Can one put a hot water urn on a timer before Shabbat so that, for instance, the water is being heated at night, stops reheating overnight and starts reheating again Shabbat morning?
When the timer is off and the water is cold, is it okay to add more water?
Would the same apply to a water boiler user for showering (assuming one can install a timer)? Or would the difference be whether or not you can tell the water is cold?
Showering would still be a problem.