If a common wire is inserted into any connector other than (C), the fuse of a HVAC system can be damaged/blown. If uncertain of what the (B) wire logic is, we strongly recommend contacting Zen Support.
The common wire, usually labeled C, should go in the C Terminal of the Zen 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 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.