As a guy with no wood boiler just dreaming, the open storage system seems to offer several advantages. Isn't Tom supposed to be selling this company to focus on other things?
Not sure what Tom's plans are. As the owner of both an open and closed systems in regards to the storage (both are closed as far as the boiler is concerned). The only slight disadvantage of the open storage with heat exchanger route is the usual transfer efficiency loss (not sure loss is the right word here) - in other words when the boiler is putting out 80C water this heat is not all transferred to storage, some is returned to the boiler and the boiler is more likely to go into overheat/smolder mode until return temps drop a bit. This never happens with my other system where all the water in the system (boiler/storage) is shared - (as long as I load the proper amount of wood for the storage temps) Now perhaps I need more open storage capacity or a bigger heat exchanger and this would not happen. As it is it works perfectly because the shop load is always taking some of the heat and I rarely see the boiler go into overheat mode. (overheat mode is not really too hot, it's just the point where the inducer fan shuts off and waits for return temps to drop about 5C). An advantage of the closed/open storage/heat exchanger set up is the closed portion is low volume (boiler capacity+piping +heat exchanger+heat emitter volume)- about 60 gallons in my system. So less volume means smaller/cheaper expansion tank and if you choose to treat the water, only purchase treatment chemicals for 60 gallons, not 1000+ gallons. Another advantage is that low volume heats fast and is available immediately to the heat emitters. Works well in a system for a shop where you really only want lots of heat when you are using it. The storage is there to soak up the excess and helps keep the shop above freezing when I'm not out there working.