Are you running antifreeze in the boiler circuits? If you do, you lose about 10% of the capacity, since it doesn't transmit heat as effectively.
I would guess you could just blow the lines out with an air compressor for the domestic water lines. You'd need to put antifreeze into the traps of the drains, but nothing else. If you were lucky, the lines were installed with the intent of draining, and you could just open a valve, but that doesn't happen too often.