Most OWB's are not "boilers",they are tea kettles, as in an open to the atmosphere , no pressure system, & as such can not be plumbed to a pressurized boiler system. Plumbing a coal "boiler"to a oil "boiler is perfectly fine & makes the most economical sense in that you can utilize the circulator/s already in place. You do not drink the boiler water,so the
boiler water & the drinking water are isolated from each other. Yes to the continuous circulation between the 2 boilers in your set up.My personal preference for this topic would have been for it to stay with your first topic as a part of your whole project & the continuity of 1 topic vs many different topics for 1 project.But that's just my preference.