If you keep having to reset it, it must be an electric model. The thermostats are bad and will need to be replaced and its a good idea to do the elements at the same time–as long as the tank is still good you dont have to replace the whole heater.: My water heater is about 10 years old. The water will get extremely hot for a couple of days and then the water will get colder until we manually reset the unit. Is ten years usually the lifespan of a water heater. And if we replace it, are tankless water heaters a good deal?