why the rocket mass heater works so efficiently

I saw this a while ago. It's counterintuitive, but these videos go into detail:

Explanation and model demonstration.

How the model was built.

I saw that and related videos a little while back. He explains the principles pretty clearly and succinctly in the first comment pinned under the video. The upshot is that that really does work, and gets energy from the following wind to go faster than the wind. It would not work in still air or a headwind.


Oops, the page jump hid my previous reply when I wrote this one

Daniel Bowman wrote:Hey Donald, thanks for the good write up. It is a lot to think about. We are getting ready to build our "forever home" with a thermal mass heater as a major design feature. One thing that I wanted to add, is I think the "size matters" claim is actually wrong. Size does matter... but smaller is actually bigger in this case. You wrote--

Donald Kenning wrote: If you were to split the wood smaller the amount of void could be a lot less. I might even be able to stack  1.3 cords of regularly chopped wood on into 1 cord volume of finer chopped wood (a guess). So optically, it looks like less wood stacked, when in fact, it is less air stacked into the wood.

But actually, cutting up the cord smaller makes for more air pockets and a larger stack of wood. You can reference this thread for some amusing back and forth-

http://www.arboristsite.com/community/threads/true-or-false-a-cord-of-rounds.141219/

Particularly good is the practical mini experiment with carrot sticks. Try it! Isn't it great when science completely bucks our intuition? If only it would buck or logs too.

Also your other point in the "size matters" section about smaller logs having a faster drying time is definitely true.

Thanks again.

I agree with the smaller = more space, or can = more space. I just did a different practical experiment, pulverizing some eggshells which I had saved up in a milk carton. There were four stacks of nested together 1/2 shells in the carton, smashed in over the course of six months. After pulverizing, they took up the same space in the carton. The "whole" shells nested together with less spaces, compared to the tiny fragments, with many more tiny spaces.... It's a bit like that life lesson of put some big, medium, and small things into a jar that is your life... if you begin with all the small things, then the big ones won't all fit in, but if you are sure to put the big ones in, then the medium and small ones can fit in the surrounding spaces.
Anything  solid, will be more dense than a fragmented version. solid stone vs. gravel for example. If the wood was "lumber" then it could be stacked without any spaces at all, but the definition of a cord of firewood is described as "well-stacked" or "compact" acknowledging that there must be some spaces due to the nature of the material.

If moisture content of the firewood is a critical factor in the efficiency of combustion, and if in cold climates we suffer from dry indoor air conditions...
Why don't we think of including an indoor firewood drying chamber (to a RMH, masonry heater, stand-alone?), for "finishing" our firewood?

I'm not thinking a kiln for green wood, but one day's/two day's/a week's supply of already "seasoned" wood...

And for that matter, does moisture content of indoor air correlate to "perceived comfort" for a given temperature? (i.e. 68*F@50%R.H. versus 68*F@75%R.H.)
Does a steaming kettle on top of the stove actually increase the efficiency of a stove? If we are talking about comfort for the people? and not just the numbers...

Although this is an old thread I thought this gave an interesting insight into how RMH can end up so efficient o wood burning. Particularly the systems efficiency being the product of design efficiency of the appliance and transport efficiency of the home environment.

A short video that covers most of this clearly.



And some more recent stuff about new efforts for further optimizations

https://permies.com/t/120/30006/pebble-style-rocket-mass-heater#1608094

and this year we will be doing a lot with the cob hat