This page includes instructions on how to wire a furnace thermostat for homeowners that want to install a new or replacement thermostat for their HVAC system.
It includes a furnace thermostat wiring diagram that is color coded to make your job simple at home.
Furnace Wiring Diagram
This gas furnace wiring diagram is a guide to wiring most thermostats that will control a gas furnace system whether or not it includes an AC unit.
You might not need all the wires. Various system configurations and thermostat wiring including 4 wire, 5 wire and 7 wire thermostats are discussed on this page.
Note on Electric furnace thermostats: This diagram is correct for most electric furnace units. However, it is always best to consult the thermostat wiring guide provided by your furnace manufacturer to ensure it is done correctly.
Furnace Wiring Color Code
Here are the wiring colors used in most furnace thermostats and the functions they serve.
Starting on the upper right side of the diagram above:
RC – Red: This wire provides 24 volt power from the furnace to the thermostat so that the thermostat can control air conditioning.
R – Red: This wire serves the same purpose as the RC wire but for heating.
Y – Yellow: Cooling Mode. This wire runs to the furnace control board which in turn controls the activation of the compressor and related components required for AC.
G – Green: This wire controls the blower motor in the furnace.
Y2 – Light Blue: This wire is necessary if you have a two-stage AC compressor. If the AC is single stage, this terminal will be empty.
W2 – Light Brown: This wire functions like the Light Blue wire but for furnaces with low and high stages of heating.
W – White: Heating Mode. For a gas furnace, it controls the sequence of furnace functions needed to fire the furnace. For an electric furnace, it controls energizing the heating coils to produce heat.
C – Dark Blue: The Common wire or C-wire completes a circuit between the thermostat and the furnace to provide continuous 24 volt power. This keeps the thermostat in constant communication with the furnace.
C-Wire Note: If your thermostat uses batteries, then the C-wire isn’t crucial. However, it makes sense to keep it connected in case your thermostat batteries fail.
What do thermostat wire colors mean? Nothing, technically. The wires inside the housing are the same. The industry has adopted these standardized colors to make thermostat installation as uniform and easy as possible for technicians and homeowners too.
The key is to connect the same wire to the proper thermostat terminal and the proper location on the furnace control board.
Do you have only one red wire?
For some thermostats used for a system with both heating and air conditioning, there is only one red wire. In this case, the red wire will likely be attached to the RC terminal. And a jumper wire will connect the RC and R terminals. Though this could be reversed – the wire attached to the R terminal and jumped to the RC terminal.
How to Wire a Furnace Thermostat
Knowing how to wire a furnace thermostat depends on whether it is a replacement thermostat or a new thermostat.
New thermostat wiring: If you’ve installed a new furnace in a new home or addition, for example, use the wiring diagram above for the new thermostat. Separate the bundle of wires coming out of the wall. Strip the housing off the last quarter-inch of the wire, and connect the wires to the terminals as shown.
Thermostat replacement: Take a picture of the thermostat wiring before you start disconnecting wires. Yes, most thermostats are wired using the wiring colors and terminal connections shown above. But in rare cases, it is different. Having a picture will ensure you wire the replacement thermostat just like the old thermostat.
How Do You Wire a Single Stage Furnace Thermostat?
How to wire a single stage thermostat is the same as the diagram given above with a few exceptions.
Single stage heating with no AC: This is 4 wire thermostat wiring. Use the R, G, W and C wires. If the thermostat is designed for a single stage system, it won’t have Y2 and W2 terminals. You can use a 2 stage thermostat with a single stage system, but the Y2 and W2 terminals won’t be used.
Single stage heating and single stage AC: Use the R, RC, Y, G, W and C terminals. This is 6 wire thermostat wiring.
If the thermostat only uses 4 or 6 wires and you have wires left over that’s OK. Not all wires have to be connected to a terminal.
How Do You Wire a Two Stage Furnace Thermostat?
See the Furnace Thermostat Wiring diagram above for reference as you go through this section. It shows the standard wire colors and connections for wiring a 2 stage thermostat for a system with 2 stage heating and air conditioning.
But not all systems with a 2 stage furnace have AC. Therefore, two stage furnace thermostats require 5, 7 or 8 wires depending on the components in the system and their performance.
2 stage heating with no AC: This system requires 5 wires. You won’t use RC, Y or Y2.
2 stage heating with single stage AC: This is a 7 wire configuration. The only wire not used is the Y2 wire for a second stage of cooling.
2 stage heating and 2 stage cooling: This installation uses 8 wires. Most thermostat wiring bundles have 8 wires, though 10-wire bundles are also used on some systems. Not all wires are used on many systems.
How do you wire a Trane gas furnace thermostat?
Wire a thermostat for a Trane furnace using these wire colors and terminals:
- Red to the R terminal for heating.
- White to the W terminal for heating – the thermostat will close the connections between the R and W terminals and contacts in the furnace to create a circuit and energize the furnace to go through the heating sequence.
- Green to the G terminal for the blower fan.
- Dark blue (sometimes black) to the C terminal for continuous power between the furnace and the thermostat.
Those are the required connections for single stage gas furnace heating.
Additional connections you might need for a system with AC or staged heating/cooling are:
- Red 2 to the RC terminal for cooling – or use a small piece of wire to jump or connect the R to the RC terminals.
- Brown to the W2 terminal for second stage heating.
- Yellow to the Y terminal for cooling.
- Light blue to the Y2 terminal for second stage cooling.
How do you wire a 4 wire furnace thermostat for a Goodman furnace?
For a Goodman furnace and most other brands, wire the thermostat this way: Red wire to the R terminal. White wire to the W terminal. Green wire to the G terminal. Blue wire to the C terminal.
What voltage is a furnace thermostat?
A furnace thermostat uses 24 volt power. The furnace is powered by 110-120 volts. A transformer in the furnace converts it to 24V to power the thermostat.
What’s the best thermostat wire for a furnace?
18-8 wire is the best furnace thermostat wire. The 18 refers to the gauge or thickness of the wire. 8 refers to the number of individual wires in the bundle.
If you have extras on your system like a humidifier or air cleaner, then you might need 18-10 wire with ten wires in the bundle.