How to install a C-wire adapter for a dual transformer system

[Mumsicles]

For the past two weeks, all 3 of my Us-based Nest thermostats began malfunctioning. Each showed the heat as not-on and set at 60F. But the radiator was clearly emitting low heat. The room temperature registered well above 60F. In another words, the thermostats didn’t turn the heat off.

Assuming the thermostat batteries are low on charge, i recharged them. That fixed one of them. The other ones remained flaky.

I reset the Nest, that worked briefly but the problem returned. I decided to get an external C-wire adapter so the thermostat could receive consistent power.

I was able to install the C-wire adapter on the heat only thermostat. I put wire-1 of the adapter to the C-terminal and wire-2 to Rc. The heating system uses W and Rh. So far, it appears to be working well.

I have a 3rd thermostat controlling a dual transformer system (separate cooling and heating wiring). In this case, Y/G/Rc/W/Rh terminals are occupied by existing wires. I understand wire-1 of the external adapter normally go into the C-terminal but I’m not sure where to put wire-2 since Rc and Rh are already occupied. How could I apply the external C-wire adapter properly to the Nest thermostat in this case?

 

[Houptee]

Is there a spare wire coming from the air conditioner that you can connect to the C terminal on the Nest and verify it is connected to the C terminal screw in the air conditioner unit.

Otherwise you can put the external transformer wire to same terminal with the Rh wire and other to the C terminal.

[Mumsicles]

Thank you for your response, Houptee. Very much appreciate it.

There is not a spare wire on the AC unit. One clarifying question. Since the AC already has a wire going to the Rc terminal, when I put the transformer wires to C and Rc respectively, Rc would end up with two wires. Is the Nest able to handle two wires going into the same terminal? Would that cause the AC to go on & off constantly in the summer time? Would I have to switch the second transformer wire to Rh in the summer (since heat is not in use) and Rc in the winter (AC not in use)?

[Houptee]

No you would not have to change any wires around.

You can only have one wire in the C terminal (the one from the external transformer) and then put the Rh and the + from the external transformer together in the same terminal on the Nest.

This keeps constant 24v to the Nest all the time to keep it powered up.

Watch this video he explains how to do it.

https://youtu.be/yi5kfn3bmEk

[Chefrichardee]

If you put the second adapter wire with the Rh wire, do you have to worry about the polarity of the wire from the adapter?

[Houptee]

The wall type transformers have small wires like speaker wire so both wires can fit into the R together.

But you can only have one C wire (in this case the one from the wall transformer).

The wires on the transformer have marks for what it Neg and Positive.

Watch this video

https://youtu.be/yi5kfn3bmEk

[Markjosephp]

Hello there,

Thanks for helping here, @Houptee.

@Mumsicles, I hope you've got the answer you're looking for. If you're still in need of assistance, feel free to let us know.

Regards,

Mark

[Markjosephp]

Hello Mumsicles,

I'm checking back in — how's it going? Still need our help? Let us know so we can assist you right away.

Regards,

Mark

[Markjosephp]

Hi Mumsicles,

I'm checking in to make sure that you've seen our response. If you have any further questions or concerns, please let us know before we lock this thread in 24 hours.

Thanks,

Mark