The most common programmable HVAC wiring systems in homes contains a furnace and an air conditioning unit. In this case, the thermostat wires are color-coded. Typically, common household thermostats only contain five terminals: two for red, one for green, one for yellow and one for white.
Modern household thermostats usually only use five terminals and four wire colors. This color-coding is standard and should be similar from system to system.
The wire colors are red, white, yellow and green. The red wire connects to two terminals because it connects to the house’s heating and cooling supply: RC (for red cooling) and RH (for red heating). The green wire, or G wire, connects to the G terminal that controls the power for the fan. The white wire, or W wire, connects to the W terminal and controls the enabling function for heating. The yellow wire, or Y wire, connects to the Y terminal, which controls the enabling function for cooling.
With familiarity with the color codes and the terminals that they attach to, an individual should be able to adequately interpret any common household HVAC system with little difficulty. However, sometimes the wires and the thermostat terminals do not match. This does not mean that the HVAC system does not work, only that the wire colors and the terminals to which the wires attach are different. This variation should be noted for future reference.