What type and size of batteries does the Honeywell Home T3 Pro use?
The T3 Pro uses 2 AA batteries, which can serve as the primary or backup power source. It is also recommended to connect the C (common) terminal for primary AC power, though the C terminal is optional if batteries are installed. (Pages 1–2)
From the official Honeywell T3 Pro Programmable Thermostat manual, page 2
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