Are batteries required for the Focuspro TH5220D to operate?
Batteries are optional for backup power if your thermostat was wired to run on AC power when installed. All settings are permanently stored in memory and do not require battery power; batteries only provide backup power for the display during power outages.
From the official Honeywell Focuspro TH5220D Thermostat manual, page 3
Have a follow-up question?
Answers from the official manual.
Common questions