Owner's Manual
Page 12
... the warning does not appear, you should replace batteries once a year, or before the batteries are optional (to provide backup power) if your thermostat was wired to run on AC power when installed. TH5320U/TH5220D - The warning flashes about two months before leaving home for more than a month. Install fresh batteries...
... the warning does not appear, you should replace batteries once a year, or before the batteries are optional (to provide backup power) if your thermostat was wired to run on AC power when installed. TH5320U/TH5220D - The warning flashes about two months before leaving home for more than a month. Install fresh batteries...