A closed system refers to a boiler system, that once filled, never needs anything .Unless there is a leak the system is closed off and really needs no maintanance. Once a boiler system is filled and bled of all air in it, you don't want to add or take any water away from it. The water in a boiler system can last for a lifetime.  It will never go bad. There is no need to flush it every year. In fact draining and adding water from your boiler every year is a bad thing to do. Everytime you do that, you are allowing fresh water into the system. When fresh water is introduced to a boiler system it brings in impurities, that will settle out of the water when the water is heated. These impurities will collect in the bottom of the boiler. When impurities sit in the bottom of the boiler they are very harmful because they create a layer of insulation to the heating surface. This insulation prevents the efficient transfer of heat and wastes heat by allowing the non transfered heat to escape out the flue. More heat up the flue takes away from the efficiency of the boiler and causes the boiler to have a tendency to overheat and prematurely leak from heat warpage. Also when introducing fresh water into the system there is oxygen in the water that separates when it is heated which causes rust to form on the inside of the pipes.