Hi wanting to connect Nest and want to install a C wire. HVAC only appears to have G,W,R so not sure where to connect C.

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  • Hello, and welcome to Home Improvement. Let's see if one of our pros has a good answer. And, you should probably take our tour so you'll know you'll know the details of contributing here. – Daniel Griscom Jun 24 '20 at 12:32
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    Is there another wiring diagram in the unit? This one tells part of the story, but it doesn't show the thermostat connections. – Greg Hill Jun 24 '20 at 17:11
  • Can you post photos of the actual control board? – ThreePhaseEel Jun 25 '20 at 0:36
  • @GregHill -- the thermostat connections are bottom center of the diagram – ThreePhaseEel Jun 25 '20 at 0:36

wiring diagram with markup

Step 1: Find the low voltage side of the transformer. Using a volt meter in AC mode, measure the voltage between the two terminals circled in red, then measure between the two circled in blue. One of these pairs should show ~240 V mains voltage; the other pair should show ~24 V low voltage.

Step 2: Measure voltage from the R terminal (circled in orange) to each of the 24 V transformer terminals identified in step 1. You'll measure ~0 V to one of them and ~24 V to the other one. This latter one is your C terminal. Splice as needed to get a wire out from there to the thermostat.

By definition the R terminal is one end of the transformer winding and the C terminal is the other end. Simple thermostats (bimetal strip, or battery-powered programmable) simply short R to W or G to call for heat or cooling. Controls makers didn't spend money and circuit board space on the unnecessary C terminal.

Now fancier thermostats, which require so much power as to make batteries infeasible do require the C terminal. Newer HVAC controls include the C terminal, but when working with older controls, the only option is to find and tap into the C circuit somewhere.

The thermostat's processor and display are powered by current flowing between R and C. Connecting the C wire and the R wire both to the R terminal leaves you with 0 volts between the wires; the thermostat can't be powered from 0 volts.

  • Thanks very much for your help Greg. May be a stupid question but is there any reason I can’t just connect the C wire to the R terminal. So I would have the R wire and C wire running both to the R terminal. – Alfred Jun 27 '20 at 5:10
  • C needs to have 24V difference to R, so you need to find the blue terminal that provides that. it might be the middle one. – Jasen Aug 15 '20 at 5:49

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