How To Troubleshooting a Furnace Thermostat E = Auxiliary heating with heat pumps(Heat strips).R = Power, 24 Volts AC (This is the hot side of the transformer).C = This is the common wire from one side of the transformer which is 24 Volts AC.The thermostat should automatically complete these connections when a set temperature is reached. When the Red-wire is disconnected from W, the furnace should turn off. The Hot-wire (24 volts), usually Red from the transformer, is the main power wire to turn on or off furnace components.įor example, if the Red wire is connected to W on the thermostat, the furnace should turn on. Terminal Letters On a Thermostat and What They Control Many times installers follow their own color coding so always follow the terminals on the thermostat and not the wiring color.īoth the furnace control board and the thermostat will usually have the same terminal letters.Įxample Furnace and AC Thermostat Wiring Diagram The two 24 volt wires can NEVER touch, or the transformer will short and need to be replaced.Įven though a red wire is supposed to go to R and a black wire to C the colors of the wires can sometimes differ.

R is the hot side, while C is the common side of the transformer. The two 24 volt wires go to the R terminal and C terminal inside the thermostat. The transformer steps down 120 volts to the 24 volts the thermostat needs, and sends out the 24 volts on two wires. Thermostats use 24 volts AC from a transformer to control a furnace. If a thermostat is thought to have a problem, then it can be bypassed with a jumper wire. How Does a Thermostat Turn on a Furnace?Ī thermostat is a switch that can automatically turn on a furnace when a set temperature is reached. Keep in mind, even though there is an industry standard for color-coding wires, that doesn’t mean the installers followed it. This way, you can always refer back to the original setup.

When working with a thermostat, the cover can be snapped off to expose the wiring.īefore doing any work on the thermostat and wiring, take a picture of the wires and their connections, or write them down. Of course, if in doubt, be sure to call a professional.

Furnace thermostat wiring falls in the DIY category that a handy type person can hook up or fix.

Step down transformer wire color code