Usually a wire labeled B goes in Nest’s O/B connector. For some systems, the B wire is actually a common wire. The common wire (usually labeled C) brings power to the system. Common wires belong in Nest’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. Look at the connector labels and take note of the ones that have wires connected.

O/B is the switchover wire that tells a heat pump whether it should be heating or cooling. If there are no wires in the O, B or O/B connectors, then your system is probably a conventional system. However for some systems, the B wire is actually the common wire. The common wire is usually labeled C. Any system, conventional or heat pump, can have a common wire. The common wire brings power to the thermostat.

IMPORTANT NOTE: If a common wire is inserted into any connector other than C, it may blow a fuse in the HVAC system and damage Nest (another reason to turn off the power during installation).
 

If you have a wire labeled B, how do you know if it’s a common wire or a traditional B wire? Here are some clues:
 

• It helps to know what kind of system you have. If you don’t know, check your heating and cooling equipment. If you have a condenser unit that is a heat pump, it will usually say “Heat Pump” or “Package Unit.” Once you’ve determined if the system is a heat pump or a conventional system, it will help you figure out which labels to use while installing Nest. 

• GE, York, American Standard and Trane are brands that are more likely to use B for the common wire.

• If you’re installing Nest to control a heat pump system and the old thermostat has both an O wire and a B wire connected, insert the O wire to Nest’s O/B connector and insert the B wire into Nest’s C connector. 

• If you’re installing Nest to control a heat pump system and the old thermostat has a B wire connected but does not have an O wire connected, connect the B wire to Nest’s O/B connector.

• If you’re installing Nest to control a conventional system - not a heat pump - and you see a B wire connected to the old thermostat, it is probably a common wire and should be inserted in Nest's C connector.

• The labels on each connector sometimes correspond to the colors of the wires (for example an orange wire is O, a green wire is G, a red wire is R). The B in O/B stands for brown. The B in common (C) stands for blue. Note that labels often aren't matched to the colors of the wires, so this is NOT a sure test. But if it looks like your B wire might be a common wire according to other clues, and it also happens to be blue, you can feel confident that you have a common wire.

Related to...

• Nest Learning Thermostat

Article #5096