Have you ever bought a new thermostat, only to open the cover and see a bunch of letters next to the terminal connections? What could they mean? Is this some kind of tribal code? Maybe it's some kind of secret heating contractor language that is top secret? Well, although heating and cooling mechanics do know what they mean, it doesn't mean you have to join a secret underground organization to learn what they mean.The terminal connections and their lettering are pretty easy to understand once you know what they connect to and why. Besides, the thermostat that you buy will give you a complete set of instructions for your particular installation, and if you can read one of the many languages that they often supply, you should be able to decipher the wiring schematic. If not, here is a simplified list of the commonly used thermostat terminal letters used.
Photo: Timothy Thiele