What power options are available for the T6 Pro thermostat?
The thermostat can be powered by inserting R and C wires into the designated terminals for primary AC power, with the C terminal being optional if batteries are installed but recommended. Alternatively, 2 AA batteries can be inserted for primary or backup power.
From the official Honeywell T6 Pro TH6210U2001 Smart Thermostat manual, page 2
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