Furnace Thermostat Wiring and Troubleshooting – HVAC How To Thermostats use 24 volts AC from a transformer to control a furnace. The transformer steps down 120 volts to the 24 volts the thermostat needs, and sends out the 24 volts on two wires. The two 24 volt wires go to the R terminal and C terminal inside the thermostat.[PDF] Standard AC with Standard Furnace Control Wiring Control Board 2 This diagram is to be used as reference for the low voltage control wiring of your heating and AC system. Always refer to your thermostat or equipment installation guides to verify proper wiring. NOTE Y2 Some AC Systems will have a blue wire with a pink stripe in place of the yellow or Y wire. **Refer to the blower chart for CFM requirements. Jump if needed to achieve correct CFM.