For Nest Thermostat 3, I am trying to identify a common wire on my old HVAC. The diagram can be found on the link below:

enter image description here

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    Can you post photos of the wiring at the furnace? – ThreePhaseEel Aug 20 '18 at 22:48
  • Upper part: photos.app.goo.gl/UPFajGs9RWwfjC219 Lower part: photos.app.goo.gl/r8t6XFmpq7EMZfJy6 By the way, there is the yellow wire from transformer that I cannot find in the above diagram. I was told that might be a common wire. – sergio Aug 21 '18 at 4:49
  • Can you post a photo of where the cable to the outdoor unit connects to all this? – ThreePhaseEel Aug 21 '18 at 11:46
  • It's in lower part: photos.app.goo.gl/r8t6XFmpq7EMZfJy6. There 5-wire cable that comes from thermostat. One wire, that is not used, is supposed to be used as common wire when I track it down in HVAC. – sergio Aug 21 '18 at 17:15
  • yeah, I was able to figure that out -- I was asking about any wires going off to your air conditioner (if you have an air conditioner that is) – ThreePhaseEel Aug 21 '18 at 22:14

Whenever a furnace has a 24V low voltage transformer, the two 24V terminals are R and C.

If you can't find any other way to do it, pick one of these two low-voltage terminals to use for C.

Worst case, you picked R by mistake, and the thermostat will see 0 volts between R and C, since you are actually bringing it R and R. In that case, use the other transformer terminal.

On your furnace, the low voltage transformer is in the smaller schematic to the middle left. Within that drawing, it shows the transformer in the middle. The R wire snakes through a limit switch to the thermostat connection terminal block on the lower left. Your furnace does not bring C out to that terminal block. You'll have to "go get it".

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  • Is there a way to find common wire using a multimeter? Can multimeter help to distinguish terminals R and C? – sergio Aug 20 '18 at 23:35
  • If you can find R at the furnace, then set range for 24 VAC and look for the transformer terminal that is 24V away from R. – Harper - Reinstate Monica Aug 21 '18 at 0:53

Grab a piece of (blue) 14AWG THHN with a piggyback crimp on one end, and run it up to the top left terminal on the transformer

It appears that the C wire from the transformer runs off to the control board on your system, then gets lost in a way I can't trace in your photos However, we know it is available at the top left terminal on the transformer as that terminal is jumpered to the chassis (visible in your photo of the upper compartment), so we can turn the unit off at the breaker, take a piece of (preferably blue) 14AWG multi-rated wire (THHN/THWN/MTW/AWM), crimp a piggyback crimp terminal to one end, remove the existing yellow and white wires from the top left terminal on the transformer, slip it on the piggyback crimp, slip the piggyback crimp back on the top left transformer terminal, then stuff the new blue wire down through to the lower compartment and nut its other end to the wire you want to use for C, and turn the power back on at the breaker.

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  • Hmm, seems kind of simple to do. But I remember I did measure the voltage between COM terminal and the ground, and the voltage was pretty high so I did not consider it as common wire. – sergio Aug 22 '18 at 18:08
  • @sergio -- I goofed up and had the transformer nomenclature turned around – ThreePhaseEel Aug 22 '18 at 22:18
  • Here's an image that should help – Tester101 Aug 27 '18 at 17:04
  • It sounds right. I just measured. Yellow and two white wires are connected together and they give 0VAC vs ground. Orange wire vs ground gives 27VAC. Thanks! – sergio Aug 27 '18 at 21:25

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