Honeywell RTH5100B Guide D'installation page 15

Connect wires (less common wire designations)
If labels do not match letters on the thermostat, 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 connect both O and
B if you have a heat pump.
Connect O wire to O/B W.
Wrap B wire with electrical
tape and do not use.
Do not use C, X or B. Wrap
bare end of wire with
electrical tape.
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RTH5100B