Thermostat Wiring Guide - Wire Colors, Terminals & Diagrams

Thermostat Terminal Labels

Terminal	Name	Typical Color	Function	Notes
R	Power (24V)	Red	24V power from transformer	Rc = Cooling, Rh = Heating (if separate)
C	Common	Blue	24V common/ground return	Required for smart thermostats
Y	Cooling	Yellow	Compressor contactor signal	Y1 = Stage 1, Y2 = Stage 2
G	Fan	Green	Indoor blower fan relay	Controls fan independently
W	Heat	White	Gas valve / Electric heat relay	W1 = Stage 1, W2 = Auxiliary heat
O/B	Reversing Valve	Orange	Heat pump reversing valve	O = Cooling energized, B = Heating energized
E	Emergency Heat	Pink/Brown	Emergency/Auxiliary heat	Bypasses heat pump, uses backup heat only
S	Outdoor Sensor	Purple/Various	Outdoor temperature sensor	S1, S2 for sensor pair

Important Notes

Rc vs Rh: Some thermostats have separate R terminals for cooling (Rc) and heating (Rh) systems. Use a jumper if you have a single transformer.
O vs B: Most heat pumps use "O" terminal (Rheem, Carrier, Trane - energized in cooling). Rheem uses "B" terminal (energized in heating).
W vs W2: W or W1 is primary heat. W2 is auxiliary/backup heat (electric strips).

Standard Wire Color Codes

Standard Color Convention

Red → R (24V Power)

Yellow → Y (Cooling)

Green → G (Fan)

White → W (Heat)

Blue → C (Common)

Orange → O/B (Reversing Valve)

Brown → W2/E (Auxiliary Heat)

Common Wire Bundles

18/2 - Two Wire

Basic heating only (R, W)

18/4 - Four Wire

Conventional heat/cool (R, G, Y, W)

18/5 - Five Wire

With common wire (R, G, Y, W, C)

18/8 - Eight Wire

Heat pump with aux heat (R, G, Y, W, C, O, W2, Y2)

Pro Tip: Always use 18/5 or 18/8 wire for new installations. The extra wires provide flexibility for future upgrades to smart thermostats or heat pumps.

System Wiring Diagrams

Heating Only System (2-Wire)

Basic configuration for gas furnace, boiler, or electric baseboard heating without cooling or fan control.

Thermostat

R

W

Furnace

R

W

R (Red): 24V power from transformer

W (White): Heat call signal to gas valve/relay

Connection Steps

1 Turn off power to furnace at breaker
2 Connect Red wire to R terminal on both ends
3 Connect White wire to W terminal on both ends
4 Restore power and test heating operation

Note: Millivolt systems (some gas fireplaces) don't use 24V transformer. They use special thermostats with direct gas valve connection.

Conventional Heat/Cool System (4-5 Wire)

Standard configuration for gas/electric furnace with central air conditioning. Most common residential setup.

Thermostat

R

Y

G

W

C

Air Handler

R

Y

G

W

C

R: 24V power (transformer hot)

Y: Cooling - activates compressor

G: Fan - controls blower

W: Heat - activates furnace

C: Common (for smart thermostats)

System Operation

Cooling Mode

Thermostat sends R→Y and R→G signals. Y energizes compressor contactor, G energizes blower relay.

Heating Mode

Thermostat sends R→W signal. W energizes gas valve (furnace control board handles fan automatically).

Fan Only Mode

Thermostat sends R→G signal only. Blower runs for circulation without heating or cooling.

Smart Thermostat Requirement

WiFi thermostats (Nest, Ecobee, Honeywell) need constant 24V power. The C wire provides the return path. Without C wire, options include:

• Run new thermostat wire with C
• Use "Add-A-Wire" kit (repurposes existing wire)
• Install 24V plug-in transformer

Heat Pump System (7-8 Wire)

Heat pump with auxiliary electric heat strips for backup. Requires reversing valve control.

Thermostat

R

Y

G

O

W2

C

Air Handler

R

Y

G

O

W2

C

R: 24V power

Y: Compressor (runs in both modes)

G: Indoor blower fan

O: Reversing valve (energized in cooling)

W2: Auxiliary/Emergency heat strips

C: Common return

O vs B Terminal

O Terminal (Most Common)

Reversing valve energized in COOLING mode

Used by: Carrier, Trane, Lennox, Goodman, Rheem (newer)

B Terminal

Reversing valve energized in HEATING mode

Used by: Rheem (older), Ruud

Auxiliary vs Emergency Heat

Auxiliary Heat (Auto)

Heat strips activate automatically when outdoor temp is too low for heat pump efficiency (typically below 35°F) or when demand exceeds capacity.

Emergency Heat (Manual)

User manually activates heat strips and locks out compressor. Use only when heat pump is broken.

Two-Stage System

Systems with two-stage compressors or two-stage furnaces for improved efficiency and comfort.

Terminal	Stage 1	Stage 2
Cooling	Y1	Y2
Heating	W1	W2
Heat Pump	Y1 + O/B	Y2

Y1/W1: First stage (low capacity) - 60-70% output

Y2/W2: Second stage (full capacity) - 100% output

Two-Stage Operation

Stage 1 (Most of the time)

System runs at reduced capacity. Quieter operation, longer run times, better dehumidification, and more even temperatures.

Stage 2 (High demand)

Activates when Stage 1 can't satisfy demand within setpoint differential time (typically 10-30 minutes).

Variable Speed Systems

Inverter-driven compressors and variable-speed furnaces use communicating protocols (like Carrier Infinity, Trane ComfortLink, or Lennox iComfort) instead of traditional thermostat wiring. These require proprietary thermostats.

Troubleshooting Common Wiring Issues

Thermostat Has No Power

• Check 24V transformer at furnace/air handler
• Check 3A fuse on control board
• Verify R wire connected at both ends
• Check for broken wires in wall
• Test voltage: R to C should read 24-28VAC

Cooling Not Working

• Verify Y wire connected at thermostat and air handler
• Check for 24V between Y and C when calling for cool
• Inspect contactor in outdoor unit
• Check if outdoor disconnect is on
• Verify thermostat is in cooling mode

Heating Not Working

• Verify W wire connected at thermostat and furnace
• Check for 24V between W and C when calling for heat
• Check furnace door safety switch
• Look for error codes on furnace board
• Verify gas valve is open

Fan Issues

• Verify G wire connection
• Check fan relay on control board
• Test blower motor capacitor
• Check for blocked filter causing high limit trips
• Verify fan switch position (Auto vs On)

Heat Pump Stuck in Wrong Mode

• Check O/B wire connection
• Verify thermostat O/B setting matches system
• Test reversing valve solenoid
• Check for stuck reversing valve (mechanical)
• Swap O and B setting in thermostat

Smart Thermostat Won't Power On

• C wire is required - verify connection
• Check if Rc and Rh need jumper
• Try charging thermostat via USB first
• Verify transformer is 24VAC (not 12V)
• Check compatibility with your system type

Voltage Testing Guide

R to C

24-28 VAC

Transformer output voltage

Y to C (Cooling call)

24-28 VAC

When thermostat calls for cooling

W to C (Heat call)

24-28 VAC

When thermostat calls for heating

Safety Warning: Always use a multimeter rated for HVAC work. While 24V is considered low voltage, always turn off power before connecting or disconnecting wires.

Smart Thermostat Compatibility

Google Nest

C Wire Required? Recommended

Heat Pump Support Yes (O/B)

2-Stage Support Yes

Wire Compatibility R, Y, G, W, C, O/B

Can work without C using battery backup, but battery drain may cause issues.

Ecobee

C Wire Required? PEK Included

Heat Pump Support Yes

2-Stage Support Yes

Wire Compatibility All terminals

Power Extender Kit (PEK) allows installation without C wire by using existing wires.

Honeywell Home

C Wire Required? Required

Heat Pump Support Yes

2-Stage Support Yes

Wire Compatibility All terminals

C-Wire Adapter available separately for systems without common wire.

Safety Precautions

Always turn off power at breaker before working on wiring
Take photos of existing wiring before disconnecting
Label wires when disconnecting for reinstallation

Never short R to C directly - this blows the 3A fuse
Verify voltage with multimeter before touching wires
If unsure, consult a licensed HVAC technician