Thermostat wire colors explained
Updated
Wire colors at the wall plate are a habit, not a rule. Learn what R, Rc, Rh, C, W, Y, G and O/B do, how to read your own wiring safely and when to stop and call a pro.
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Colors are a convention, not a standard
Honeywell’s wiring guide says it plainly: there is no standard for which color wire controls which function, so each wire should be identified by the terminal it connects to, never by its color. Google’s Nest help page agrees. It notes that orange often means O/B and blue often means common, but that you should not rely on color alone.
The reason is simple. Thermostat cable usually holds four to eight thin conductors, and whoever installed your system used whichever color was next in the bundle. A previous owner may have changed a connection and reused a wire of a different color. Treat a color as a hint and the letter as the fact.
What each terminal letter does
| Terminal | What it does | Common color (not reliable) |
|---|---|---|
| R | 24 VAC power from the system’s transformer. Many homes have one R wire. | Red |
| Rc and Rh | Rc is 24 VAC power from the cooling transformer, Rh from the heating transformer. They matter only on systems with two transformers; otherwise one R wire serves both. | Red |
| C | Common: the return path that lets a smart thermostat draw steady power. | Blue or black |
| W or W1 | Calls for first-stage heat (furnace, boiler or electric heat). | White |
| W2 or AUX | Second-stage or auxiliary heat. | Brown, black or another color |
| Y or Y1 | Calls for cooling. On a heat pump it runs the compressor for both heating and cooling. | Yellow |
| Y2 | Second-stage compressor. | Varies |
| G | Runs the indoor fan (blower). | Green |
| O/B | Reverses a heat pump between heating and cooling. Whether the system wants O or B depends on the equipment. | Orange (O), blue or dark (B) |
| E | Emergency heat on a heat pump. | Varies |
| S | Connection for a wired indoor or outdoor sensor. | Varies |
Not every thermostat prints every letter. A few brands use their own labels for humidifier, ventilator or heat pump fault inputs. The thermostat’s manual and the maker’s compatibility checker tell you what to do with unusual ones.
Read your wiring safely in five steps
- Turn off power to the heating and cooling system at the breaker or at the furnace service switch. Then change the thermostat setting and confirm nothing starts.
- Pull off the faceplate and take a sharp photo of the terminals and the wires attached to them. Take a second photo with the faceplate in the frame so you remember which side was up.
- Label every wire with masking tape and the terminal letter it was connected to before you disconnect it. Write the letter, not the color.
- Look for extras: unused wires folded back into the wall, and any short jumper wire between Rc and Rh.
- Check the cable. You should see thin, flexible wires. If you find thick wires joined with wire nuts or a label reading 120 V or 240 V, stop: see the section on line voltage below.
Common wire sets and what they usually mean
| Terminals with a wire | Usual system | What to check |
|---|---|---|
| R, W | Heat only: a furnace, boiler or heater with no AC | Often no C-wire; see our guide to smart thermostats without a C-wire |
| R, W, Y, G | Furnace or air handler with central AC | Look for an unused wire that could become C |
| R, W, Y, G, C | Same system with a common wire | Ready for most smart thermostats |
| R, Y, O/B, G, usually C | Heat pump | Choose a model that supports heat pumps, and set O or B correctly; see heat pump thermostats |
| R, Y, O/B, G, W2/AUX, E | Heat pump with backup heat | Keep the auxiliary and emergency wires; the thermostat must support them |
| Two thick wires, 120/240 V | Line-voltage electric heat | A different type of thermostat; see smart thermostats for baseboard heat |
The Rc and Rh jumper
If you have a single R wire, it normally goes to the Rc terminal (or to R on thermostats that have only one) and a small jumper connects Rc and Rh inside the thermostat. If you have two power wires, one on Rc and one on Rh, the system probably has two transformers. Do not connect those two together; follow the manual of your new thermostat for a two-transformer system. Photographing the old thermostat before you remove it shows how the jumper was set.
Mistakes that cause trouble
- Matching by color. It is the most common error and it can leave the fan running with the heat off or the air conditioner stuck on.
- Working with power on. Touching two bare conductors together can blow a small fuse on the furnace control board, which then needs a repair visit.
- Bent or loose ends. Use about 1/4 inch of bare copper. If the tip is corroded or kinked, trim it and strip a fresh end. Push each wire all the way in so no copper shows.
- Mixing up O and B. The wrong setting makes a heat pump cool when you ask for heat. Check the equipment manual or ask the installer.
- Moving wires to fake a C-wire without checking the furnace. That is covered in the C-wire guide, and it can disable fan control.
When to stop and call a professional
Low-voltage wiring is a common DIY job once power is off. Call an HVAC technician if the labels in the wall do not match anything in the manual, if a wire is hot or sparking, if you smell burning, if the system has a digital communicating bus rather than separate wires, or if you cannot get your equipment listed in the maker’s compatibility checker. If the thermostat is line voltage, turn off the breaker, and hire a licensed electrician if you are unsure.
Shop by need
You want to label wires before disconnecting
What to look for: pre-printed thermostat wire labels or numbered cable markers, adhesive that survives a few weeks in a wall box
Typical price: $5–$12
See options on AmazonYou want to confirm 24 V is present at the wall
What to look for: digital multimeter with AC voltage range, test probes with fine tips, auto-ranging option
Typical price: $15–$40
See options on AmazonA damaged or short cable needs replacing
What to look for: 18-gauge solid copper thermostat cable with 5 or 8 conductors, CL2 or CL3 rating, 25 to 50 ft length
Typical price: $15–$40
See options on AmazonFrequently asked questions
Can I trust wire colors when I install a thermostat?
No. Colors are common habits, not a standard. Identify each wire by the terminal letter it is connected to, and photograph and label the wires before you disconnect anything.
What is the difference between R, Rc and Rh?
All three carry 24 VAC power. Many systems have a single R wire. Rc and Rh are separate power terminals for cooling and heating on systems that have two transformers.
What does the C wire do and which color is it?
C is the common wire that completes the 24 V circuit so a smart thermostat gets steady power. It is often blue or black, but not always, and it is sometimes missing from the wall.
Why is my thermostat wire connected to B instead of O?
On a heat pump, O/B controls the reversing valve. Some equipment energizes it for cooling (O) and some for heating (B). Use the equipment manual or ask the installer, and set the thermostat to match.