Thermostat Wiring Diagram: A Complete Guide for American Homeowners

A thermostat is the heart of your home’s heating and cooling system. Whether you’re installing a new thermostat, upgrading to a smart device like Nest or Ecobee, or troubleshooting your HVAC system, understanding a thermostat wiring diagram is essential.

For most American homes, thermostat wiring follows a standard wire color code, but variations exist depending on the furnace, AC unit, or heat pump you have. In this detailed guide, we’ll explain thermostat wire colors, terminal letters, C-wire connections, furnace control board terminals, and brand-specific diagrams (Honeywell, Ecobee, Nest, Sensi).

By the end, you’ll be able to:

• Read and understand a thermostat wiring diagram.
• Identify and connect thermostat wires properly.
• Troubleshoot wiring issues like “thermostat not working after installation.”
• Handle special cases such as heat pumps, multi-stage systems, and systems without a C-wire.

What Is a Thermostat Wiring Diagram?

A thermostat wiring diagram is a visual map that shows how thermostat wires connect between your thermostat terminals and your HVAC control board (inside the furnace or air handler).

Why It Matters

• Prevents wiring mistakes.
• Ensures efficient heating/cooling.
• Helps troubleshoot power loss or fan issues.
• Required when upgrading to smart thermostats like Nest, Ecobee, Honeywell Home, or Sensi.

Thermostat Wire Colors Explained

Although thermostat wires are often color-coded, color isn’t always a guarantee of function. Always verify wiring with the terminal letters on the control board.

Tip: Many Americans ask, “Are thermostat wire colors standardized?” The answer: colors are just a guide, but always label wires vs colors when disconnecting your old thermostat.

Thermostat Terminal Letters Explained

Thermostat terminals are usually labeled R, Rc, Rh, W, Y, G, C, O/B, etc.. Here’s what they mean:

• R / Rc / Rh – 24V power (Rc = cooling power, Rh = heating power). Sometimes Rc and Rh are jumpered together.
• W / W1 / W2 / Aux – Heat stages or auxiliary heat (for heat pumps).
• Y / Y1 / Y2 – Cooling compressor stages.
• G – Fan control.
• C (Common Wire) – Provides power for smart thermostats.
• O/B Wire – Reversing valve for heat pumps. O = cool mode, B = heat mode.
• E – Emergency heat.
• L – Service light (rare).
• * / K – Extra functions (Nest uses this).

Types of Thermostat Wiring Diagrams

Different HVAC setups require different thermostat wiring. Let’s break them down:

1. 2-Wire Thermostat Wiring (Basic Heat Only)

• W (Heat) and R (Power).
• Common in older furnaces or boilers.

2. 3-Wire Thermostat Wiring

• Adds a C-wire for power.
• Found in some newer heating systems.

3. 4-Wire Thermostat Wiring

• R, W, Y, G – Supports basic heating and cooling.
• Works in most U.S. homes with furnace + AC.

4. 5-Wire Thermostat Wiring (Most Common in U.S.)

• Adds C-wire to support modern smart thermostats.
• Standard for most Honeywell and Nest thermostat wiring diagrams.

5. 6-Wire Thermostat Wiring Diagram

• Includes additional wires for two-stage heating or cooling.

6. 7-Wire Thermostat Wiring Diagram

• Supports advanced multi-stage HVAC systems with heat pumps.
• Example: 7-wire Honeywell thermostat wiring diagram.

Brand-Specific Thermostat Wiring Diagrams

Honeywell Thermostat Wiring

• Popular in many American homes.
• A 5-wire Honeywell thermostat wiring diagram is the most common.
• Color code Honeywell thermostat wiring usually follows industry standards, but confirm on your furnace control board.

Honeywell Home Thermostat Wiring Diagram (HS/LK – Honeywell Home)

• Official Honeywell Home support recommends labeling by terminal, not just colors.
• Common setup: R, Rc, W, Y, G, C, O/B.

Nest Wiring Diagram (Google Help)

• Nest thermostats use fewer wires due to built-in power management.
• Nest C wire is optional, but recommended.
• Terminals: R, Y, W, G, C, O/B, * / K.

Ecobee Thermostat Wiring (Ecobee Support)

• Requires a C-wire or an Ecobee Power Extender Kit (PEK).
• O/B wire (reversing valve) is critical for heat pumps.
• Heat pump O vs B setting determines heating/cooling mode.

Sensi Thermostat Wiring (Emerson)

• Uses a C-wire adapter if the furnace board doesn’t have a free C terminal.
• Sensi common wire setup is common in U.S. retrofits.

Special Cases in Thermostat Wiring

Furnace Thermostat Wiring Diagram

• Connections: R, W, C, G, Y.
• If you don’t see a C terminal, you may need a C-wire adapter.

Heat Pump Thermostat Wiring Diagram

• Adds O/B wire for reversing valve.
• W1/W2 (Aux Heat) is important for cold U.S. regions.

AC Thermostat Wiring

• Standard R, Y, G, C setup.
• Works with central air conditioning.

Most U.S. homes use 24V thermostat wiring.

• Safe for DIYers.
• Found in furnaces, heat pumps, and air handlers.

High-voltage thermostat wiring (120V/240V) is for baseboard heaters and not covered here.

Control Board & C-Wire (Common Wire)

The C-wire is one of the most searched topics in America when it comes to thermostat wiring.

• Provides constant 24V return power.
• Found on the control board C terminal inside the furnace.
• If your system doesn’t have one, options include:
  • Ecobee Power Extender Kit (PEK).
  • Sensi C-wire adapter.
  • Running a new wire from furnace to thermostat.

To find the C terminal on the furnace board, look for a labeled terminal strip near wires R, W, Y, and G.

Common Thermostat Wiring Issues & Troubleshooting

1. Thermostat not working after installation
   • Check that R wire has 24V power.
   • Verify Rc vs Rh jumper is installed (if needed).
2. Fan won’t turn on
   • Check G wire connection.
3. AC won’t cool
   • Inspect Y wire and outdoor unit connection.
4. Heat pump not switching modes
   • Verify O/B wire setting (O = cooling, B = heating).
5. Smart thermostat won’t power up
   • Missing or disconnected C-wire.
   • Use PEK or C-wire adapter.

Safety Tips for Americans Doing DIY Thermostat Wiring

• Turn off furnace power before wiring.
• Label wires before removing old thermostat.
• Use the thermostat wiring diagram that matches your HVAC system.
• If unsure, hire a licensed HVAC technician.

Thermostat Wiring Diagram For Heat Pump With Aux Heat (HS/LK)

A heat pump with auxiliary heat requires more wires than a standard system. Typical terminals:

• R/Rc – 24V power
• C – Common wire
• Y1 – First-stage cooling/compressor
• O/B – Reversing valve (cooling or heating depending on system type)
• W2/Aux – Auxiliary heat (electric heat strips or gas furnace backup)
• E – Emergency heat (often tied to Aux)
• G – Fan

Tip: Always confirm at the furnace or air handler board since color coding can vary. Many Americans search this because wiring heat pump + Aux heat is the trickiest configuration.

How To Find The C Terminal On A Furnace Control Board (HS/LK)

The C terminal (common) is located on the furnace or air handler control board, usually labeled C, COM, or 24V Common.
Steps:

1. Open the furnace access panel.
2. Look for the terminal strip where thermostat wires connect.
3. Identify the R terminal (power). The C terminal is normally nearby.
4. Trace the wire bundle—often a blue or black wire is attached to C.

If unused, the C terminal may be empty, requiring you to connect a spare wire from the thermostat cable.

Rc Vs Rh Jumper: Do I Need It? (HS/LK)

• Rc = Cooling transformer (for air conditioner).
• Rh = Heating transformer (for furnace).
• Many American homes have a single transformer, so Rc and Rh are internally the same. In that case, a metal jumper (factory-installed) connects them.
• If your system has separate heating and cooling transformers, keep Rc and Rh separate—no jumper.

What Color Is The C Wire On A Thermostat? (HS/LK)

The C wire (common) does not have a universal color, but most often it is blue or black in U.S. homes. Always confirm by tracing back to the furnace control board’s C terminal instead of relying solely on color.

Aire Serv / Meyer Heating & Air

Both are American HVAC service companies that publish guides and FAQs on thermostat wiring and C wire solutions. Referencing Aire Serv and Meyer Heating & Air in your content can give credibility, since they explain color coding, Rc vs Rh, and C wire identification for U.S. systems.

O/B Wire Heat Pump—Use O Or B (HS/LK)

The O/B wire controls the reversing valve in a heat pump:

• O = Energize reversing valve in cooling mode (common in Carrier, Trane, Goodman).
• B = Energize reversing valve in heating mode (less common, Rheem/Ruud).
  Your thermostat setup menu will ask whether the O/B wire should be set to O or B. Always check the equipment manual.

Ecobee Support – No C Wire Thermostat Solutions (Adapter/PEK) (HS/LK)

If you don’t have a C wire, Ecobee provides a Power Extender Kit (PEK) that allows installation without running new wires. Other solutions include:

• Using a C-wire adapter (3rd party or brand-specific).
• Using the G wire as a C wire (not recommended if you want fan control).
• Running new thermostat cable.

Sensi also offers a C-wire adapter for homeowners facing the same problem.

Nest Thermostat Wiring Diagram 5 Wires (HS/LK) – Google Help

For a 5-wire setup, the Nest typically uses:

• R (or Rc) – Power
• C – Common (recommended for stability)
• Y1 – Cooling
• W1 – Heating
• G – Fan

Google Help has brand-specific wiring guides and diagrams that walk users through identifying wires and terminals.

Honeywell Home diagrams typically follow:

• Red (R/Rc) – Power
• White (W/W2) – Heat
• Yellow (Y/Y2) – Cooling
• Green (G) – Fan
• Blue/Black (C) – Common

But again, wire colors aren’t guaranteed standards, so always confirm at the HVAC control board. Honeywell’s website has official diagrams for conventional and heat pump systems.

Ecobee Wiring With C Wire – Ecobee Support

When a dedicated C wire is available, simply connect it to the C terminal on both the furnace board and the Ecobee thermostat. Ecobee strongly recommends a C wire for reliable Wi-Fi and power, even though their PEK adapter can be used as a workaround.

[Brand] Furnace Board C Terminal Location (Goodman/Trane/Carrier) (HS/LK)

• Goodman: Typically on the lower-right terminal strip labeled C.
• Trane: Found on the integrated furnace control (IFC) board, labeled C (Common).
• Carrier: On the control board terminal block, often grouped with R, Y, W, G.

If in doubt, look at the service diagram inside the furnace panel.

Why AC Won’t Turn On After Replacing Thermostat—Wiring

Common causes:

• C wire missing or miswired.
• Rc/Rh jumper not installed where needed.
• Y wire not connected.
• Blown 3- or 5-amp fuse on furnace board due to shorted wires.
• Thermostat not compatible with system type (heat pump vs conventional).

How To Wire 2-Stage Cooling Y1/Y2

• Y1 = First-stage cooling (runs compressor at lower speed).
• Y2 = Second-stage cooling (runs compressor at full speed).
  Your thermostat must support 2-stage cooling and be programmed accordingly. Y2 only energizes during peak demand for efficiency.

Emergency Heat E Vs Aux W2 Wiring

• Aux (W2): Automatically kicks in when heat pump can’t meet demand.
• E (Emergency Heat): Manually selected on the thermostat—runs backup heat only, bypassing the heat pump.
  In many thermostats, E and W2 are tied together, but some systems keep them separate.

Fan G Wire Not Connected—What Happens

• For smart thermostats, lack of a G wire can prevent proper fan control or C-wire adapter use.
• Without the G wire, you cannot manually turn the fan on from the thermostat.
• System fan may still run automatically with heating/cooling cycles, depending on board settings.

FAQs

Do Thermostat Wire Colors Matter Or Do The Letters Matter

The letters on the terminals matter more than the wire colors. Colors are a common guide (e.g., red for power, white for heat, green for fan), but there’s no universal standard. Always match each wire to the correct terminal letter on both the thermostat and the furnace/air handler control board.

Which Wire Is The C Wire And What Color Is It

The C wire (common) provides continuous 24V power for smart thermostats. It’s most often blue or black in U.S. homes, but the color isn’t guaranteed. To be certain, trace it back to the C terminal on the furnace or air handler control board.

How Do I Add A C Wire If I Don’t Have One

Options include:

1. Use a C-wire adapter (offered by Ecobee, Sensi, Honeywell, etc.).
2. Install Ecobee’s PEK (Power Extender Kit).
3. Repurpose the G (fan) wire as C if you don’t need manual fan control.
4. Run new thermostat cable with more conductors.

What Is The O/B Wire On A Heat Pump

The O/B wire controls the reversing valve in a heat pump system:

• O → Energized in cooling mode (most systems, like Trane, Goodman, Carrier).
• B → Energized in heating mode (some Rheem/Ruud systems).
  The correct setting depends on your equipment.

Do I Need An Rc-Rh Jumper, When

• If your system uses a single transformer for heating and cooling, you need the Rc-Rh jumper (often factory-installed).
• If you have separate heating and cooling transformers, do not use the jumper—Rc and Rh will each connect to their respective transformer wires.

How Do I Read A Thermostat Wiring Diagram

1. Identify the terminal letters (R, Rc, Rh, Y, W, G, C, O/B, etc.).
2. Match each thermostat terminal to the corresponding furnace control board terminal.
3. Understand that letters represent functions, not wire colors.
4. Look for optional connections (Aux, E, Y2, W2) based on system type.

Why Did My HVAC Stop After Installing A New Thermostat

Possible reasons include:

• Miswired connections (common with Rc/Rh, O/B, or C).
• Missing C wire for smart thermostats.
• Shorted wires causing a blown low-voltage fuse (3- or 5-amp) on the furnace board.
• Thermostat not compatible with system type (heat pump vs conventional).

Can I Use Wire Color To Identify Function

Not reliably. While many installers follow color conventions (red = power, white = heat, yellow = cooling, green = fan, blue/black = C), these aren’t enforced standards. Always confirm by checking the control board terminals.

Does Nest/Ecobee Require A C Wire In 2025

• Nest: Some models can run without a C wire but work more reliably with one. Google still recommends a C wire.
• Ecobee: Almost always requires a C wire. If unavailable, Ecobee includes a PEK adapter for older systems.
  In 2025, both brands strongly encourage using a C wire for stable Wi-Fi and power.

How Do I Wire W2/Aux And E For Emergency Heat

• W2/Aux = Auxiliary heat (kicks in automatically when heat pump can’t meet demand).
• E = Emergency heat (manually selected on thermostat, bypassing heat pump).
  Some thermostats tie W2 and E together, while others have separate terminals. Follow the manufacturer’s wiring diagram.

Where Is The C Terminal On My Furnace Control Board

The C terminal is located on the furnace or air handler control board alongside other thermostat terminals (R, Y, W, G). It’s usually labeled C or COM. A blue or black wire may be attached, or it may be empty if your system was installed without a C wire.

What Gauge Cable For Thermostat Wiring

Most U.S. thermostat wiring uses 18 AWG copper conductors, usually 18/5 cable (five wires). Markings like 18/5 thermostat wire or 18/8 are common. CL2 or CL3 rated cables are recommended for in-wall use.

Is Thermostat Wiring Low Voltage And Safe To DIY

Yes. Thermostat wiring is low voltage (24V AC), so it is generally safe for DIYers. However:

• Always turn off power at the furnace breaker before handling wires.
• Incorrect wiring can blow a fuse or damage your HVAC control board.
• If unsure, call an HVAC professional.

Summary

• A thermostat wiring diagram is key for safe, correct thermostat installation.
• Thermostat wire colors provide guidance but always match wires to terminal letters.
• American homes commonly use 5-wire thermostat wiring, with C-wire for smart devices.
• Honeywell, Nest, Ecobee, and Sensi thermostats each have brand-specific wiring needs.
• Troubleshooting often involves checking C-wire, Rc vs Rh jumper, O/B wire settings, and control board terminals.

Conclusion

For American homeowners, learning thermostat wiring isn’t just a DIY skill—it’s a way to save money, improve comfort, and ensure your HVAC system runs efficiently. Whether you’re working with a Honeywell thermostat wiring diagram, Nest C-wire connection, or Ecobee Power Extender Kit, the principles remain the same: match wires to terminals, use diagrams, and always prioritize safety.

If your thermostat wiring colors don’t match the diagram, don’t panic—label wires, trace them to the furnace control board, and consult manufacturer resources like Honeywell Home, Ecobee Support, or Google Help.