... non-pressurized solar water heater 1,vacuum tube length:47/1500mm,58/18002,vacuum tubes:15pcs,18pcs,20pcs,24pcs,30pcs3,Outer tank material:202stainless ... Inner tank material:SUS304-2Bstainless steelthickness :0.41mm or 0.31mm5,Insulation:foaming polyurethane thickness:50mm or 55mm6,Optional electric heater:220v ... freely .it is suitable for the many residentbuildings. non-pressurized solar water heater