C-Wire for Nest Thermostat

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My ancient Honeywell thermostat died on me, and I decided I would get with the times and replace it with a Google Nest Thermostat. I have a York commercial rooftop unit with a single cooling option and three wires, so I installed it according to the configurator in the Nest app:
  1. Yellow to Y1
  2. Green to G
  3. Red to RH
The RH part did confuse me, considering there is no heating option, and I wondered why it wasn't RC, but I went with it anyway and the unit appears to be functioning as far as cooling is concerned, although certain features such as "Wake on Approach" does not seem to work.

While it took a while for the battery to hit 3.84 volts through "power stealing", the LIN reading is 20 mA, which probably explains why certain features do not work. There is no common wire, and I decided I would not mess with running wires, considering I won't be here for much longer. So I went and got myself a 24V 2-wire adaptor, which suggested the following configuration:
  • 1 wire to C
  • 1 wire to RC
The thermostat did not seem to like this setup and would not function with these two wires connected, so I pulled them out. I did a bit of research, which gives me the following options:
  1. Disconnect, cap and substitute the Red wire with one of the White 24V adaptor wires
  2. Wire in one of the 24V White wires with the Red wire to RH
Wondering if we happen to have any HVAC technicians onboard and which of the two (or any other option for that matter) would work best? Thanks!
 
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As you're already aware, the best and proper solution to get it to work 100% correctly is to run new wiring. These adaptors that they sell are a "work around", at best. With an adaptor, you may need to be prepared to give up on some features. I finally gave in and pulled new wiring, when I was one wire short.

Hint: With some time and patience, you can use the old wiring to pull through the new wiring. That's exactly how I installed my new wiring.... although I'm not keen on Kuwaiti construction practices. I'm familiar with wood studs and wallboard.
 
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Best option is use a plug-in power supply to power up the nest. Dont know for certain in your situation but in some cases you can move the g-fan wire use as a common and then the fan would lose the ability to run independently "will only run when ac is on" I've mostly seen this done in 4 wire heat/cool systems. 20ma? seems kind of wimpy.
 

Falcon_LS

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Thank you for all the input!

As you're already aware, the best and proper solution to get it to work 100% correctly is to run new wiring. These adaptors that they sell are a "work around", at best. With an adaptor, you may need to be prepared to give up on some features. I finally gave in and pulled new wiring, when I was one wire short.

Hint: With some time and patience, you can use the old wiring to pull through the new wiring. That's exactly how I installed my new wiring.... although I'm not keen on Kuwaiti construction practices. I'm familiar with wood studs and wallboard.

Absolutely, and I agree 100%! But local construction practices are horrible to say the very least, and landlords are worse than most HOAs put together in the U.S. - especially when it comes to doing anything to the property. Since my time here slowly drawing to a close, I was hoping I could get away by using a plug-in adaptor, which I haven't even gotten around to installing yet. That way, I can just rip it off when time comes to pack.

I've seen some folks suggest switching the G wire over to the C, which means I'd lose the fan option, but that is not a concern considering the AC runs 24/7.

Not sure if this is compatible but I also used to have a honeywell thermostat and the wiring was incompatible with nest.

Found this and my HVAC guy said it should work with the existing wiring.

It's been over a month and the nest has been flawless.

Take a look at the installation guide and see if it might work for you too:
https://storage.googleapis.com/support-kms-prod/WJrWiEaeDuoZnn7t6LNYcRa50GQKt4dECMKH

https://store.google.com/product/nest_power_connector

Thanks! I was actually looking at that initially, but since it involves messing with the rooftop unit, I shied away to avoid any possible confrontation with the landlord. 🤦‍♂️

Best option is use a plug-in power supply to power up the nest. Dont know for certain in your situation but in some cases you can move the g-fan wire use as a common and then the fan would lose the ability to run independently "will only run when ac is on" I've mostly seen this done in 4 wire heat/cool systems. 20ma? seems kind of wimpy.

I'm going to give this ago, and see if it works. I've read that come units need the R wire from the unit to function and a workaround is to wire one of the 24V adaptor wires with the R wire, although Google doesn't like that. Hopefully that won't be the case here!
 

Falcon_LS

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So I went ahead and wired the external adaptor in, which bumped up the LIN reading to 200 mA and everything seemed to be hunky dory until I turned on the AC and it did not run.

It appears this unit requires the 24V wire from the transformer to run. I've watched a video about connecting both the Red and one of the external adaptor wires to the RH terminal, but it appears to be something Nest frowns upon and I'm not sure if it could be a possible safety concern?
 
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Only thing I can think of there’s not enough oomph to keep the low voltage side of the Contactor pulled in.
 
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I ran new wire, it wasn't difficult to pull it through, but the old wire eas nailed inside a vent so I had to take the vent off. But pulling the new wire us easy.

You could try an Ecobee instead of a Nest. I had one thatcwent bad after several years, and wentcwith Nest only because I got it free. Ecobee are nice and come with an adapter that works well if you don't have a C wire.
 
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