Sure easy Question YOU DO NOT FLUSH your hydronics system, not unless you want to kill it prematurely.
Think about this all fresh water contains oxygen, Oxygen causes oxidation to ferrous metals BOILERS, CIRCULATORS, Black piping etc.
Once this oxygen is out the system should never need to be drained.
What should be serviced is the burners if gas every 2 years, The relief valve tested, the circulator slightly oiled if applicable and the expansion tank checked for the proper air cushion.
The flue pipe for proper draft. Oh yeah keep flammable stuff away from the burner and double check your free air openings are unobstructed
Steam boilers should be “blown down weekly” and at the end of the season the hand holes opened and the stay bolts checked etc.