Nest C wire, central air, transformer


Old 08-31-19, 08:15 PM
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Nest C wire, central air, transformer

I installed a Nest thermostat. I had to replace a contactor. I think it is Nest taking power from the red wire that is doing it. My wiring: a red wire, and a yellow wire. On the old (battery-backed up thermostat), then on the Nest thermostat, red wire went to Rc and yellow wire went to Y. Nest seemed to work but in a few hours, the contactor was buzzing and nothing more. My idea to fix the missing C wire issue: buy a 24 volt transformer, and install one wire into the C connector on Nest and the other wire into Rh. The Nest if for air conditioning only. Connecting the transformer into the C and Rh wire plugs will allow the Nest to be powered separately from the Rc air conditioner wire. Comments are appreciated. Additional info: I have access to air handler in attic but I do not see a C terminal. To make the Nest powered up on a constant basis, and have the battery charged, and to not kill the condenser contactor, I think the 24 volt transformer is the way to go. The Nest will never have heat hooked up to it, so the Rh and C wire ports are essentially never going to be used ...except maybe for the hookup to the 24 volt transformer. And yes, I already do not like Nest but it was free so I figured I would try it out. The same problem awaits me for heat I guess if I ever swap out my current battery backup thermostats. I have to find a C wire solution. Thanks everyone.
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Old 08-31-19, 08:36 PM
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Why keep it if you don’t like it?
The Nest is arguably the worst choice one can make for a thermostat. And they have a long history of bizarre problems, including shorting the subbase which damages the equipment electronics.
The C wire is mandatory even though Nest says it’s not.
Old 09-01-19, 08:05 AM
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I agree with you. I am sending it back. Getting it to work is like trying to make a cow meow.
Old 09-01-19, 08:27 AM
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For what it's worth, I recently purchased a Honeywell touchscreen thermostat. It has the same modern look and feel, wifi connectivity and app, but without the "too-smart-for-its-own-good" features.

It has a way to install without a C-wire, but takes a little doing. I tried a wifi thermostat without the c-wire, but it burned through batteries about once a month. Not really sustainable in my opinion.

Anyway, just a suggestion!
Old 09-01-19, 03:44 PM
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Hi, go Honeywell, post some pics of the control center in the AH, there must be a common terminal there somewhere, can you get a new cable up there from the Tstat?

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