Thermostat Wire Colors and Terminal Letters, Explained

Before you touch anything

Kill the power at the equipment first. Use the switch on the furnace or air handler, or flip its breaker, because turning the thermostat itself off doesn't count and Google's Nest Help and ecobee's install instructions both put this step ahead of everything else, since a shorted 24-volt wire can blow the fuse on the control board.

Take a photo of the old wiring before anything comes loose, and label each wire with the letter it was on. ecobee puts labels in the box for exactly this reason. Keep the old thermostat. Nest suggests hanging on to it and any jumper wire, along with that photo, in case you ever need to put things back the way they were.

What each terminal does

These descriptions follow Google's Nest Help article on identifying thermostat wires and the terminal table in Resideo's T6 Pro installation guide. The color column is just the usual convention. Google puts it plainly: you shouldn't rely on color alone to determine what a wire does.

What common wiring setups look like

Two wires, R and W, means heat only. That's often a boiler or an older furnace, with no fan or cooling control and usually no C.

Four wires (R, W, Y, G) is a furnace with central air, which is about as common as it gets and also the setup most likely to be missing a C wire. Add a fifth wire for C? Most smart thermostats install directly.

A heat pump usually shows R, C, Y, G and O/B, plus Aux/E. There's no W for the main heat because the compressor heats through Y, and the O/B wire flips the valve.

Rh, Rc and the jumper

Most homes have one transformer, and its single R wire powers heating and cooling both, so thermostats with separate Rh and Rc terminals tie them together with a small jumper wire or a built-in slider. Nest's help page notes that jumpers are fairly common on these two terminals.

• One red wire means a single transformer. On Resideo's T6 Pro, you set the R slider to the one-wire position. Nest Learning Thermostats handle it with an internal jumper, and Google says not to connect the old jumper wire to a Nest.
• Separate wires on Rh and Rc means two transformers. You see this a lot when a boiler and a separate air conditioner share one thermostat. Keep them apart. On the T6 Pro, that's the two-wire slider position.

O or B?

A heat pump's reversing valve has a resting position, and the thermostat energizes it to switch to the other mode. Brands disagree. O energizes the valve in cooling, and B energizes it in heating. Most modern thermostats have one O/B terminal and a setting to pick between them. Resideo's T6 Pro calls it "O/B on Cool" or "O/B on Heat" in installer setup. If your new thermostat heats when you ask for cooling, change this setting before you touch the wiring.

Getting a C wire

Smart thermostats run Wi-Fi radios and screens, so they need steadier power than an old mechanical one ever did, and the C wire is what supplies it. No C wire? Work down this list:

1. Look for a spare conductor. Thermostat cable often has more wires than anyone used. An unused one folded back into the wall can be hooked to C at both the thermostat and the equipment's control board.
2. Use the maker's adapter. ecobee's Power Extender Kit goes at the furnace. Once it's in, the G wire goes to the thermostat's C terminal and the Y wire to its PEK terminal. ecobee says it isn't compatible with dual-transformer, fan coil or communicating systems. Resideo's THP9045A adapter works the same way. Its guide describes combining the Y and G functions onto the K terminal, which frees up the leftover wire to be your common.
3. Run new cable if neither of those works. It's the cleanest fix, but it means fishing a wire through the wall.

After you wire it

Power back on. Now try every mode, meaning heat, cool, fan only and emergency heat if it's a heat pump, using the setup or test menu most thermostats include for exactly this. If something doesn't respond, shut the power off again and check the new terminals against your photo before you change any settings.

Sources

• Google Nest Help: Identify a thermostat wire.
• Google Nest Help: Learn about thermostat jumper wires.
• Resideo T6 Pro Smart Thermostat installation guide: terminal table, slider tabs and installer setup 218.
• ecobee: Installing your Smart Thermostat Essential.

Thermostat wiring FAQ

What do the letters on a thermostat mean?

Each letter is a job, not a color. R is 24-volt power (Rh for the heating transformer, Rc for cooling). W calls for heat, Y calls for cooling and G runs the fan. C is the common wire that completes the 24-volt circuit, and O/B switches a heat pump between heating and cooling. W2 and Y2 are second stages. Aux and E cover a heat pump's backup and emergency heat.

Can I go by wire color?

No. Colors are habit, not a rule, and installers use whatever conductors are free in the cable. Google's Nest Help says you shouldn't rely on color alone to determine what a wire does. Go by the terminal letter each wire sits on at your old thermostat, and label every wire before you disconnect it.

What is a C wire and do I need one?

C, the common wire, gives the thermostat a constant return path so it can pull steady 24-volt power. A lot of smart thermostats need it. If you don't have one, look for an unused conductor in the cable first. Failing that, adapters like ecobee's Power Extender Kit or Resideo's THP9045A work with only four wires.

What is the difference between Rh and Rc?

Rh powers the heating circuit and Rc powers cooling. With one transformer they're the same power, so a jumper or slider ties them together. With two transformers they have to stay separate. If your old thermostat had one red wire, you almost certainly have a single-transformer system.

Should O/B be set to O or B?

Depends on the heat pump. O means the reversing valve is energized in cooling mode, and B means it's energized in heating mode. Most thermostats let you pick. On Resideo's T6 Pro the setting reads 'O/B on Cool' or 'O/B on Heat'. If heating and cooling come out backwards after the install, this setting is usually the reason.