You can’t run out of hot water with tankless water heaters - unless, of course, the power supply is off or goes out. However, a tankless water heater can have trouble keeping up with demand if too many things require hot water all at once. For example, if several appliances are operating at the same time (the dishwasher, the laundry machine, etc.), and you go to have a hot shower, you may find that your tankless unit isn’t capable of providing enough hot water. Therefore, it’s important to ensure that you install a tankless water heater that has a large enough capacity to handle your demand for hot water.