Thermostats & Wiring
Terminal letters, wire colors and low-voltage circuits.
Thermostat Blank or No Power: Finding the Missing 24 Volts
How to troubleshoot a blank thermostat: batteries, the C wire, the furnace door switch, a blown board fuse, a tripped float switch, the transformer and power to the air handler.
Thermostat Wiring: Terminal Letters and Wire Colors Explained
What each thermostat terminal does (R, Rc, Rh, C, W, W2, Y, Y2, G, O/B, E, AUX), the usual wire colors, and how to verify a hookup.