Wiring Reference Guide

Thermostat Wire Color Code Guide
What Each Wire Color Means

Understand every thermostat wire color, from red (R) power to blue (C) common. This complete reference explains what each wire does, which terminal it connects to, and how to troubleshoot when something goes wrong.

Last updated: February 2026

Safety Warning

Turn off power at the circuit breaker before inspecting or working with thermostat wires. Never assume a wire is safe to touch based on its color — always verify with a voltage tester. Thermostat wires carry 24V AC, but furnace and air handler connections carry 120V/240V which can cause serious injury.

Complete Thermostat Wire Color Code Chart

The chart below shows the standard thermostat wire color conventions used by most HVAC installers in the United States. These colors are not legally mandated — they are industry-standard practices that make installation and troubleshooting easier. Most residential thermostat cable (18/5, 18/8, etc.) uses these color-coded conductors inside a single jacket.

Standard Thermostat Wire Color Codes

Red (R/Rh/Rc)R

24V power — connects to transformer hot lead

White (W/W1)W

Heating — signals furnace/boiler to start

Yellow (Y/Y1)Y

Cooling — signals AC compressor to start

Green (G)G

Fan — controls indoor blower fan independently

Blue (C)C

Common — 24V return path for continuous power

Orange (O/B)O/B

Reversing valve — switches heat pump between heat and cool

Brown (W2/AUX)W2

Auxiliary heat — activates backup heat strips

Black (varies)varies

May be C wire, spare, or brand-specific connection

Note: Older homes and non-standard installations may use different colors. Always verify by terminal letter, not color alone.

How to Identify Your Thermostat Wires

Before working on your thermostat or upgrading to a new model, you need to correctly identify every wire. Follow these six steps to document your wiring safely and accurately.

1

Turn off power at the circuit breaker

Before touching any thermostat wiring, go to your electrical panel and flip the breaker labeled "HVAC," "furnace," or "air handler" to the OFF position. Even though thermostat wires carry low voltage (24V AC), the furnace itself operates on 120V/240V. Cutting power at the breaker protects you and prevents short circuits while you work.

2

Remove the thermostat faceplate

Most thermostats have a removable faceplate that pulls straight off the wall plate, or is held by a screw or tab at the bottom. Gently pull the faceplate forward — you should see the wiring terminals on the wall plate or base behind it. On Nest thermostats, twist the display counterclockwise to remove it. On Honeywell models, look for a release tab at the bottom edge.

3

Note which color wire connects to which terminal letter

Look at each wire and write down which terminal it is connected to. For example: "Red wire on R," "White wire on W," "Yellow wire on Y," etc. Pay special attention to wires that don't match standard colors — an installer may have used a black wire for the C terminal or a brown wire for W2. The terminal letter is what matters, not the wire color.

4

Take a close-up photo for reference

Use your phone to take a clear, well-lit photo of the wiring before disconnecting anything. Get close enough so that both the wire colors and the terminal letters are clearly visible. This photo will be your safety net — if anything goes wrong during reinstallation, you can always refer back to it. Take photos from multiple angles if wires overlap.

5

Check the furnace or air handler for matching connections

Go to your furnace or air handler and locate the thermostat wire terminal strip on the control board. You should see the same wire colors connected to matching terminal letters (R, W, Y, G, C, etc.). Verify that the connections match what you documented at the thermostat end. If they don't match, the wires may have been spliced or extended with different colors somewhere in the wall.

6

Label any non-standard colors with tape

If any of your wires use non-standard colors, wrap a small piece of masking tape or electrical tape around each end and write the terminal letter on it with a marker. For example, if a black wire is connected to the C terminal, label it "C" at both the thermostat and furnace ends. This simple step prevents confusion during future thermostat upgrades or troubleshooting.

If these steps resolved your issue, your thermostat should now be working correctly.

Red Wire (R) — 24V Power

The red wire is the most important wire in your thermostat system. It carries 24V AC power from the transformer (usually located inside your furnace or air handler) to the thermostat. Without a functioning red wire, your thermostat has no power and nothing works.

Rh vs Rc — What's the Difference?

Some thermostats have two R terminals: Rh (heating power) and Rc (cooling power). This split design exists for homes with separate transformers — one for the furnace and one for the AC unit. In most residential systems, there is a single 24V transformer that powers everything. If you have only one red wire, connect it to the R or Rc terminal and install a jumper wire between Rh and Rc. Many thermostats include this jumper pre-installed.

If the Red Wire Has No Power

  • Check the 3A or 5A glass fuse on the furnace control board — this is the #1 cause of power loss
  • Verify the circuit breaker for the HVAC system is ON
  • Test the transformer output with a multimeter — you should read 24-28V AC
  • Check for a short circuit caused by bare wires touching each other or the metal junction box
  • Ensure the furnace door safety switch is fully engaged (door closed firmly)

The red wire is found on every thermostat system — from basic 2-wire heat-only setups to complex multi-zone heat pump systems. It is always the first wire to check when your thermostat screen is blank.

White Wire (W) — Heating

The white wire signals your heating equipment to turn on. When the thermostat calls for heat, it sends 24V through the white wire to the furnace, boiler, or electric heat strip relay, telling it to fire up and start heating your home.

W1 vs W2 — Staging

W1 controls your primary heat source (stage 1) — this is the main furnace or heat pump. W2 (sometimes labeled AUX or E) controls auxiliary or emergency heat, which is typically electric heat strips that activate when the primary heat source cannot keep up. In a standard single-stage furnace, only W1 is used. Heat pump systems with backup electric heat use both W1 and W2.

Wire Bypass Test for Heating

If your heat is not working and you suspect the thermostat, you can perform a bypass test: turn off power at the breaker, remove the thermostat, and twist the R (red) and W (white) wires together. Restore power briefly. If the furnace starts, your thermostat is faulty and needs replacement. If the furnace does not start, the problem is in the furnace itself — check the limit switch, flame sensor, gas valve, or ignitor.

Common Causes When Heat Doesn't Work

  • Dirty flame sensor — clean with fine sandpaper or steel wool (most common furnace failure)
  • Tripped high-limit switch — caused by restricted airflow from a clogged filter
  • Failed hot surface ignitor — glows but cracks over time, needs replacement (~$15 part)
  • Gas valve not opening — could be a faulty gas valve solenoid or gas supply issue
  • Loose wire connection at the furnace control board W terminal

Yellow Wire (Y) — Cooling

The yellow wire controls your air conditioning system. When the thermostat calls for cooling, it sends 24V through the yellow wire to energize the contactor in your outdoor condensing unit, which starts the compressor and outdoor fan.

Y1 vs Y2 — Two-Stage Cooling

Y1 activates first-stage (standard) cooling. Y2 activates second-stage cooling on two-stage air conditioning systems, which runs the compressor at a higher capacity for faster cooling. Most residential systems are single-stage and only use Y1. Two-stage systems are more common in homes over 3,000 square feet or in extreme climates.

The 5-Minute Compressor Delay

Most HVAC systems have a built-in 5-minute compressor delay to protect against short cycling. If you turn the AC off and back on quickly, the system waits 5 minutes before the compressor restarts. This is normal and prevents damage to the compressor from rapid pressure equalization. Some thermostats display "Wait" or "Delay" during this period.

If the AC Doesn't Start

  • Check the outdoor unit disconnect switch — it may have been turned off
  • Inspect the contactor for pitting or burned contacts (a buzzing sound means the contactor coil works but contacts are bad)
  • Test the run capacitor — a failed capacitor is the #1 cause of AC not starting in summer
  • Verify refrigerant levels — low refrigerant causes the low-pressure safety switch to trip
  • Check for a frozen evaporator coil — caused by dirty filter or low refrigerant

Green Wire (G) — Fan

The green wire controls your indoor air handler blower fan independently of heating and cooling. This wire allows the thermostat to run the fan by itself for air circulation, or to control when the fan runs during heating and cooling cycles.

"Fan ON" vs "Fan AUTO"

When your thermostat fan setting is set to ON, the green wire is energized continuously, and the blower fan runs 24/7 regardless of whether the system is heating or cooling. When set to AUTO, the fan only runs when the heating or cooling system is actively operating. Most HVAC professionals recommend the AUTO setting to reduce energy usage and prevent humidity problems in cooling season (a continuously running fan can re-evaporate moisture from the evaporator coil).

Heat Pump Fan Control Note

In many heat pump systems, the air handler controls the fan directly — the G wire may not function the same way as in a conventional furnace setup. Some heat pump air handlers always run the fan during heating and cooling regardless of the G wire state. If you have a heat pump and the fan behaves unexpectedly after a thermostat change, check the air handler's dip switch or jumper settings.

If the Fan Doesn't Run

  • Check the blower motor itself — it may have a failed capacitor or burned-out motor
  • Inspect the fan relay on the furnace control board
  • Verify the green wire is securely connected at both the thermostat and furnace terminals
  • Look for a separate fan speed switch or dip switches on the air handler board

Blue Wire (C) — Common

The blue wire serves as the common (return) side of the 24V AC circuit. It completes the electrical loop back to the transformer, providing a continuous path for current to flow. While older thermostats could operate by "stealing" power through the R wire, modern smart thermostats require a dedicated C wire for reliable power.

Why Smart Thermostats Need a C Wire

Smart thermostats like the Nest, Ecobee, and Honeywell Home need constant power to run their Wi-Fi radios, touchscreen displays, processors, and sensors. Without a C wire, these devices attempt to trickle-charge through the HVAC circuit, which causes problems — delayed system starts, thermostat reboots, Wi-Fi disconnections, and the dreaded "low battery" warnings. A proper C wire connection provides clean, continuous 24V power and eliminates all of these issues.

What If You Don't Have a C Wire?

If your thermostat cable doesn't include a blue C wire, you have several options:

  • Use a spare unused wire in the cable — repurpose it as the C wire by connecting it to the C terminal on both ends
  • Install an add-a-wire adapter (like the Venstar ACC0410) — it uses existing wires to create a virtual C wire circuit
  • Install a Nest Power Connector (for Nest thermostats) — it installs at the furnace and powers the Nest through existing wires
  • Run a new 5-conductor (18/5) thermostat cable — the most reliable solution but requires fishing wire through walls

For a comprehensive guide on adding a C wire to your system, see our complete C Wire Guide.

C Wire Label at the Furnace

At the furnace control board, the C terminal may be labeled "C," "COM," "COMMON," or sometimes "X" or "B" on older boards. Refer to your furnace's wiring diagram (usually printed on the inside of the access panel) to confirm the correct terminal.

Orange Wire (O/B) — Reversing Valve

The orange wire is only used in heat pump systems. It controls the reversing valve, which is the component that allows a heat pump to switch between heating and cooling modes. If you don't have a heat pump, you won't have (or need) this wire.

O Terminal vs B Terminal

This is one of the most confusing aspects of heat pump wiring because different brands handle the reversing valve differently:

O Terminal (Most Brands)

Reversing valve energized in COOLING mode.

Used by: Carrier, Trane, Lennox, Goodman, York, Amana, Bryant, American Standard

B Terminal (Rheem/Ruud)

Reversing valve energized in HEATING mode.

Used by: Rheem, Ruud (and some older Nordyne/Frigidaire systems)

What Happens If O/B Is Wrong

Getting the O/B setting wrong is a common mistake when installing a new thermostat on a heat pump. The symptom is unmistakable: your heat pump blows cold air when set to heat and blows warm air when set to cool. The system runs fine mechanically — it is just operating in the opposite mode. The fix is simple: swap the O/B setting in your thermostat configuration (most smart thermostats have a software toggle for O vs B).

Other Wire Colors

Beyond the six primary thermostat wire colors, you may encounter additional colors in your cable bundle. Here is what each one is typically used for:

Brown

W2 / AUX / E

Auxiliary or emergency heat. Activates backup electric heat strips in heat pump systems. Sometimes also used for humidifier control.

Black

C / varies

Often used as the C (common) wire, especially in Honeywell systems. May also be a spare wire or brand-specific connection. Always trace to verify.

Pink / Purple

Spare / HUM / ACC

Typically spare wires. Sometimes used for humidifier, dehumidifier, or whole-house ventilation control. Can be repurposed as a C wire if unused.

Extra / Unused Wires

None (yet)

Any unused wire in your cable is valuable. It can be repurposed as a C wire when upgrading to a smart thermostat. Never cut spare wires short.

Pro Tip

If you have an unused wire in your thermostat cable, don't cut it short or discard it. Wrap it around the cable bundle and tuck it into the wall. You may need it later when upgrading to a smart thermostat that requires a C wire.

When Wire Colors Don't Match Standard

One of the most important things to understand about thermostat wiring is this: wire colors are NOT standardized by any electrical code. The colors we have discussed — red for power, white for heat, yellow for cooling — are industry conventions that most professional installers follow. But they are not legally required.

Situations Where Colors May Not Match

  • Older homes (pre-1990) where the original installer used non-standard cable
  • DIY installations where the homeowner used whatever wire was on hand
  • Spliced or extended runs where different cable was joined mid-way through the wall
  • Multi-zone systems where the installer ran out of one cable type and switched to another
  • International installations — wire color conventions differ outside North America

The Golden Rule

Always trace wires to terminals, not by color. The terminal letter on the furnace control board and the terminal letter on the thermostat base are what define a wire's function — not the color of its insulation. A black wire connected to the W terminal is a heating wire, period, regardless of what color it "should" be. When in doubt, disconnect both ends, use a multimeter set to continuity mode, and trace each wire individually from thermostat to furnace.

Label wires by terminal letter, not by color. Use masking tape and a marker to label each wire at both ends (thermostat and furnace). Write the terminal letter — R, W, Y, G, C, O — directly on the tape. This eliminates all confusion and makes future work straightforward for you or any technician.

Troubleshooting by Wire Color

If a specific part of your HVAC system is not working, the wire associated with that function is the first place to look. Here is a quick diagnostic reference organized by wire:

Red wire — No power at all

Thermostat screen is blank, nothing works

Check transformer, 3A fuse on furnace board, circuit breaker, furnace door switch

White wire — No heat

Thermostat calls for heat but furnace won't fire

Check flame sensor, limit switch, ignitor, gas valve. Bypass test: twist R + W wires

Yellow wire — No cooling

Thermostat calls for AC but compressor won't start

Check run capacitor, contactor, outdoor disconnect, refrigerant. Wait 5 min after restart

Green wire — No fan

Blower fan won't turn on when set to ON or during a cycle

Check blower motor, fan capacitor, fan relay on control board, G wire connections

No C wire — Smart thermostat issues

Wi-Fi drops, delayed starts, battery warnings, thermostat reboots

Add C wire: use spare wire, add-a-wire adapter, Nest Power Connector, or run new cable

Orange wire — Wrong mode (heat pump)

Heat pump blows cold in heat mode or warm in cool mode

O/B setting is reversed in thermostat config. Swap O to B or B to O in settings

For a complete step-by-step troubleshooting process, see our main Thermostat Not Working guide.

Related Wiring Guides

Dive deeper into thermostat wiring with our detailed guides:

Thermostat Wiring Guide

Complete overview of thermostat wiring for all system types

Thermostat Wiring Diagram

Visual wiring diagrams for 2-wire through 8-wire setups

C Wire Guide

Everything about the common wire and how to add one

Thermostat Not Working

General troubleshooting for all thermostat problems

Frequently Asked Questions

Written by

ThermostatFixer Editorial Team

Our team of HVAC enthusiasts and DIY experts creates detailed thermostat troubleshooting guides, wiring diagrams, and repair tips to help homeowners fix common thermostat issues without calling a technician.