Furnace Fan Wire Colors Guide

The following guide explains common furnace fan wire colors, their functions, and practical tips for troubleshooting and safe testing. This resource helps homeowners and technicians identify wires for thermostats, blower motors, and ECM controls so repairs and upgrades are accurate and efficient.

Wire Color	Common Terminal/Function	Notes
Red (R)	24V Hot	Power from transformer; may be R or Rh/Rc
White (W)	Heat Call	Calls furnace burner; sometimes W1/W2 for stages
Yellow (Y)	Cooling Call	Sends signal to AC compressor contactor
Green (G)	Fan/Blower On	Controls fan relay or variable-speed fan
Blue/Blue-White (C/B)	Common	Completes 24V circuit for thermostats and controls
Orange (O/B)	Heat Pump Reversing Valve	O or B designates energize on heat/cool depending on system

How Furnace Fan Wiring Works

Furnace fan wiring links the thermostat, control board, and blower motor so the HVAC system responds to temperature and mode changes. The thermostat sends low-voltage signals (typically 24V AC) over color-coded wires to energize relays or solid-state controllers that run the fan and other components.

Key concept: The furnace control board acts as the central hub; wires from the thermostat go to labeled terminals (R, C, W, Y, G), and the board controls power to the blower motor and other outputs.

Common Furnace Fan Wire Colors And Their Meanings

Red (R, Rh, Rc)

Red is the 24V hot feed from the transformer. It powers the thermostat and is the source for all control calls. Some systems use Rh and Rc if heating and cooling have separate transformer feeds; jumpers connect them on single-transformer systems.

White (W)

White is the heat call wire. When the thermostat calls for heat, it connects R to W, signaling the furnace to start the heating sequence. Multi-stage furnaces may have W1 and W2 wires for first and second stages.

Green (G)

Green controls the blower or fan relay. When the thermostat requests fan-only or when calling for heat/cool (fan may be auto), the G terminal energizes the fan circuit. On variable-speed furnaces the G wire may signal speed or be used differently with ECM motors.

Yellow (Y)

Yellow is typically the cooling call to the outdoor compressor contactor. While Y does not directly run the furnace fan, the system often runs the indoor blower when Y is active to circulate cooled air.

Blue, Black Or Brown (C – Common)

The common wire provides the return path for the 24V transformer and powers electronic thermostats or smart thermostats. Without the C wire, many smart thermostats require a power extender or alternative wiring to function reliably.

Orange/Blue (O/B)

For heat pumps, O or B wires control the reversing valve to switch between heating and cooling modes. These colors are not used in conventional gas/electric furnaces but are common in combined systems.

Furnace Fan Motor Wiring: PSC vs ECM/EC Motor Differences

Two primary blower motor types affect wiring behavior: PSC (permanent split capacitor) motors and ECM/EC motors (electronically commutated motors). Wiring conventions differ between these technologies and affect fan control.

PSC Motors

PSC motors typically use the G wire to energize a relay that applies line voltage to the motor. Speed is often determined by selecting a specific tap or wire color on the motor harness (e.g., black-high, blue-med, yellow-low), which is set at the furnace or control board.

ECM/EC Motors

ECM motors are variable-speed and use a control signal from the furnace control board or thermostat to set fan speed. Many ECMs accept a simple G signal for on/off and a separate communication or 0-10V signal for speed. Wiring color may not reflect speed taps; refer to motor and control documentation.

How To Identify Furnace Fan Wires Safely

Before testing or touching any wires, turn off power to the furnace at the breaker and the furnace switch. Use a multimeter rated for low-voltage to verify voltages across R and C, and confirm terminal labels on the control board before disconnecting wires.

When labeling or replacing a thermostat, match each thermostat wire to the corresponding terminal at the furnace: R to R, W to W, G to G, Y to Y, and C to C. If wires are unlabeled, use a wire tracer or note wire color and terminal function before removing.

Troubleshooting Common Fan Wiring Problems

No Fan On Thermostat Fan Command

Check the G wire connection at the thermostat and furnace control board. Verify 24V between R and G when the thermostat calls for the fan. If no voltage, inspect the thermostat settings, fan relay, and fuses on the control board.

Fan Runs Constantly

If the fan runs nonstop, the G circuit may be stuck closed, the thermostat may be set to ON instead of AUTO, or a relay on the control board may be welded. Disconnect G at the thermostat and see if the fan stops to isolate the problem.

Thermostat Loses Power Or Goes Dark

A missing or faulty C wire causes loss of power for smart thermostats. Verify the C wire is properly connected to the furnace’s C terminal. If a C wire is unavailable, a common workaround is a power extender kit or using the furnace’s internal 24V adapter where compatible.

Testing And Verifying Wire Functions

With power on and the thermostat set to call for the fan, measure voltages: R to C should read around 24VAC. R to G should read 24VAC when the fan is requested. R to W reads 24VAC during heat call; R to Y reads 24VAC during cooling call.

Use a continuity tester with power off to trace wires and confirm they connect between the thermostat and control board. Label wires with tape for future reference.

Replacing Or Rewiring The Thermostat Fan Wire

When replacing a thermostat, ensure the G wire is connected to the G terminal, and the C wire is on C. If an extra wire is not run, consider pulling a new thermostat cable or using a wireless adapter or power kit to supply continuous power.

For older furnaces without a C terminal, some technicians tap the transformer’s common or install a new transformer with a dedicated C output. Any modification should follow manufacturer instructions and local code.

Color Exceptions And Regional Variations

Wire colors are conventions, not guarantees. Installers or previous owners may reuse colors differently. Always confirm function at the terminals rather than relying solely on color.

Examples of exceptions include blue used as G, black used as C, or red used for more than one transformer feed. Multi-transformer systems can use different colors for separate circuits.

When To Call A Professional

If wiring appears damaged, multiple unfamiliar wires are present, or there is uncertainty about motor types and control boards, a licensed HVAC technician should be called. Professionals can safely test, replace, or reconfigure wiring and ensure compatibility with thermostats and ECM motors.

Practical Tips For Smart Thermostat Installations

• Verify presence of a C wire before purchasing a smart thermostat.
• If no C wire exists, consider a power extender kit supplied by the thermostat maker or install a common wire adapter at the furnace.
• Confirm fan control behavior after installation: fan on AUTO should operate with thermostat-controlled calls, and ON should run the blower continuously.

Frequently Asked Questions About Furnace Fan Wire Colors

What If The G Wire Is Missing?

Without G, many thermostats cannot directly control the fan. Some thermostats can use the heat/cool call to indirectly run the fan, but typically a G wire must be present or a relay/power kit used.

Can Wire Colors Be Changed?

Yes; colors can be changed when rewiring or extending cables, but labels should be updated at both ends. For clarity, follow the R, C, W, Y, G convention when possible.

Is Blue Always The Common Wire?

Blue often serves as the common, but black or brown can be used. Confirm at the control board to determine which conductor is the true C connection.

Reference Table: Quick Wire Function Summary

Terminal	Color Typically Seen	Primary Function
R	Red	24V Hot Supply
C	Blue/Black/Brown	24V Common
W	White	Heat Call
Y	Yellow	Cooling Call
G	Green	Fan/Blower Control

Using the guidance in this furnace fan wire colors guide helps ensure correct identification, safer testing, and smoother thermostat or fan motor upgrades. When in doubt, verify functions at the furnace control board and consult a licensed HVAC technician for complex or high-voltage wiring tasks.