Wire letter guide

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.

What happens if B is wired wrong

If a common (C) wire labeled B goes into the O/B connector, the thermostat connects R to C when it switches the valve. That shorts the 24 V transformer and usually blows the 3 or 5 A fuse on the furnace board, or damages the transformer. If this has happened, turn off power, move the wire to C, and replace the fuse with the same rating.

Take a photo of the furnace board before you change anything. The terminal the B wire lands on there (C, COM or B for common; O or B next to the heat pump wires for the valve) is the most reliable answer.