I know this issue has been beaten to death but i am stumped and nest support has no clue how to help me.

I thought i had the issue resolved but that was wishful thinking. Here is my issue:

I have a nest hooked up to a separate heating and cooling system. The ac has 3 wires ( rc, g, y1) the boiler has two wites rh and w1. The nest worked great until the weather started getting warmer and the boiler stopped kicking on as much. My battery started dying and i would lose connection to the stat.

I called nest they said i would need to use a c wire from the boiler. I had a spare wire behind the wall so i traced it to the boiler and hooked it up to the other side of the transformer. I got 24v across rh and c. But the nest says no power to Rh and the battery still does not charge.

Thinking somemore about this issue i know i wired it correctly so i removed the rc wire from the stat and reconnected the nest. The battery charged and the heat worked. All the numbers were perfect.

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When i hook up the rc the battery does not charge and all my numbers drop. And it says i have no power to the c wite.

enter image description here

It seems to me that the nest is trying to utilize the rc and c to generate constant 24v which of course will not work because the wires are from different transformers.

Nest says i need to pull the c from the ac unit. ( but she only mentioned that after i told her what i thought was going on)

Do i need 2 c wires? If so where do i hook that into the stat and why wouldnt the rh and c circuit work? Getting to the ac unit in the attic is not really that easy.

How do i fix this issue. Or should i just throw this damn nest against the wall which i am thinking of doing.

I should add that i reconnected my old wifi thermostat including the c wire and it worked perfectly.

Is there some way to force the nest into using rh and c as the constant power supply to thw nest?

  • Can you run your C wire from the AC instead of the boiler? – ThreePhaseEel May 30 '17 at 3:47
  • Have you looked at their documentation/FAQ on the subject? nest.com/support/article/Help-with-wiring-problems – SDsolar May 30 '17 at 8:51
  • Thank you for your response. I have read their help pages and watched their videos and unless i over looked something I cant seem to find the answer on the nest website. Getting into the attic to access the air handler is a job and a half. It is so old i cant even tell what brand it is. I was hoping to avoid that if at all possible. – Kevin d May 30 '17 at 10:39
  • If I'm not mistaken, I think most thermostats use Rc and C for power, when there are separate heat and cool systems. I'd assume (and it appears) the NEST is the same. – Tester101 May 30 '17 at 18:21
  • Is the AC and the heater controls being fed by one control transformer or does each unit have it's own? Second you say you have an rc, g and y1 on the AC and, rh and c on the heater. Does your AC hace a c terminal? Last on your low voltage side of the control transformer. Is the common terminal grounded to the enclosure of your unit? If not your voltage from your transformer may be floating and you need a reference. – Retired Master Electrician May 30 '17 at 18:25

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