Presumably, you're referring to an immersion heater. Yes, when switched on, the water in an IH should stay at a constant temperature, controlled by the thermostat. But thermostats can fail, resulting in water over-heating, and in any case it would be a waste of electricity to leave an IH switched on while you're away. Personally, because there are only two of us, and we have a large IH tank, we tend to switch the heater on for 40 mins or so each morning, and then switch it off - which works well for us.