Why it matters
A basic thermostat needs very little power and can run on batteries. A smart thermostat has a screen and a Wi-Fi radio, and many models need steady 24-volt power to run them. That power comes from the C-wire, short for common.
Without it, some smart thermostats borrow power through the heating or cooling wires. That can work, but in some systems it makes the equipment click on and off or leaves the thermostat going dark.
How to look
Switch off the power to your heating and cooling system at the breaker or the service switch. Then pull the thermostat face off its base; most lift or slide off without tools.
Look at the letters printed beside each screw terminal. Take a clear photo before you touch anything, so you can put every wire back where it was.
What the letters usually mean
R or Rh and Rc carry the 24-volt power. W calls for heat, Y calls for cooling, G runs the fan, and O or B switches a heat pump between heating and cooling. C is the common wire that completes the circuit and gives steady power.
Colors are a habit, not a rule. A blue wire is often the C-wire, but go by the terminal letter, not the color.
If there is no C-wire
Look into the wall opening: there may be a spare wire folded back that is not connected at either end. An installer can connect it at the thermostat and at the furnace or air handler.
Some smart thermostats come with a power adapter that is wired in at the equipment. Otherwise a technician can run a new thermostat cable.
If the wires are thick
Two thick wires joined with wire nuts, often on a thermostat attached to an electric baseboard heater, carry 120 or 240 volts. Standard smart thermostats are not built for that. Look for a model made for line voltage, and have an electrician install it.
General information only. For wiring, refrigerant or gas work, use a licensed professional.
