How do I power the T4 Pro thermostat and what are the battery requirements?
The thermostat can be powered by either the HVAC system's 24-volt power source or through 3 AAA alkaline batteries. Batteries are optional if the thermostat is wired to run on AC power, but are required if no AC wiring is connected. A battery backup is also included to retain settings during power outages. (Page 4, 11)
From the official Honeywell T4 Pro Programmable Thermostat (TH4110U2005/U) manual, page 4
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