Of course, if in doubt, be sure to call a professional.
When working with a thermostat, the cover can be snapped off to expose the wiring.
Before doing any work on the thermostat and wiring, take a picture of the wires and their connections, or write them down. This way, you can always refer back to the original setup.
Keep in mind, even though there is an industry standard for color-coding wires, that doesn’t mean the installers followed it.
How Does a Thermostat Turn on a Furnace?
A thermostat is a switch that can automatically turn on a furnace when a set temperature is reached.
The transformer steps down 120 volts to the 24 volts the thermostat needs, and sends out the 24 volts on two wires.
The two 24 volt wires go to the R terminal and C terminal inside the thermostat.
The two 24 volt wires can NEVER touch, or the transformer will short and need to be replaced.
Even though a red wire is supposed to go to R and a black wire to C the colors of the wires can sometimes differ.
Many times installers follow their own color coding so always follow the terminals on the thermostat and not the wiring color.
Example Furnace and AC Thermostat Wiring Diagram
Terminal Letters On a Thermostat and What They Control
The thermostat should automatically complete these connections when a set temperature is reached.
Thermostat Terminals Meanings Explained
- C = This is the common wire from one side of the transformer which is 24 Volts AC.
- R = Power, 24 Volts AC (This is the hot side of the transformer)
- G = Turns the fan on.
- W = Call to turn on heat.
- Y = Call for cooling.
Other Possible Terminals Often Not Used
How To Troubleshooting a Furnace Thermostat
When the thermostat reaches a set temperature, it connects R and W terminals which will turn on the furnace.
Keep in mind; it could also be an open wire; although this is not common, it does happen.
The wiring connections can go bad for any number of reason but eliminate the thermostat first by changing the battery and making sure it is on the right setting (heat) and temperature to activate the call-for-heat to the unit.
Then jumper the wires R and W at the thermostat terminals to rule out the wiring.
Replace the thermostat as the last option after checking everything else.