RT310RF : Low Battery Warning (4.2.1) .
Low Battery Voltage warning Indicator.
The battery voltage in your thermostat is checked every minute. When the battery voltage drops below a certain level, the ‘Low Battery Warning Indicator’ appears on the screen.
The thermostat will function normally during low battery. However, you must change the batteries as soon as possible before the battery is too weak for the normal operation to be assured.
When you change the batteries, you have about 30 seconds to do so without losing your thermostats settings. If the batteries are out for too long or have drained completely and your thermostat is left for any time, not only will you lose your settings, but you may have to ‘pair’ your devices again when new batteries are installed.