Pleas allow me to provide some clarification. The the HVAC operates the control system using a 24 Volts AC power supply. This power comes from a step-down transformer in the HVAC equipment. It steps down 120 VAC or 240VAC to the needed 24VAC. There are two wires on the primary side of the transformer. For 120VAC LINE and Neutral or for 240VAC Line-1 and Line-2. On the secondary side of the transformer there is the Line and COMMON. The secondary side LINE is connected to R and the COMMON is connected to the C. Until programmable and smart thermostats came along, the COMMON was not needed at the thermostat.
However, batteries were all that was needed in the early days of programmable thermostats. Today of course we have lithium rechargeable batteries built into the thermostats. To charge them you need the COMMON to be brought to the thermostat.
You may find some color conductors of your thermostat cable that are not connected to anything in the thermostat. One of those can be used for COMMON. You will need to open the service panel of your HVAC a equipment and locate the control board. Not all equipment has a control board, but furnaces is definitely do. When you open the service panel follow the thermostat cable that goes inside the cabinet, you’ll find one of the wires is connected to either an R terminal on the control board, or it is wire nut it to the red wire, typically, of the thermostat cable. If you follow that wire back to the transformer, you will find the common at the transformer. It may be cross connected to the common on the control board, but it is definitely cross connected going to the outdoor unit.
to get the common to your thermostat, choose one of the unused wire, hopefully blue or black, and connected to the common side of the transformer or joint with the common wire going to the outdoor unit under a wire nut. Then take the same color wire at your thermostat and connected to the C terminal. Turn the power back on to your HVAC system then test between R and C and you should see 24 V AC, approximately, it could be 3V or 4 V higher or lower.
I hope this helps you to understand the system better.
Ken, The AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
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