rocket mass heater build

Hello
I am building a rocket mass heater as part of a community gardens project.
Have a few q's I need clearing up...

In the book Rocket mass heaters by Ianto Evans and Leslie Jackson it states that the gap between the heat riser and the outer barrel should be 1 1/2 inches. what is the reason for this and how important is it? I've seen some stoves where the gap looks much bigger.

-is it ok to have the bottom of the outer barrel at floor level and then cut a hole in it for the exit flue? does this make it better/worse, and why is this?

Many thanks
jim

Jim Sanders : 1 and 1/2 inch is the minimum for a 6'' system, and 2'' is the minimum for an 8'' system, I freely admit that this is a problem !

Assuming that you are building on a concrete surface and not on wood, you still want a layer of insulation under the bricks that make up the
bottom of the Burn Tunnel/Combustion Chamber, Have you done the math to prove to yourself that you can have a full sized Feed Tube, and
Burn Tunnel and have a Heat Riser that is 3 Xs as tall in total as the feed tube is deep? You must also add in the proper gap to the barrel!

Even if you eliminate an ash pit at the bottom of the Feed Tube, and you should you are making it necessary to feed your R.M.H. with very
short wood more often !

To answer your last question, yes you can cut a hole in the side of the barrel here is a good place to 'go big or go home " 2-3 Xs the regular
Cross Sectional Area ! The inside walls of this area need careful attention, I would build a form out of hardware cloth and use a little fine
chopped straw, for your first coat, finished off with a fine sand Clay coat as smooth as sheet rock, this is the hardest pert of the build to get right! BIG AL