Thermostat Wiring Guide
Color Codes, Diagrams & Instructions
Everything you need to wire a thermostat yourself — from basic 2-wire heating systems to advanced heat pump setups. Understand every wire color, terminal, and connection.
Last updated: February 2026
Safety Warning
Always turn off power at the circuit breaker before working with thermostat wiring. While thermostat wires carry low-voltage 24V AC, the furnace and air handler connections involve 120V/240V which can cause serious injury or death.
Thermostat wiring connects your thermostat to your HVAC system using color-coded low-voltage wires. Each wire serves a specific purpose — controlling the heat, air conditioning, fan, and power supply. Understanding these connections is essential whether you are installing a new thermostat, upgrading to a smart thermostat, or troubleshooting a system that has stopped working. This guide covers every wire color, every system type, and walks you through the complete wiring process step by step.
Thermostat Wire Color Quick Reference
Thermostat wires follow an industry-standard color coding system. Each color connects to a specific terminal on both the thermostat and the furnace or air handler control board. Here is the complete reference for all thermostat wire colors and their functions:
Standard Thermostat Wire Colors & Terminals
Carries 24V AC power from the transformer to the thermostat. Rh powers heating; Rc powers cooling. Often jumpered together.
Signals the furnace, boiler, or heating system to turn on. Present in every system that has heating capability.
Signals the air conditioning compressor or heat pump to turn on in cooling mode. Not present in heating-only systems.
Controls the indoor blower fan independently of heating or cooling. Allows the fan to circulate air without activating the HVAC system.
Provides the 24V return path to complete the circuit. Required by most smart thermostats for continuous power.
Controls the heat pump reversing valve to switch between heating and cooling modes. O energizes in cooling; B energizes in heating.
Activates auxiliary or emergency heat strips in heat pump systems when the heat pump alone cannot meet demand.
Activates the second stage of cooling in two-stage AC systems for additional cooling capacity on extremely hot days.
Important Note About Wire Colors
While the colors above are the industry standard, there is no universal law requiring installers to follow them. In some homes — especially older ones or those wired by non-professionals — you may find non-standard wire colors. A brown wire connected to the Y terminal is still a cooling wire, regardless of its color. This is why labeling wires by terminal letter (not color) is critical when replacing a thermostat. Always trust the terminal label on the old thermostat over the wire color. For a deeper dive into every color variation, see our thermostat wire color code guide.
Understanding Your HVAC System Type
The number of wires at your thermostat tells you what type of HVAC system you have. Each system type requires a different wiring configuration. Identify your system below to understand which wires you need and how they connect.
Heating Only (2-Wire System)
Wires: R + W
The simplest thermostat wiring configuration. Found in homes with only a furnace or boiler and no air conditioning. The red wire (R) supplies 24V power, and the white wire (W) signals the heating system to turn on. When the thermostat calls for heat, it completes the circuit between R and W. Many older homes with baseboard heaters or wall furnaces use this basic 2-wire setup. If you want to upgrade to a smart thermostat, you will likely need to add a C wire or use a thermostat with power-stealing capability.
Cooling Only (3-Wire System)
Wires: R + Y + G
Found in warm climates where homes have central air conditioning but no furnace. The red wire (R) carries power, the yellow wire (Y) activates the AC compressor, and the green wire (G) controls the indoor fan. This is less common than combined heating and cooling systems but appears in condos, apartments, and homes in southern states where heating is provided by electric baseboard units on a separate circuit.
Heating + Cooling (4-Wire System)
Wires: R + W + Y + G
The most common residential HVAC wiring configuration in the United States. Includes power (R), heating (W), cooling (Y), and fan (G). This setup supports a gas or electric furnace paired with a central air conditioning system. The thermostat independently controls heating, cooling, and fan circulation. This 4-wire system works with most basic programmable thermostats. For smart thermostats, you will want to check if there is an unused 5th wire in the cable bundle that can serve as the C wire.
Heating + Cooling + C Wire (5-Wire System)
Wires: R + W + Y + G + C
The ideal wiring configuration for modern smart thermostats. Includes all four standard wires plus the common wire (C) which provides a continuous 24V power return path. The C wire allows Wi-Fi thermostats to power their displays, processors, and radios without draining the system. If your home was built or rewired after 2010, you likely have a 5-wire cable. This setup is fully compatible with every thermostat on the market including Nest, Ecobee, Honeywell, and all programmable models. Learn more in our C wire guide.
Heat Pump System (5 to 8 Wires)
Wires: R + Y + G + O/B + C, optional W2 and Y2
Heat pump systems use more wires because the same outdoor unit provides both heating and cooling. The orange wire (O/B) controls the reversing valve that switches the heat pump between modes. The C wire is particularly important for heat pump thermostats. Systems with auxiliary heat strips add a W2 wire for backup heating on cold days. Two-stage heat pumps may also have a Y2 wire for second-stage cooling. Heat pump wiring is the most complex residential thermostat configuration — see the dedicated section below for full details.
Pro Tip
Before disconnecting any wires, take a clear photo of the existing wiring with your phone. Label each wire with masking tape and a marker — this simple step prevents 90% of wiring mistakes.
Step-by-Step: How to Wire a Thermostat
Whether you are replacing an old thermostat or installing a brand-new smart thermostat, the wiring process follows the same fundamental steps. This guide works for Nest, Ecobee, Honeywell, Emerson, and every other residential thermostat brand. The entire process typically takes 30-60 minutes.
Turn off power at the circuit breaker
Locate your electrical panel and turn off the breaker(s) for your HVAC system. There may be separate breakers for the furnace/air handler and the outdoor AC unit — turn off both. Use a non-contact voltage tester to confirm power is off at the thermostat before proceeding. Never skip this step, even though thermostat wires are low voltage, because the furnace control board connects to 120V/240V power.
Remove the old thermostat faceplate
Most thermostats have a faceplate that snaps or pulls off from the wall plate (also called the backplate or mounting plate). Gently pull the faceplate straight out from the wall. Some models have a small release tab at the bottom. If your thermostat is battery-powered, remove the batteries first. Set the faceplate aside — you will not need it again if you are installing a new thermostat.
Label and photograph all wires
This is the most critical step. Before disconnecting a single wire, take a clear close-up photo of the wiring with your phone. Then, using small pieces of masking tape or the wire labels included with your new thermostat, label each wire with the terminal letter it connects to (R, W, Y, G, C, etc.). Write clearly on each label. The terminal letter matters more than the wire color because installers sometimes use non-standard colors.
Remove the old wall plate
Disconnect each wire from the old wall plate terminals by loosening the screws or pressing the release clips. As you disconnect each wire, wrap it around a pencil or tape it to the wall so it does not fall back into the wall cavity. Then remove the mounting screws and take the old wall plate off the wall. If there is a large hole in the wall behind the plate, stuff some non-combustible insulation around the wires to prevent drafts from affecting temperature readings.
Mount the new thermostat wall plate
Thread the labeled wires through the wire opening on the new wall plate. Hold the plate level against the wall (most new thermostats include a built-in bubble level) and mark the screw holes with a pencil. Drill pilot holes if mounting into drywall, and insert the included wall anchors. Secure the wall plate with the provided screws. Make sure the plate is firmly seated and does not wobble.
Connect wires to the correct terminals
Match each labeled wire to the corresponding terminal on the new thermostat wall plate. Insert the stripped wire end (about 1/4 inch of exposed copper) into the terminal connector and tighten the screw or push it into the quick-connect slot until it clicks. Give each wire a gentle tug to confirm it is secure. Connect R to R (or Rh/Rc), W to W, Y to Y, G to G, C to C, and O/B to O/B. If your new thermostat has separate Rh and Rc terminals and you only have one R wire, install the included jumper wire between Rh and Rc.
Attach the thermostat to the wall plate
Carefully push any excess wire back into the wall cavity. Align the thermostat faceplate or display unit with the wall plate and press it into place until you hear or feel it snap securely. For smart thermostats, you may need to rotate or slide the unit to align with mounting pins. Do not force the connection — if it does not seat easily, check that no wires are pinched between the faceplate and wall plate.
Restore power and test the system
Turn the circuit breaker back on. The thermostat should power up within a few seconds. Follow the initial setup prompts to configure your system type, Wi-Fi connection (for smart thermostats), and heating/cooling preferences. Test both heating and cooling modes: set the temperature well above room temp to trigger heat, then well below to trigger cooling. Verify the fan runs when set to "on" mode. Allow 3-5 minutes between mode switches for the compressor protection delay to clear.
If these steps resolved your issue, your thermostat should now be working correctly.
Understanding Rh and Rc Terminals
Some thermostats split the R terminal into two: Rh (power for heating) and Rc (power for cooling). This design supports systems with two separate transformers — one in the furnace and one in the air conditioning unit. In most residential homes, there is only one transformer, and a single R wire powers both. If your new thermostat has Rh and Rc terminals but you only have one red wire, connect it to Rh and install the jumper wire (usually a short metal clip or wire included with the thermostat) between Rh and Rc. This bridges the two terminals so a single power wire feeds both heating and cooling circuits.
If the jumper wire is missing and you only have one R wire, you can use a short piece of thermostat wire to connect Rh to Rc manually. Never leave Rc disconnected if you have air conditioning — the thermostat will not be able to call for cooling without power on the Rc terminal.
Heat Pump Wiring Explained
Heat pump systems are unique because the same outdoor unit can both heat and cool your home by reversing the flow of refrigerant. This requires an additional wire — the O/B wire — to control the reversing valve. Understanding O versus B terminal wiring is critical because connecting to the wrong terminal will cause your heat pump to heat when it should cool, and cool when it should heat.
Heat Pump Wiring Configuration
Power supply from the air handler transformer. Connect to R or Rh terminal.
Activates the heat pump compressor for both heating and cooling operation.
Controls the indoor air handler blower fan for air circulation.
Energizes the reversing valve in COOLING mode. Used by most brands: Carrier, Trane, Lennox, Goodman, York.
Provides continuous power return path. Essential for smart thermostats on heat pump systems.
Activates backup electric heat strips when outdoor temperature drops too low for the heat pump alone.
O Terminal vs. B Terminal: What's the Difference?
The O and B terminals both control the reversing valve, but they work in opposite ways. The difference comes down to when the valve is energized:
O Terminal (Most Common)
Energizes the reversing valve in cooling mode. The valve is de-energized (default position) for heating. Used by the vast majority of heat pump brands including Carrier, Trane, Lennox, Goodman, York, Amana, and Bryant.
B Terminal (Less Common)
Energizes the reversing valve in heating mode. The valve is de-energized (default position) for cooling. Used primarily by Rheem and Ruud heat pumps, and some older American Standard models.
If you are unsure which terminal to use, check the label on your heat pump outdoor unit or the installation manual. Most smart thermostats like Nest and Ecobee ask you during setup whether you have an O or B wire and configure themselves accordingly. If your system heats when it should cool (or vice versa), the O/B setting is likely reversed and can be changed in the thermostat settings without rewiring.
When Does Auxiliary Heat Activate?
The W2/AUX wire connects to backup electric resistance heat strips in your air handler. These activate automatically when the outdoor temperature drops below a certain point (typically 30-35 degrees F) and the heat pump alone cannot maintain the set temperature. Auxiliary heat is much more expensive to operate than the heat pump — roughly 2-3 times the energy cost — so it should only run when necessary. Your thermostat monitors the temperature differential and engages auxiliary heat when the heat pump falls behind by more than 2-3 degrees. If you notice "AUX" or "emergency heat" running frequently on mild days, your heat pump may need servicing. See our troubleshooting guide for diagnosis steps.
6 Common Thermostat Wiring Mistakes
Even experienced DIYers make wiring errors that can prevent the HVAC system from working or, worse, damage the control board. Here are the most frequent mistakes and how to avoid them:
Not labeling wires before disconnecting
The number one mistake. Without labels, you are relying entirely on wire colors which may not follow the standard code. Always label each wire with the terminal letter it was connected to on the old thermostat before removing a single wire. Use masking tape and a permanent marker.
Mixing up R, Rh, and Rc terminals
If your old thermostat had one R terminal but the new one has Rh and Rc, you need to connect the R wire to Rh and install a jumper between Rh and Rc. Forgetting the jumper means the thermostat can power heating but not cooling (or vice versa). The jumper wire is usually included in the thermostat packaging.
Leaving stripped wire exposed outside the terminal
When connecting wires, make sure only the stripped end is inside the terminal connector. If bare copper extends beyond the terminal, it can touch other terminals or wires and cause a short circuit that blows the 3-5 amp fuse on your furnace control board. Strip wires to exactly 1/4 inch for screw terminals or 3/8 inch for push-in connectors.
Wiring the C terminal incorrectly at the furnace
The C wire must connect to the C terminal on both the thermostat and the furnace control board. A common error is connecting the C wire to the thermostat but forgetting to connect the other end to the furnace. If you are adding a C wire to an existing setup, you must make the connection at both ends of the wire run.
Confusing O and B terminals on heat pumps
Connecting an O wire to a B terminal (or setting the thermostat to B when you need O) reverses your heat pump operation — it will heat in cooling mode and cool in heating mode. Always check your heat pump brand: most use O (energize in cooling), while Rheem and Ruud use B (energize in heating).
Not turning off power before wiring
Working on thermostat wires with the power on risks electric shock at the furnace end (120V/240V) and can damage the thermostat or furnace control board if wires touch accidentally. Always turn off the breaker and verify with a voltage tester before starting any wiring work.
Troubleshooting After Wiring
If your thermostat does not power on or your HVAC system does not respond after wiring, work through these checks in order:
- 1Verify the breaker is turned back on. It sounds obvious, but it is the most common oversight.
- 2Check for a blown fuse on the furnace control board. Look for a small 3A or 5A automotive-style fuse near the wire terminals. If it is blown (you can see the broken filament through the glass), a wiring short occurred. Replace the fuse and double-check all connections before restoring power.
- 3Confirm all wires are fully inserted and terminal screws are tight. A loose wire that makes intermittent contact can cause the thermostat to power cycle or the HVAC to behave erratically.
- 4Verify the wire labels match the terminal connections. Compare your wiring photo (from before the swap) to the current connections.
- 5Check that the Rh/Rc jumper is installed if you have a single R wire on a dual-terminal thermostat.
If none of these steps resolve the issue, the problem may be with the furnace control board, transformer, or the thermostat itself. See our complete thermostat troubleshooting guide for additional diagnosis steps, or consider consulting an HVAC technician for complex wiring issues.
Tools You'll Need
Thermostat wiring requires only a few basic tools. Having the right equipment makes the job safer and faster. Here are the essential items we recommend:
Klein Tools NCVT-1 Voltage Tester
$19Non-contact voltage detector that alerts you to live wires without touching them. Essential for confirming the breaker is off before working on any electrical connections.
- Non-contact voltage detection
- Audible and visual alerts
- Auto power-off
- Pocket clip for easy carry
Klein Tools 11061 Wire Stripper
$12Multi-purpose wire stripping and cutting tool for 10-20 AWG wires. Strips thermostat wire cleanly to the exact length needed for secure terminal connections.
- Strips 10-20 AWG wire
- Built-in wire cutter
- Spring-loaded self-opening
- Comfortable grip handles
You will also want a Phillips-head screwdriver (for mounting screws and terminal screws), a small flathead screwdriver (for push-in terminal release slots), a pencil (for marking screw holes), masking tape and a marker (for labeling wires), and a drill with a 3/16" bit (if installing wall anchors). For a comprehensive cost breakdown of thermostat installation, see our thermostat replacement cost guide.
Choosing the Right Thermostat Wire
If you need to run new thermostat wire — whether you are adding a C wire, extending a run, or doing a complete rewire — here is what to look for:
| Specification | Recommendation |
|---|---|
| Wire Gauge | 18 AWG (standard). Use 16 AWG for runs over 100 feet. |
| Number of Conductors | 5-conductor minimum for modern systems. 8-conductor gives future flexibility. |
| Wire Type | Solid copper conductors (not stranded) for screw terminals. |
| Rating | CL2 or CL3 rated for in-wall installation. CL2P for plenum spaces. |
| Jacket | PVC outer jacket rated for dry/damp locations. White is standard. |
| Max Run Length | 18 AWG: up to 100 ft. 16 AWG: up to 200 ft. Beyond 200 ft, consult an electrician. |
We always recommend running more conductors than you currently need. An 8-conductor cable costs only slightly more than a 5-conductor cable but gives you spare wires for future upgrades — like adding a C wire for a smart thermostat or a W2 wire for a heat pump system.
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.