... 500L 3.vaccum tube work over 15 years split solar heater system:1. that type have CE,ISO9001,CCC, certificates ... , work wirth work station, that together complete autumatic, and can work in bad weather(-30), if one tube ... 25mm,work principle the water tank have coil,connect with the solar energy, inside coil have special liquid for recirculate ... more details you can contact us anytime solar system