A water distribution system having a dual use water treatment unit. The same water treatment unit can be used to treat water supplied from a fill port to a storage tank during a fill operation, as well as water leaving the storage tank for supply to one or more points of water use during a supply operation. When a diverter valve is in a fill position, water supplied to the fill port can flow through the diverter valve and to the water treatment unit for treatment then to the storage tank for storage. When the diverter valve is in a supply position, the water from the storage tank can be delivered using means for pressurizing water to the same treatment unit for treatment, then through the diverter valve and to a point of water use device.