Hard water has salts like calcium, magnesium in dissolved state. They can cause corrosion and form rust inside the walls of the geyser. If you have hard water supply at home look for a geyser with a corrosive resistant tank and glass coated copper Incoloy heating element for greater durability and longer life span. You can also install a water softener that will turn hard water into soft water before passing it to the geyser.