The Nest Thermostat is compatible with 2 wire low voltage HVAC systems that are either heat only systems or cooling only systems.

Where does black wire go on nest?

Hi Salman Zuberi, Welcome to Google Nest Community Forum. Your old thermostat wiring will serve as a reference for identifying wires and correctly installing your Nest thermostat. The black wire going to the Y terminal on your old thermostat should be connected to the Y1 terminal of the Nest Thermostat.

Can I use the G wire for the C wire?

Option #3: Use a G-Wire as a C-Wire If there is no hidden C-wire, you can use a G-wire instead. However, you will not be able to use your fan independently when the heating or cooling isnt running. Also, many HVAC systems are incompatible with this solution, including: HVAC systems using electric heat.