Honeywell T8112C Owner's Manual page 11

To remove batteries, press down on the left
ends. If the new batteries are installed within 20
to 30 seconds of removing the old ones, the
thermostat does not have to be reprogrammed.
If the display is blank, the batteries are dead or
incorrectly installed and the thermostat has to
be reprogrammed. See Programming Your
Thermostat section for instructions.
11 69-0862—4