Unpressurized rocket water mass heater

Hi,
 I live in Michigan on a mostly wooded 10 acre property that has hot water heat.  I had an wood boiler addon that would occasionally get too hot, so I know all about the BoomSquish problems with pressurized water heat.  I would like to make a rocket mass heater that would heat water for an unpressurized water heat battery (Like you would use for DYI thermal solar) that would then have a heat exchanger for heating the hot water heating in the house. (and domestic hot water.) I one of the permies videos that I watched, Paul talked about the unpressurized hot water heater and how they were possibly going to upgrade that to a version 2.0.  How can I find information about that?