Furnace Thermostat Wiring and Troubleshooting

By | August 18, 2019

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

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.
Furnace Thermostat  Troubleshooting
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.

If a thermostat is thought to have a problem then it can be bypassed with a jumper wire.
Furnace Thermostat Wiring and Troubleshooting Color Codes
Thermostats use 24 volts AC from a transformer to control a furnace.

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
Furnace Thermostat 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 and Troubleshooting Contorl Board
Furnace Thermostat Wiring Diagram
Furnace Thermostat Wiring and Troubleshooting Diagram
Terminal Letters On a Thermostat and What They Control
Furnace Thermostat Wiring
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.

Thermostat Terminals

  • 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.

  • 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

Troubleshooting A Furnace Thermostat
When the thermostat reaches a set temperature it connects R and W terminals to turn on the furnace.

If a bad thermostat is suspected then these to terminals can be jumpered at the furnace to rule it out.
Furnace Thermostat Wiring and Troubleshooting
If the furnace starts after jumpering R and W the thermostat is likely bad.

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.



5 thoughts on “Furnace Thermostat Wiring and Troubleshooting

  1. Tom blackford

    I have a older Coleman furnace the white n red are connected to plug on furnace but green wire is off inside the furnace were do I connect it to get fan to run it lites burner but no fan

  2. Alice Carroll

    Thanks for this very thorough guide; I appreciate how you also included diagrams that are easy to understand. My furnace recently hasn’t been regulating the temperature I set in my thermostat so I think, based on how you explained it, there’s something wrong with the hot wire. I’m a bit nervous to open up my thermostat myself, so I guess I should just opt to get a repair service.

  3. DM Hogan

    Interesting and informative. Learning is accomplished. Good work! And, thanks.

  4. James

    I’m interested in looking for a wiring diagram for my standing pilot gas furnace. It has one 3 wire zone valve and one thermostat. It also has an AQUASTAT, gas valve and circulator pump as well as a vent damper. It recently stopped working and I’m trying to follow the power through the devices. The AQUASTAT has a 24v transformer but there is also another transformer hooked up

  5. C

    So the C on my transformer is the common and can be used as the common for my nest thermostats?



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