Honeywell RTH5100B Guide D'installation page 9

RTH5100B
Identify wires you will not use
If any wires are not attached to terminals in your old thermostat, do
not connect them to your new thermostat.
If a wire is attached to a terminal marked "C" or "C1," disconnect it.
You will not connect it to your new thermostat.
Wrap the bare metal ends of each unused wire with electrical tape,
so it cannot touch and short out other wires.
Do not use unconnected wires.
Do not use C or C1 wire.