i use a header system ..wheremy lines come in the house i have a incoming (hot header) with multiple outlets to go to allmy different appliances) and a return header right below it for the cooler water to return to the OWB. i have valves on all ports so i (if needed ) can adjust flow (so far i have run them all wide open all the time) i have one 1/2 line for a floor loop, one 1/2 line for a small water to air exchanger in my fire place (no more wood in the house) two 3/4 lines to the hot water heat and the furnace.
if i were to run all these diferent lines in series i would be restricting the flow so much i would never have temps above 150 returning to the furnace and that could lead to a lot of corrosion at the OWB