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 off 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.
Furnace 24 Volt Transformer
R is the hot side while C is the common side of the transformer.
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.
Both the furnace control board and the thermostat will usually have the same terminal letters.
Furnace Thermostat Wiring Diagram
Terminal Letters On a Thermostat and What They Control
The Hot wire (24 volts) usually Red from the transformer is the main power wire to turn on or off a furnace components.
For example, if the Red wire is connected to W on the thermostat the furnace should turn on.
When the Red wire is disconnected from W the furnace should turn off.
The thermostat should automatically complete these connections when a set temperature is reached.
- 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.
- V = Used instead of R on some furnaces
- Rh = Call for heat dedicated
- Rc = Call for cooling dedicated
- W2 = Second stage heating
- Y2 = Second stage cooling
- E = Auxiliary heating with heat pumps(Heat strips)
- 0 = Reversing valve
Other possible terminals often not used.
Troubleshooting A Furnace Thermostat
When the thermostat reaches a set temperature it connects R and W terminals to 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.