... 500L 3.vaccum tube work over 15 years split solar heater system:1. that type have CE,ISO9001,CCC, certificates, ... together complete autumatic, and can work in bad weather(-30), if one tube is broken, still can work, no ... principle the water tank have coil,connect with the solar energy, inside coil have special liquid for recirculate ... anytimeFor more details you can contact us anytime solar system