How to tell if a B wire is really a common or C wire

Wires labeled B typically should go into a Google Nest thermostat's O/B connector. But for some systems the wire in the thermostat's B connector is actually a common or C wire. Common wires should go in the Nest thermostat's C connector.

Most of the time a wire labeled O, B, or O/B  controls whether a heat pump is in heating or cooling mode. However, on some thermostats a wire labeled B is actually the common wire. On most thermostats, the common wire is labeled C.

Because the common wire is used to provide consistent power to the thermostat, it’s important to determine what function a wire labeled B serves before connecting it to a Nest thermostat.

  • Read through the manual for your old thermostat.
    If you have the manual for your thermostat (or can find it online), it may describe the function associated with the B connector and help you determine if it’s meant for a heat pump wire or a common wire.
  • Check if your thermostat has a C connector
    Whether or not there is a wire attached to it, if your thermostat has a separate connector labeled C in addition to a connector labeled B, then the wire labeled B should be connected to the O/B connector when installing a Nest thermostat.
  • If you have a heat pump
    It helps to know what kind of system you have. If you have a heat pump and you don’t have a separate wire labeled O, then the wire labeled B should be connected to the Nest thermostat's O/B connector.
  • Check the B wire color
    While the color of the wires connected to a thermostat often don’t adhere to industry standards, they can provide a clue in situations where you’re still uncertain about the function of a wire labeled B. If you’re thermostat doesn’t have separate connector labeled C and you’re not sure what type of system you have, make a note of the wire color connected to B. A brown wire is mostly likely a heat pump wire and should be connected to the O/B connector on the Nest thermostat. But if the B wire is blue, it should most likely be connected to the C connector.

IMPORTANT: If a common wire is inserted into any connector other than C on your Nest thermostat, you may blow a fuse on your HVAC system and/or damage the thermostat. If you're at all uncertain what type of wire you have, we strongly recommend that you find a Nest Pro installer near you.

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