A closed loop water purification system comprising a feed water line (42) to supply feed water, a high pressure feed pump (62), a filter assembly (66) having an inlet supplied by the high pressure feed pump, a distribution line (68) and a reject water line (70), the reject line returning reject water from the filter assembly to the feed water line, the distribution line supplying at least one purified water take off point (86) and returning to a junction (50) in the feed water line upstream of the high pressure feed pump. The reject water line is connected to the junction in the feed water line upstream of the high pressure feed pump. A first backflow prevention device (48) is on the feed water line upstream of the junction. A second backflow prevention device (100) is on the distribution line upstream of the junction and a third backflow prevention device (80) is on the reject water line upstream of the junction. A heater system (56) is in one of the distribution line, the reject line or the feed water line.