Nest thermostat

[Electrician252]

• Good morning, I’ve recently seen a wiring diagram showing an external 24vac transformer (plug in) connected to a nest power connector. Correct me if I’m wrong but if I have  a  2-wire system on an old furnace  with no (c-wire) common wouldn’t you have an issue with two different power supplies on the red wire going to the thermostat? It seems that way considering there’s already power going on the red wire, just curious, thanks.

[zoeuvre]

Hi Electrician252, 

Thanks for reaching out to us. Yes, you are correct. Connecting an external 24VAC transformer to a 2-wire system on an old furnace without a common wire (C-wire) can cause issues. In a typical HVAC system, the red wire (R) supplies power from the furnace or air handler to the thermostat, while the common wire (C-wire) provides a return path. In systems without a C-wire, power is sourced only from the red wire. Introducing an additional 24VAC transformer could create a conflict and potentially damage the Nest thermostat or other HVAC system components.

You have the option to use the Nest Learning Thermostat or Nest Thermostat E instead, as these types of thermostats may operate without a C wire since they have an internal built-in battery that can supply power to the thermostat. Another option is to run a new C-wire from the furnace or air handler to the thermostat. This may involve running additional wiring or using alternative methods like power stealing or wireless adapters to create a C-wire connection.

Let us know if that helps.

Best,

Zoe