Fred61
Minister of Fire
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.