There's some interesting comments being posted on here.
Just to re-iterate what I've mentioned before -
linked a combined Batch Box rocket mass heater/water into an existing 'conventional' oil fired central heating hydronics system, and it works
The BB rocket is located inside the house and provides mass heating storage for the room it's located in (and to some extent, adjoining rooms) as well as hot water.
The boiler I use with the BB is rated at 45,000btu/hr.
The boiler is located around 10" above my 5" heat riser and it doesn't flash steam.
The boiler thermo-syphons into the central heating system (via a 'Neutralizer' multiport device) without a pump.
The whole water system side is open vented - in the event of a power failure there is no pressure build up and excess heat is soaked up by a gravity fed 'heat leak' radiator.
Mine is not an ideal system. If I had the finances and the time I would build perhaps an 8" BB Rocket to drive a large boiler that would thermo-syphon via large diameter piping to a 2,000 litre (or larger) highly insulated thermal store. The BB Boiler would be on an independant open vented water coil loop that would only require a small volume of water. The water in the thermal store would store this heat and domestic hot water and the central heating would be fed from this indirectly - that is, these systems would be in their own enclosed pipe loops. The water in the store would just exchange the heat into them. (Tim Barker has done something similar with a domestic hot water/shower rocket)
It's nothing new. A lot of mainland European countries already use this type of arrangement and have done successfully for many years. The difference until now is that most of the existing systems use pellet or wood gassification boilers to heat the stores which are very expensive and relatively complicated. They rely on a lot of electronics, sensors, forced air fans, etc to ensure they function and the combustion is correct.
What I love about the BB Rocket is its simplicity and its ability to keep working in the event of a power outage. It doesn't need sensors, fans or valves - just dry wood and a bit of manual attention now and then. I really think that a BB rocket would be in it's element in this application. The large volume of water in the thermal store is ideal to 'smooth out' the btu output from a BB and store that heat for many hours. Maybe one day...