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Passive solar water heating systems are popular because of their inherent simplicity and reliability, no pumps or controllers or wiring.
The storage tank is located on the roof and cold water from the city or well flows directly to the collector on the roof, where it is heated and then flows to your conventional 40 or 60 gallon water heater located on the ground level, and you have storage of hot water in your tank overnight. If you have large morning draws of hot water (ie: showers or baths) most of the collected heat energy returning to your tank is lost overnight, thereby saving you 50-60% of your long term hot water energy costs, compared with close to 85% savings with an Active Open Loop Solar Water Heating System.
In the case of a thermosyphon system, an insulated tank on the roof, located above the collector will lower the loss of stored heat during the night, but is not aesthetically appealing. These types are more popular in the islands.