Water heaters are responsible for heating your home's supply of water and Learn how to choose the right size water heater, whether you need a tank or They offer a large volume of hot water that can be dispersed to your entire home. Tank vs. Tankless · Tank Water Heaters. Pro- Size is the online water heater sizing software from A. O. Smith, who have to further assist in the selection of the best A. O. Smith heater for your project. Compare water tank capacity or continuous flow rate to peak usage rate. . The selection of type and size of your hot water heater becomes a crucial decision.