Air Conditioner wiring color codes (Central Air Conditioning)

I posted this to respond to a question, but it's good, so I blogged it.

In air conditioning language:
The labels you will find inside the machines are the first letter of the alleged wire color in comm use in, "The Trade".

Green (G) is the interior fan command. If 24 VAC, then fan.
Yellow is the compressor command.
White is the first stage heat command. If you have a second stage of heat the labels will be W1 and W2.
Red is the 24 VAC supply.
C is, "common" of the 24 VAC supply, not, "cool".
Common is not used at the thermostat if there is no need to charge batteries or run a microprocessor.
Rc means, "Red/cool" and Rh means "Red/heat" for when you have two separate systems for heat and cool. If you only have one system, you jumper Rh to Rc and supply them both from one 24 VAC transformer.

Orange and Blue (as labels on the equipment) are there to run the reversing valve on the Freon system. If you have a reverse cycle air conditioner, you reverse the Freon flow in the winter. If you have a heat pump, you reverse the Freon flow in the summer. They are rare enough around here that I never memorized whether it's blue or orange for which one. I just hook one up. If the Freon system runs backwards, it must be the other one.:D

The colors of the wires in the wall are OFTEN whatever was on the truck that day. The labels on the machine are God. If Joe Blow used blue for "common" at the thermostat, believe the label, not the wire color. The first thing I do on a job with control problems is check to see what colors the installer used and memorize them for as long as it takes to finish that job.
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