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enter image description hereI want see if someone help me on hooking up this new Honeywell thermostat with Wifi I just purchased. I have an old ac unit and I only have 3 connections to my circuit board which does not have an option to connect my wire. Any help would be appreciated. Thanksenter image description here

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  • often C connects to the body of the air-condtioner. is there a wiring diagram on the inside of the cover? – Jasen Sep 30 at 2:27
  • Disconnect R and measure voltage from R on the board to the body of the air conditioner - should be approx 24V AC – Jasen Sep 30 at 2:37
  • Hello Jasen. Thanks for giving me some input. There is a wiring diagram on the cover. I will also test the R wire. I had seen another post where they connect to one of the blue wires that go to the body which I tried but my fuse blew out when I connected the thermostat and I made sure that all the wires we're connected properly. – Guillermo Avina Sep 30 at 16:17
  • if you can add the wiring diagram to the question that will help. – Jasen Sep 30 at 19:08
  • Thanks for following up. I'll add the wiring diagram soon as I get a chance. Thanks again – Guillermo Avina Oct 2 at 5:25
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I think you've already figured out that older units often didn't run the C wire. Power from 'R' was switched to 'G' (fan) and/or 'W' (heat) by a simple switch in the thermostat. The older thermostats needed no power, so there wan't any point in running the common wire 'C'.

Older "smart" thermostats worked off the difference between 'R' and 'W' when the heat wasn't on, and then used batteries to ride through when it was on. That was fine before we wanted WiFi connected thermostats, but the new stuff wants more (and constant) power.

The wire labeled 'C' on the diagram is what you want. That gives the other end of the 24VAC transformer.

This site has great coverage of all this: https://smartthermostatguide.com/thermostat-c-wire-explained/

  • Problem is, connecting C from the t-stat to chassis blows a fuse on his unit... – ThreePhaseEel Oct 3 at 4:07

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