What type and size of batteries does the T3 Pro thermostat require?
The T3 Pro requires 2 AA batteries, which can be used for primary or backup power. If a C-wire common connection is used, batteries serve as backup; if no C-wire is present, batteries act as the primary power source. (Page 2)
From the official Honeywell T3 Pro Programmable Thermostat manual, page 2
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