•All of the dashed wires shown in the following wiring diagrams are either optional, or their usage depends upon your specific system type or brand. For example: Diagram #1 shows the fan wire as optional. If your system does not have a fan, than this terminal will not be used.
•The optional “C” terminal is used for powering the thermostat by the 24 Volt system power. This can be used alone, or in addition to installing batteries.
•If “Y” and “C” wires are both present, then “C” is most likely a system common wire.
•For Heat Pump systems, use either the “O” terminal or the “B” terminal, but not both. If an “O” and a “B” wire are both present, “B” is likely a system common and may be connected to the “C” terminal. Connecting system common power to this thermostat’s “B” terminal may damage the thermostat, and also your system.
•If replacing a Honeywell
•If replacing an old thermostat that has a mechanical clock, there may be two wires labeled as “C” for the clock power. Tape off these wires and do not connect them to the “C” terminal of this thermostat.
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