Furnace Thermostat Wire Colors Explained — DIY Guide

Published: July 23, 2026 — BC Wide Home Services Ltd, doing business as BC Wide Heating & Air Conditioning — Greater Vancouver, BC

Standard Wire Colors

R (red): 24V power. W (white): heat call. Y (yellow): cooling call (for AC). G (green): fan control. C (blue or black): common wire for smart thermostat power. Some systems have additional wires for heat pumps (O/B for reversing valve, AUX for auxiliary heat).

Identifying Your Wiring

Remove the thermostat faceplate to expose the wiring terminals. Note which color wire connects to each terminal label. Common labels: R, RC, RH (power), W, W1, W2 (heat), Y, Y1, Y2 (cool), G (fan), C (common). Take a photo before disconnecting any wires.

Smart Thermostat Compatibility

Most smart thermostats need a C-wire for continuous power. If your system lacks a C-wire, options include: using a power extender kit (Ecobee includes one), installing a plug-in transformer, or running new thermostat wire. Check compatibility before purchasing a smart thermostat.

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Safety Reminder

If you smell gas, suspect carbon monoxide or believe there is an immediate danger, leave the property and contact emergency services or the appropriate gas emergency authority. Do not remain inside — exit the building immediately and call for help from outside.