What is the difference between a B and C wire?

The C wire in most cases is for the Common wire, which provided direct power to the thermostat. A wire labeled O, B, or O/B is used to control whether a heat pump is in heating or cooling mode. However, on some thermostats, a wire labeled B is actually the common wire C. In most cases, the common wire is labeled C. Because the common wire is used to provide direct power to the thermostat, it’s important to determine what function a wire labeled B serves, before connecting it to the Zen Thermostat.

For a more detailed description of the C and B wires please see the attachment below.

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