Standard Thermostat Wire Colors
R/Rh/Rc: 24V power. G: Fan. Y: Cooling. W: Heating. C: Common (for power). O/B: Heat pump reversing valve. These are standard but not universal.
Basic Wiring for Heating Only
A simple furnace only needs R (power), W (heat), and G (fan). Many older homes have just two or three wires.
Heating and Cooling Wiring
Add Y (cooling) and typically Rc (separate cooling power) for a central AC system. Five wires cover most conventional systems.
Heat Pump Wiring
Heat pumps use O or B to control the reversing valve. Some thermostats are configured for O-energized cooling, others for B-energized heating.
Common Wire for Smart Thermostats
Many smart thermostats require a C wire for continuous power. If you lack a C wire, options include using a spare wire, installing an add-a-wire kit, or using a power extender.
When to Call a Professional
If you're unsure about wiring, have a complex system, or encounter unexpected wire configurations, call a professional to avoid damage.