible. With all that being said, do you think there is any danger in me hooking up the blue C wire to the air handler board, piggy backing on the AC unit? In reading this forum and others, that is exactly what I should be doing.

  2. #2
    HVAC Thermostat Specialist
    Join Date
    Feb 2012
    Posts
    395
    Quote Originally Posted by Boomn4x4 View Post
    Sorry for being so long, but I wanted to make sure I put out as much information as possible. With all that being said, do you think there is any danger in me hooking up the blue C wire to the air handler board, piggy backing on the AC unit? In reading this forum and others, that is exactly what I should be doing.
    Piggy backing the commons from the thermostat and outdoor units to the indoor unit common terminal is how the wiring is typically connected. Sound like you had the common connected properly. You could reconnect the Nest and with front removed and only the terminal of the Nest showing turn the power back on and verify the voltage between the RH an C terminals.

  3. #3
    Junior Member
    Join Date
    Jul 2012
    Posts
    3
    Quote Originally Posted by HVAC View Post
    Piggy backing the commons from the thermostat and outdoor units to the indoor unit common terminal is how the wiring is typically connected. Sound like you had the common connected properly. You could reconnect the Nest and with front removed and only the terminal of the Nest showing turn the power back on and verify the voltage between the RH an C terminals.

    Got the new unit tonight, hooked the Blue back up to the C terminal on the board and installed the new Nest. Turned the breakers on and it powered right up. Everything was fine. I had it all right in the first place and the problem with just with a bad unit.

    Thank you for taking the time to respond.

  4. #4
    Junior Member
    Join Date
    Jul 2012
    Posts
    10
    That's good news, Boom. Glad you got it going and you'll like the Nest. I really works great once it's been going a week or so and has learned your patterns.

    -Charles

 

 

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