The ESRTD3 has a built in auto-rechargeable battery to power the display when the system is between heating cycles. To conserve power this will run for eight hours and if the heating intervals are more than eight hours apart, the display will “go to sleep” – although the thermostat is still active. When mains power is restored, the battery auto-recharges. This is a design feature to ensure the delayed start thermostat clears its memory at the end of every heating cycle.
