Doesn't an open hearth achieve the same end as a rmh assuming a reasonably well insulated home? Both involve the heat being all soaked up and the smoke leaving at a low temperature through a small hole. Unless an open hearth isn't legally possible for you I just don't see the point of spending money building a rocket mass heater and having it take up space in your house. Plus you can't cook with them as far as I'm aware.
Sorry to those of you who are really invested in these things. To be fair open hearths might never catch on due to being perceived as dangerous or illegal in too many places. I guess it's also less practical for people who live in conventional homes built without the forethought that someone might want one.
But still, for people building their own cabin, underground house, cob/straw bale house or what have you I think a hearth is the way to go.
Open wood fires are about 20% efficient. A rocket, can be in the range of 90%
Heat recovery, and ambient air loss is also at play.
Heat recovery in an open hearth, might be in the order of 10% of the heat recovered. And if the fumes are cold, this is not due to only the fumes going up the chimney, but because of air dilution. The chimney sucks huge amounts of hot air, with your smoke. So you're cooling the home, to heat the home.
This is why stoves have been invented, and refined.
This is really simple. With an open fire be it a pit or a fireplace. You must keep that fire lit to stay warm. Fire goes out cold comes in ...
With a rocket mass heater (which can be cooked on) you build your fire for as long as you think you need and you let it go out. Fire goes out mass sheds heat , you sleep toasty warm all night .
Open fire takes armloads of wood and waking up several times to stay warm . RMH takes one armload and your warm all night with NO CHANCE of rolling into your fire overnight.