Gas furnaces require two types of wiring: low voltage and line voltage. Low voltage wiring connects the furnace to the thermostat and air conditioning unit. Line voltage wiring runs to a junction box placed within 3 feet of the furnace. It operates the blower and has a disconnect switch.
The low voltage runs from the furnace control board, which is just inside or on the outside of the furnace, to a terminal on the thermostat. Standard connections on the control board are color-coded. The red, white and green wires from the control board connect to corresponding terminals on the thermostat. The blue wire, which is labeled C, connects to the C terminal on the thermostat and has a white wire connected to it that links to the air conditioning unit. The Y, or yellow wire, on the control board connects to the Y terminal on the thermostat and has a red wire connected to it that also links to the air conditioner. Furnace wiring systems include safety devices that function prior to the connections which ignite the main blower and automatic shut offs should temperatures exceed standards. Wiring should be verified against manufacturer instructions prior to turning on the power.