Furnace Thermostat Wiring and Troubleshooting

By | November 18, 2015

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.
Even though there is a industry standard for color coding wires doesn’t mean the installers followed it.
How Does a Thermostat Work?
A thermostat is a switch that can automatically turn on a furnace when a set temperature is reached. If a thermostat is thought to be a problem then it can be bypassed with a jumper wire.
Thermostats use a 24 volt 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 change from furnace to furnace so it is best to examine the transformer and the thermostat furnace wiring for the correct color of the wires. Many times installers follow their own color coding.
Inside the furnace connections are made between it and the thermostat. Both the furnace and the thermostat will have the same terminal letters so they are wired correctly.
Here Are the letters On a Thermostat And What They control.
Furnace Thermostat Wiring
C———This is the common wire from one side of the transformer which is 24 Volts AC.
R———The other side of the transformer also 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 jumperd at the furnace to rule it out.
Furnace Thermostat Wiring and Troubleshooting
If the furnace does start after jumpering R and W but wont with the thermostat then keep in mind it could also be the wiring 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.
Swap out the thermostat as the last option after checking everything else.

3 thoughts on “Furnace Thermostat Wiring and Troubleshooting

  1. 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

  2. C

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


  3. DM Hogan

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


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