I stumbled across this today and thought it looked very much like a rocket style device. They used forced air for combustion, and a heat exchanger for hot water heat. From page 6, "The heat exchanger and combustion can are connected to a storage system through pipes large enough to permit gravity circulation. An open-to-the-atmosphere expansion tank will prevent overpressure." I think Ernie and Erica will be able to glean a lot of useful information from it, it gives a lot of information regarding efficiency and exhaust gas makeup.