Each thermostat terminal controls one function: R supplies 24-volt power, C completes the circuit, W calls for heat, Y calls for cooling, G runs the fan, and O/B controls a heat pump’s reversing valve. Wire colors usually match the terminal letter (red for R, white for W, yellow for Y, green for G), but installers don’t always follow the convention. Always go by which terminal the wire is connected to at the furnace or air handler, not just by color.
Quick answers
- Turn off power at the breaker before touching thermostat wires.
- Photograph the existing wiring before disconnecting anything.
- Thermostat wires are low voltage (24V), but thick wires at 120V or 240V mean a line-voltage thermostat: don’t use a standard thermostat with those.
- Heat pumps use O or B for the reversing valve; set this correctly in the thermostat menu.
Terminal guide
| Terminal | Typical wire color | Function |
|---|---|---|
| R / Rh / Rc | Red | 24V power. Rh for heating, Rc for cooling; often joined with a jumper on single-transformer systems |
| C | Blue or black (varies) | Common wire; completes the 24V circuit to continuously power smart thermostats |
| W / W1 | White | Heat call (furnace or first stage of heat) |
| W2 / AUX | Brown or other | Second stage heat or auxiliary heat |
| E | Varies | Emergency heat (some heat pump systems) |
| Y / Y1 | Yellow | Cooling call (compressor), and heating too on heat pumps |
| Y2 | Light blue or other | Second stage compressor |
| G | Green | Fan |
| O | Orange | Reversing valve energized in cooling (most brands) |
| B | Dark blue or brown | Reversing valve energized in heating (some brands) |
Common wiring setups
Gas furnace with central AC (conventional)
- R (red), W (white), Y (yellow), G (green), and ideally C.
- Thermostat set to “conventional” or “furnace” system type.
Heat pump with electric backup
- R, Y (compressor), G (fan), O or B (reversing valve), AUX/W2 (backup heat), C.
- Thermostat set to “heat pump” system type, with the O/B setting matching your equipment.
Heat only (furnace, no AC)
- R and W, often with G and C.
Rh and Rc explained
Some systems have separate transformers for heating and cooling (common with a boiler plus a separate AC). In that case, the thermostat keeps Rh and Rc separate. Most forced-air systems have one transformer, so Rh and Rc are connected with a jumper.
O vs B: getting it right
If a heat pump blows cold air in heat mode after installing a new thermostat, the O/B setting is probably reversed. Most brands energize the reversing valve in cooling (O). A few brands use B (energized in heating). Check your equipment’s manual or the existing thermostat’s settings.
How to wire a thermostat safely
- Turn off the breaker for the furnace or air handler.
- Remove the old thermostat cover and take a clear photo of the terminals.
- Label each wire with its terminal letter (most new thermostats include stickers).
- Disconnect wires and wrap the bundle around a pencil so it doesn’t fall into the wall.
- Mount the new base, connect wires to matching terminals.
- Attach the thermostat, restore power and configure the system type.
Full walkthrough: How to Replace a Thermostat Yourself.
What if the colors don’t match?
Open the furnace or air handler (power off) and look at the control board. The wire color connected to the board’s “W” terminal is your heat wire, and so on. That’s the reliable way to identify wires.
Frequently asked questions
What if I don’t have a C-wire?
Many smart thermostats need one. Options include an unused wire in the bundle, an adapter, or running a new cable. See Do I Need a C-Wire for a Smart Thermostat?
Can I damage my system with wrong wiring?
Yes. Shorting R to C can blow the control board fuse or damage the transformer. Keep power off while wiring.
Can I use a regular thermostat with baseboard heaters?
No. Electric baseboards usually use line-voltage thermostats rated for 120V or 240V.