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I am replacing the existing thermostat with a nest thermostat E, and it gives me e195 error on the nest device. Looks like I am missing a C wire, so I checked the air handler in the attic (made by First Co, model number 18HBXB-HW), I see the brown and blue wire is not connected to anywhere on the unit, I am thinking I can just connect them to the unit to get the 24v AC, but the question now is which wires on the unite provides 24v? the manual of the air handler doesn't seem to be very clear to me. here is the link: https://www.firstco.com/documents/ProductDocuments/hbxb-hw1117.pdf

Below is the picture of the wires coming out(or in) from/to the air handler, from the manual above it is not clear which wire is for what, or I am mis-reading it? enter image description here

the wiring diagram from its manual: enter image description here

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  • Can you post photos of the wiring diagram on the air handler, or is there not one posted there?
    – ThreePhaseEel
    Dec 29, 2020 at 1:44
  • Thanks for your reply, I attached the diagram found from their manual. But I didn't find any diagram on the unit itself..
    – diabloooo0
    Dec 29, 2020 at 14:27

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You want the brown wire

Based on the diagrams in the manual, the brown wire is the C wire you're after, so connect the blue wire in your thermostat cable to the existing bundle there. We can also tell it's your C wire from the fact the cable off to your air conditioner's outdoor unit connects to that wire in addition to the yellow Y wire from your thermostat.

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  • Thank you! so the cable #2 (I edited the photo) goes to outdoor AC unit! that answers my another question I didn't ask in the post. Any idea what cable #1 is for? I was also thinking to use an external 24v transformer (those can be found on amazon for $15 or so) to supply the power for the thermostat (I happen to have a 120v outlet close to the air handler in the attic), because on some forum I see people saying nest thermostat sometime can cause problems because it "periodically steals" power from the air handler, do you see any problems going with the external 24v way? Thank you again!
    – diabloooo0
    Dec 29, 2020 at 22:10
  • @diabloooo0 -- you shouldn't need to faff about with an external 24VAC transformer in your situation since your air handler provides a C wire. BTW: Cable #1 goes off to a zone board or zone valve that manages the hot water flow to your air handler.
    – ThreePhaseEel
    Dec 29, 2020 at 22:32
  • Sounds like a plan! Thank you, much appreciated!
    – diabloooo0
    Dec 30, 2020 at 2:08
  • Hi ThreePhaseEel, happy new year! I checked the brown wire today with a multimeter, interestingly there is no 24v AC on it... Any idea why is that? And if I measure it with the RED wire, there is 24v (actually 28.x).. doesn't make much sense to me.. shouldn't the brown C wire showing 24v?
    – diabloooo0
    Jan 1, 2021 at 19:54
  • Red to Brown (R to C) should measure 28V -- the C wire should be close to ground, because it's the return path for the power used by the thermostat/compressor contactor coil/... to get back to the 24V transformer in the air handler
    – ThreePhaseEel
    Jan 1, 2021 at 21:00

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