Does the 3iE have a battery backup?
3iE Battery Backup
The 3iE uses a capacitor to back up the date/time and programming.
In the event of a power outage, the capacitor will hold the charge for approx 1 hour, after which time the device will require re-programming.
When power is resumed the capacitor will recharge, ready to work in the event of another power outage. If your outage has outlasted the capacitors and you need to reprogram your thermostat, consult the programming manual.