With so many new functions appearing on today’s modern thermostats (t-stats), there is of course, many more wires to connect up nowadays, when installing a new or replacement t-stat. 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. So here, we show examples of Honeywell Thermostat wiring color code for these higher wire count t-stats.
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 AC). Many also have a wire to turn on the compressor change over 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 t-stat. This light shows heat pump status. Each of these newer functions must have its own wire from the t-stat to tell it when to operate. So, these color codes help keep all the wires straight. With them you can avoid matching a wire to the wrong t-tat terminal.
|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 Honeywell Thermostat Wiring Color Code
The table above provides a more comprehensive list of Honeywell thermostat wiring colors and their uses. But here is a list of the most common wire color to function mappings, as seen in many four and five wire t-stat setups.
- 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 t-stat.
- 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 flip the change-over valve that assures that the compressor is running in cooling mode, depending on the HVAC system involved.
- W – Heat (white wire). In gas systems, this triggers the lighting process; opens the valve, turns on 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 t-stat terminals, and in fact, sometimes even for the above mentioned common color assignments, the exact purpose of a wire is often not clear.
So again, if putting in a new t-tat, DO NOT rely on the wire colors alone! Repeating: DO NOT RELY ON THE WALL WIRE COLORS ALONE TO FIND EXACT WIRE FUNCTION. Instead, take a picture or write down which lug on the old t-stat that each wire goes with. For each wire, jot down its color. Plus, write the letter of the terminal on the old t-stat that it connects with. If the correct wire functions are still not clear, then trace the wires. You need to track the wires back to the HVAC unit itself. Then, find out which parts the various colors of wires attach to.
Do not attempt to wire a t-stat yourself if you feel unsure with working with electricity. Further, wrong wiring can break the t-stat or furnace. This can result in costly repairs.
We cannot assume responsibility for damages due to faulty wiring of any t-stat.
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- 2017-03-08: Originally published.