I currently have a harman sf160 boiler with two zones and baseboard. My other heater is a oil fired hot air furrnace which I am getting rid of soon. My plan is to buy an oil fired boiler and tie in to my coal boiler. So the coal manual simply states to tie into return and add a circulator between and install tee into feed side. Is this best way to plumb it? What purpose is this additional circulator?