What to do with the existing water boiler?  
     
 
As a standard in USA, every house is built with an existing water boiler. Solar water heating never conflicts with the existing boiler. Solar heater is connected upstream of the existing boiler as a pre-heater to provide as much heat as possible, whereas the existing boiler acts as backup heater to cover the unavailability of solar heating. If only the existing boiler is used, it has full coverage of all times, but the energy bill is high; if only solar heater is used, no energy cost is billed, but there is no coverage without extra-storage when sunshine is not available; when both heaters are used in combination, the best result is achieved.
 
     
 
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