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How to tell if your B wire is really a common wire in disguise

While usually a wire labeled B goes in the Nest Learning Thermostat's O/B connector, for some systems the B wire is actually the common wire. The common wire, usually labeled C, should go in the Nest Thermostat's C connector.

To see what labels you have on the wires attached to your heating and cooling system, remove the cover from your current thermostat. You should see a series of labeled connectors and take note of the ones that have wires connected.

The vast majority of the time, 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. On most thermostats, 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 Nest Learning Thermostat.

How to tell if your B wire is really a common wire in disguise:

  • 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.
  • Does your thermostat have a connector labeled C?
    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 the Nest Learning Thermostat.
  • Do 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.
  • What color is the wire labeled B?
    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 Learning Thermostat. But if the B wire is blue, it should most likely be connected to the C connector.

IMPORTANT NOTE: 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 your B wire is, we strongly recommend that you find a Nest Pro installer near you.