In a standard thermostat wiring scheme, the heating control is wired to terminal

• A. R
• B. Y
• C. G
• D. W

Answer: (D)

Calling for heat is done through the heat terminal, labeled W. The thermostat is fed power from the R circuit, and when heat is needed it closes the circuit to W, sending 24 volts to the furnace control to start the heating sequence. The other terminals serve different roles: Y controls cooling (compressor), G controls the blower fan, and R is simply the power supply for the thermostat. So the heating function is wired to the W terminal.