The best way to use water heater controls is to determine the desired water temperature, and leave the control settings in place. Adjusting the temperature frequently wastes energy and can strain the water heater and piping.
Older water heaters typically had only one control interface: the temperature control. This mechanical thermometer caused the water heater to stop producing heat once the target temperature was met, and the unit would only be triggered when the temperature dropped due to hot water being used or energy radiating from the system. Newer models sometimes provide digital interfaces, but these displays are often used to provide feedback about the system's operation instead of control interfaces.
Because of their complex operation, tankless water heaters are more likely to provide digital interfaces than their traditional counterparts. Since problems that occur with tankless water heaters are often caused by components within the unit that are difficult to inspect visually, they often have diagnostic modes. The operation of these displays is typically explained in operation manuals.
Experts differ on what the best temperature for hot water should be. Higher temperatures present a risk for scalding, which causes many experts to advise keeping the system at or slightly below 120 degrees Fahrenheit. However, lower temperatures can cause bacteria, especially Legionella, to grow, presenting health risks.