Water heaters can only be incorporated within the supply sub-loops of domestic hot water plant loops so you first need to go to the supply sub-loop of the domestic hot water plant loop to which the water heater is to be added. The Add Water Heater tool icon will then appear on the toolbar. To place a water heater, click on the Add Water Heater tool and then move the mouse cursor to the required position and click the mouse button to place the water heater.