Tank less water heaters also known as On Demand water heaters click on any image to see full details and prices for all units. For simple use these tank on demand water heaters are great. but there are limitations and potential problems. hard water with out a water softener can clog the coil. You are limited to a specific Gallon per minute of water flow, if you exceed that flow rate you will not get hot water, this can be a problem filling up a bathtub or when using a power shower setup.