With so many new functions appearing on today’s modern thermostats, there is of course, many more wires to connect up nowadays, when installing a new or replacement thermostat. Gone are the two-wire days, when all you needed was one wire from the furnace transformer, and the other to feed power to a gas valve or relay to turn the heat on and off. State-of-the-art HVAC systems these days, in addition to the original single stage heating provision, often include a second stage heater, as well as one or two stages of cooling (central air conditioning). Many also feature a wire to operate the compressor changeover valve (switch it between heating and cooling mode), as well as a light (L) wire that often has a light connected to it within the thermostat that indicates heat pump status of some sort. Each of these newer functions must have its own wire from the thermostat to tell it when to operate. So, to help keep all the wires straight and to avoid connecting a wire to the wrong terminal on the thermostat, they’ve devised various thermostat wire color codes and conventions.
|Wire Color, Typical||Letter Designation||Purpose|
|A||Power present here when any heating or cooling is operating in the HVAC system.|
|Aux/E, W2||Operates 2nd heating stage, or when controlling a heat pump, the emergency heat stage.|
|Blue||B||Activates the changeover to heat relay / valves in HVAC systems.|
|Brown, blue, purple, black.||C||24 volts AC. The common lead of the transformer. All furnace switched components have one side of their power connected to this lead.|
|Brown||E||Activates emergency heat relay|
|Green||G||Operates the internal fan|
|K||May activate emergency heat relay in some installations. Also used between a thermostat and Honeywell wire savers.|
|L||Heat pump monitor light. May turn on when emergency heat is operating.|
|Orange||O||Activates the changeover to cool relay / valves in HVAC systems.|
|O/B, W, W1||Operates first heating stage. For heat pumps, activates the compressor in heat mode.|
|P||Defrost operation in progress lamp; comes on when either the HVAC compressor outside has switched into defrost mode.|
|Red||R||24 volts AC ????|
|Red||RC||24 volts AC supply to cooling relay in HVAC system.|
|Red||RH||24 volts AC supply to heating relay in HVAC system.|
|White||W1, W, O/B||Operates first heating stage. For heat pumps, activates the compressor in heat mode.|
|W2 - Aux/E||Operates 2nd heating stage, or when controlling a heat pump, the emergency heat stage.|
|X||Malfunction alert lamp. Voltage present when the HVAC system detects a problem in its components.|
|Yellow||Y1, Y||Operates first stage compressor cooling.|
|Y2||Operates 2nd cooling stage.|
|Y, Y1||Operates first stage compressor cooling.|
|S1, S2||Outdoor temperature sensor (S) wires.|
Common Thermostat Wire Color Code Assignments
The table above provides a more comprehensive list of thermostat wire colors and their uses. However, here is a list of the most commonly found wire color to function mappings, as seen in many four and five wire thermostat installations.
- C – 24 VAC Common (You might see blue, purple, or brown typically used for this wire).
- G – Fan (green wire). Turns on the circulating fan in most forced air heating and cooling systems. Allows the fan to be run independently of whether or not actually heating or cooling is called for by the thermostat.
- R – 24 VAC / R and Rc (red). Supplies the high side of the 24 volt AC line from the cooling power transformer in an HVAC system.
- Y – Compressor / cooling (yellow). Turns on the compressor in cooling mode. May also activate the change-over valve that assures that the compressor is operating in cooling mode, depending on the particular HVAC system involved.
- W – Heat (white wire). In gas systems, this triggers the ignition process; opens the valve, energizes the ignitor, and fires up the burners in the furnace. In electric heating furnaces, this lead turns on the heating elements. And, in heat pump systems, the white wire starts the compressor in heating mode.
For all other thermostat terminals, and in fact, sometimes even for the above mentioned common color assignments, the exact purpose of a particularly colored wire are often inconsistent.
So again, if you’re installing a new thermostat, DO NOT rely on the wire colors alone coming out of the wall! Repeating: DO NOT RELY ON THE WALL WIRE COLORS ALONE TO DETERMINE EXACT WIRE FUNCTION. You instead, need to either photograph or write down to which terminal on the old thermostat that each wire is attached. For each wire, jot down its color, along with the letter designation of the terminal on the old thermostat. If the correct wire functions are still not clear, then you need to trace the wires back to the furnace / HVAC unit itself, and see which components the various colors of wires attach to.
Do not attempt to wire a thermostat yourself if you feel uncomfortable with working with electricity. Further, improper wiring can damage the thermostat or the furnace, resulting in expensive repairs by a professional.
We cannot assume responsibility for damages resulting from incorrect wiring of any thermostat.
- 2017-03-08: Originally published.