Swapping a thermostat is pretty common DIY work, and most of the time it's straightforward. The headache starts when you think wire colors are some kind of code. They're not. The guy who installed your system could've used any color he had in the truck. What actually matters is the letter on the terminal.
Kill power at the equipment. Flip off the furnace or air handler switch. It looks like a regular light switch on or near the unit, or just hit the breaker. Yeah, thermostat wiring is low voltage, but short it out and you'll pop a fuse or fry the control board.
Before you touch a wire
- Pull the old thermostat off its base and snap a clear photo of the wiring. Every terminal letter needs to be readable.
- Wrap tape around each wire and write the terminal letter on it. Not the color. The letter.
- Look for a jumper between R terminals. Get that in your photo too.
What the letters mean
| Terminal | Usual color | What it does |
|---|---|---|
| R (Rh, Rc) | Red | 24-volt power from the system transformer. Rh and Rc are separate heat and cool power, usually jumpered together. |
| C | Blue or black | Common. The return side of the 24-volt circuit. Smart thermostats need it for steady power. |
| W (W1) | White | Calls for heat. |
| W2 / AUX / E | Brown or other | Second-stage, auxiliary, or emergency heat. On heat pumps this is usually the backup heat strips. |
| Y (Y1) | Yellow | Calls for cooling. Starts the compressor. On a heat pump it runs the compressor in both modes. |
| Y2 | Light blue or other | Second-stage compressor on two-stage systems. |
| G | Green | Turns on the indoor blower fan. |
| O/B | Orange or dark blue | Heat pumps only. Switches the reversing valve. Most brands energize O in cooling. A few energize B in heating. |
The C-wire question
Pretty much every Wi-Fi thermostat wants a C wire. If there's an extra wire shoved back in the wall bundle, check if it's hooked to C at the furnace control board. If it is, you're golden. If not, you can connect it there yourself, or use the power adapter kit that comes with a lot of thermostats.
Heat pump gotchas
- Set the thermostat's O/B setting to match your equipment. If heat and cool are backwards after you finish, this is almost always the culprit.
- A heat pump thermostat has to be set up as a heat pump. Otherwise it might treat W as your main heat and run the strips all winter long.
Mini splits are different. Most ductless systems don't use a standard 24-volt thermostat at all. They talk to their own remote or wall controller. Check the manufacturer's compatible controller list before you buy anything.