When I cleaned out my heat exchanger
, I just popped the hose from the front and drained if from there. It's hazardous stuff so dispose of it properly. Then, I just filled up the 50:50 coolant at the top where the cap is located. I have an overflow tank, but the fill level stays at low so that when the coolant heats, it slightly fills up that tank. That's it. I did not need to bleed the coolant side. Would be interested to hear if I should have done anything differently. My engine is a 3GM30F.
My hot water tank is completely out of the picture now, too, because it rusted out and I need to replace the heater element. I have closed off all the hoses so the engine water heat cycling doesn't run through the water tank any more.
Hope this is helpful.