If the boiler tubes are burried in the masonry the heat needs to be transferred to the concrete and then to the copper to the water. If you have a bucket of water and a concrete block of equal size heated to the same temperature with a copper tube running through each, which will heat the flow through the copper faster. I don't understand how you can take the heat off fast enough while firing at 500,000 btus. Concrete is not a good transfer medium.At risk of playing devil's advocate against myself, I _can_ envision situations in which a hybrid masonry heater/ boiler might make sense- such as when the masonry heater part of the unit is located in a rather large relatively open space/ spaces (so that it can spread its heat easily and directly by unrestricted radiation and convection) but you also want to be able to send heat to a different area that would not be effectively warmed directly by the masonry heater. Seems like you lose the stone-simple near-failproof characteristics of the masonry heater in the process though, without fully gaining the ability that a high efficiency boiler + well insulated water storage yields of bring able to efficiently"bank" large quantities of heat for release only when or at the rate that is desired. Different strokes for different folks.
I do have to say though that Heiss beat me to my lifelong dream. They figured out how to burn water.