Rocket Mass Heater

I presume if they worked and were safe to use, there would be a company (or multiple companies) making and selling them for serious money. Given they are not, I think it safe to assume they are ineffective, undesirable to have around, or unsafe, or all of the above. If it sounds too good to be true, it probably is.

From poking around they look to be a giant version of a wood gasification cooking stove.

That said, I'm not sure how that really differs in practice from a high efficiency wood stove. Essentially, a woodstove creates wood gas and burns it, followed by a charcoal burning stage.

Not sure how those things are really different, plus I can't say they're very attractive. Maybe interesting to play with for the academic exercise, but I doubt they could be better than and EPA approved stove, especially all the $$$ that companies like VC and Harmon etc... put in the R&D.