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.
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.
If a thermostat is thought to have a problem, then it can be bypassed with a jumper wire.
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
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 and AC 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 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 Meanings
- 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.
If a bad thermostat is suspected, these terminals can be jumpered at the furnace to rule it out.
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.
my boiler goes on when i put thermostat to heat but does not turn off when i have desired temp set i pulled off cover and checked with volt meter reads 120 volts is my transformed bad and there is no ac on this system
I have a Honeywell thermostat 2wires where do they go on the furnace one to the R and one to the C ??
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
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.
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
So the C on my transformer is the common and can be used as the common for my nest thermostats?
Interesting and informative. Learning is accomplished. Good work! And, thanks.
I’m in Massachusuetts in an old home. I have a Rh and W pair going to what is now a battery operated thermostat as there is no C wire. This has worked well for years but I’m tired of buying batteries. I bought a transformer designed to give a C wire feed..it has two wires designed it says to be a C and R line. If I disconnect the Rh and use the transfomer’s R and C, then the thermostat shows it’s calling for heat, but the boiler never goes on–perhaps no surprise there. But I am not sure I should attach BOTH the old Rh wire and the new R wire (from the new transformer) lest I sent too much electricity back to the boiler from the old Rh and new R. Yet I can’t see how the system would work if I use only the C wire from the transformer alone. Suggestions?
Thanks for this guide! Hoping you still review these comments. I jumped R and W at the control board and the furnace did come on, but only for about 4 minutes before shutting off. With these jumped should it run continuously?
Yes it should run continuously .It may shut off if the heat exchanger gets too hot. Check if your filter is dirty as a furnace needs good air flow to not over heat.