Your hot water tank may be equipped with a tank timer. There are positive and negative aspects of using a timer on a boiler system. The timer controls a switch that will turn on and off a hot water heater during different parts of the day, much like how a thermostat works on a home climate control system.
Pros- Energy Efficiency
A timer enables the home owner to be able to control energy consumption because the heater is not working on its own schedule, but instead will only work based on the needs of the household. This will help reduce the homeowner's energy bills.
Pros- User Friendly
The water will remain hot in the tank for the intended time. If for some reason there is a need for additional heated water over that which is held in the tank a manual over-ride switch can be activated to heat the extra water needed.
If a timer on a hot water tank fails it can cause either a lack of hot water altogether, or it may fail in an open state and continue to run the water heater continuously, causing water that is too hot and increasing energy consumption.
Cons- Inconsistencies between Household Members.
If the timer is set to a particular member's schedule then hot water may not be available for all members of the home.