B Wire on a Thermostat: Common Wire or Reversing Valve?

This is the most dangerous label mix-up in thermostat wiring. On Rheem and Ruud heat pumps, B is the reversing valve (energized in heat). On many Trane, American Standard and older Carrier systems, B is the common wire, usually blue.

Google warns that putting a common wire on the wrong connector can blow a fuse on your HVAC system or damage the thermostat. How to tell:

• If your old thermostat also had a wire on O, then B is almost certainly common.
• If you have no heat pump, B is common.
• If you have a heat pump and no O wire, B is probably the valve (Rheem/Ruud style).
• When in doubt, follow the wire to the furnace or air handler and see which terminal it lands on.