There are some common wire colors, but there are exceptions, and there are people that just grab some wire and put whatever color they want on a given terminal. Your thermostat is a bunch of switches. A 24 VAC wire feeds the thermostat switches, and different colored wires go to the other side of each switch so that 24V can be applied to the appropriate contactor. There is usually a switch for heat, cool, and fan, and the thermostat decides which one(s) should be energized and when. So 4 wires would be required (24 VAC source, heat, cool, fan) for the typical modern thermostat. These terminals on the thermostat are typically named by the color of wire that goes on that terminal (e.g. R = Red). The other side of the 24 VAC source rarely goes to the thermostat, but does go to the controlled equipment. Usually, the only time the other side of the 24V is brought to the thermostat is if it has lights or other things in it that consume power that is more than batteries can supply for a significant time. Most LCD thermostats can run off their batteries for a long time.
Common wire colors are:
R: Red Wire, 24 volts hot from transformer (usually in the air handler)
Y, Y1: Yellow wire, AC Compressor first stage
Y2: Blue or Pink, AC Compressor second stage
W, W1: White wire, heating or 1st stage of backup heat in a heat pump
G: Green wire, fan blower relay.
C: Brown or sometimes black, 24 volts common from transformer
Heat pumps need more wires, and the color usage is not universal, so be careful if you have one of those. You can be a manual thermostat by connecting the red and white wires together. This should turn the furnace on until you break that connection. Likewise, if you connect the red and yellow together, the AC should come on. The air handler usually determines for itself when the fan should come on. The green wire is usually used only when you want the fan on when there is no call for heat or cool. Connect red to green to make that happen.
If you have a Carrier Infinity or Bryant Evolution system, then all of the above is wrong, as they use a digital control method over 4 wires that works for even a complicated multi-staged heat pump setup. It is 24 VAC source, 24 VAC return, Digital +, Digital - (and I'm not sure of the colors).
Last edited by suemarkp : July 5th, 2006 at 04:53 PM.