Honeywell RTH4300B Guide D'installation page 15

Connect wires (less common wire designations)
If labels do not match letters on the thermo-
stat, check the chart at right and connect to
terminal as shown here (see notes, below).
If wires will be connected to both
R and Rc terminals, remove metal
jumper (see page 12).
Do not use C, X, or B. Wrap bare
end of wire with electrical tape.
Do not connect both O and B
if you have a heat pump. Connect
O wire to O. Wrap B wire with
electrical tape and do not use.
RTH4300B Programmable Thermostat
3