Swapping a thermostat is one of the most common DIY HVAC jobs, and it's usually simple. The trouble comes from assuming wire colors mean something fixed. They don't. The installer can use any color on any terminal. What matters is the letter the wire is connected to.
Kill power at the equipment. Turn off the furnace or air handler switch, which usually looks like a light switch on or near the unit, or turn off its breaker. Thermostat wiring is low voltage, but shorting it can blow a fuse or damage the control board.
Before you touch a wire
- Take the old thermostat off its base and photograph the wiring clearly, with every terminal letter visible.
- Wrap a piece of tape around each wire and write the terminal letter on it, not the color.
- Check for a jumper between R terminals, and note it in your photo.
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, often jumpered together. |
| C | Blue or black | Common. The return side of the 24-volt circuit. Smart thermostats use 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. It 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
Most Wi-Fi thermostats want a C wire. If there's an unused wire tucked back into the wall bundle, check whether it's connected to C at the furnace control board. If it is, you're set. If not, you can connect it there, or use the power adapter kit that many thermostat makers include.
Heat pump gotchas
- Set the thermostat's O/B setting to match your equipment. If heat and cool are backwards after the swap, this is almost always why.
- A heat pump thermostat has to be configured as a heat pump. Otherwise it may treat W as the main heat and run the strips all winter.
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.