Purpose of a Coolant Overflow Tank

So what is the real purpose of a coolant overflow tank? Is it different with expansion tanks? Well, the true overflow tank is an add-on part that is mainly used with a conventional cooling system. This conventional cooling also includes radiator. So let me explain a bit of something I know. This cooling system is packed through the filler neck on the radiator and it must be filled with almost half inch to one inch below the bottom of the filler neck. Now, if that has been filled any higher, of course it will be full so the excess coolant will be then vented. Then the overflowing coolant is then expelled to either the atmosphere or into the coolant overflow tank. And if one can see it, the actual flow of excess coolant is on one direction only, and that is to the coolant overflow tank. Thus, an overflow tank provides us a catchment basin, but not like an extra coolant reservoir. The only think lacking is that the coolant in the overflow tank won’t pour back into the cooling system due to the reason that liquid can’t pour back from a lower pressure to a higher one. So I guess that’s about it. So if you are thinking of adding a coolant overflow tank, it’s not actually a waste of money.