You really don't want to be drinking heating system water even if there are no additives, as is usually the case in residential.

When it comes from the water supply provider it has disinfectants in it, either chlorine or chloramine, but those disappear quickly and the bugs begin to grow. Bacteria, slime, etc. Look at what comes out of a fire sprinkler system sometime, it's black and slimy. Water has a shelf life that is long exceeded in a heating system.