When we had a new hot water boiler installed, we had a cut-off valve put in the pipe that feed cold water into the tank. Before Shabbos, we turn off the valve, cutting off the cold water feed. We then use the hot water remaining in the tank until it runs out (usually by Shabbos afternoon – fyi we do not take showers on Shabbos)
It sure makes washing ourselves and the serving platters more pleasant !!