Picture of a Honeywell WiFi thermostat wall plate, showing a typical wiring hookup.

Honeywell Thermostat 4 Wire Wiring Diagram

Today’s thermostats have so many new functions. So there are many more wires to connect up when installing a new or replacement t-stat. Gone are the two-wire days.  In those times, all you needed was one wire from the furnace transformer.  Plus, you had another to feed power to a gas valve or relay to turn the heat on and off. But as mentioned, today’s t-stats, with their ever growing feature list, have more wires to ring out.  So here, we show examples of Honeywell Thermostat Wiring Diagram 4 Wire for these higher wire count t-stats.  Here, we focus on the 4 wire t-stats.


Sometimes even for the above common color uses, the exact purpose of a wire is often not clear.  Not every four wire t-stat setup follows these “standards.”

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.

Multi Stage Heating and Cooling Systems

State-of-the-art HVAC systems these days, in addition to the original single stage heating provision.  These often have a second stage heater. Plus, they also may have one or two cooling stages. Many also use another wire to turn on the compressor change over valve.  This valve switches the unit between heating and cooling mode).  There also may be a light (L) wire with a light connected to it on 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 turn on. So, these color codes help keep all the wires straight. With them you can avoid matching a wire to the wrong t-tat terminal.  You wouldn’t want the heat to come on when the air was intended.

Honeywell thermostat wiring diagram 4 wire. Picture of a typical Seven Conductor Thermostat Cable, showing the different colors of each wire within the cable.
Honeywell thermostat wiring diagram 4 Wire. Typical seven conductor thermostat cable, showing the color of each wire inside.
Wire Color, TypicalLetter DesignationPurpose
APower present here when any heating or cooling is operating in the HVAC system.
Aux/E, W2Operates 2nd heating stage, or when controlling a heat pump, the emergency heat stage.
BlueBActivates the changeover to heat relay / valves in HVAC systems.
Brown, blue, purple, black. C24 volts AC. The common lead of the transformer. All furnace switched components have one side of their power connected to this lead.
BrownEActivates emergency heat relay
GreenGOperates the internal fan
KMay activate emergency heat relay in some installations. Also used between a thermostat and Honeywell wire savers.
LHeat pump monitor light. May turn on when emergency heat is operating.
OrangeOActivates the changeover to cool relay / valves in HVAC systems.
O/B, W, W1Operates first heating stage. For heat pumps, activates the compressor in heat mode.
PDefrost operation in progress lamp; comes on when either the HVAC compressor outside has switched into defrost mode.
RedR24 volts AC ????
RedRC24 volts AC supply to cooling relay in HVAC system.
RedRH24 volts AC supply to heating relay in HVAC system.
WhiteW1, W, O/BOperates first heating stage. For heat pumps, activates the compressor in heat mode.
W2 - Aux/EOperates 2nd heating stage, or when controlling a heat pump, the emergency heat stage.
XMalfunction alert lamp. Voltage present when the HVAC system detects a problem in its components.
YellowY1, YOperates first stage compressor cooling.
Y2Operates 2nd cooling stage.
Y, Y1Operates first stage compressor cooling.
S1, S2Outdoor temperature sensor (S) wires.

Honeywell Thermostat Wiring Diagram 4 Wire Examples

The table above provides a more complete list of Honeywell thermostat wiring colors and their uses. But here is a list of the most common wire color mappings, as seen in many four 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 run separate from whether heating or cooling is called for by the t-stat.  That is, you can run the fan without either heating or cooling, just to circulate the air.
  • 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.  The change over valve assures that the compressor runs in cooling mode.  Whether the valve is open when on or closed  depends on the HVAC system design 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.
Honeywell thermostat wiring diagram 4 wire. Picture of the wall plate for a 4 wire smart thermostat installation, using the green wire as the C wire instead of the G (fan) wire.
Honeywell thermostat wiring diagram 4 wire. Wall plate for a 4 wire smart thermostat installation. This one uses the green wire as the C wire instead of the G (fan) wire.

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Suggested Reading

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References for Honeywell Thermostat 4 Wire Wiring Diagram

  1. 4 Wire or 5 Wire Thermostat Wiring Problem, from HighPerformanceHVAC.com 
  2. Honeywell Thermostat Wiring Instructions, from DIYHouseHelp.com 

Revision History

  • 2019-04-24: Purged ad scripts, tweaked target focus, and added tags.
  • 2018-05-05: First published.
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