Rocket Mass Heater questions

See the attached picture for my proposed design.

*fire brick dimensions are appr ~ 9 X 4.5 X 2.5

Question 1: Do I need to extend the burn tunnel distance? This is a proposed 8" system with appr 7" feed tube, heat riser and burn tunnel with 8"exhaust.

Question 2: I've seen people on permies using loose packed vermiculite to insulate their heat risers. Is that advisable? Seems like the easiest way to do it, but I don't know about their long term success.

Question 3: "The Book" says the burn tunnel cross sectional area should be less than the cross sectional area of my heat riser, the area above the heat riser and barrel top, and the exhaust port. So if I make my burn tunnel 6.5" X 6.5" (42.25 sq in), my heat riser 7" X 7" (49 sq in) and my exhaust 8" duct (50.24 sq in) would that work?

Thanks for you help!
Wiley the soon to be pyro!

Willie Shannon : Not long after the American public found out that in Abraham Lincoln they had elected a Great orator that sounded
magnificent within the written press but sounded like a country Rube in person, The McGuffey Reader was introduced to America!

It used a common syntax and grammar and for the first time one American could reasonably expect to be understood by another
American! With a few exceptions, the Rocket Mass Heater RMH Book can be clearly understood, The slight ambiguity of stating
that the Cross Sectional Areas of all other areas should be of the same size or larger than the Burn Tunnel, is simply a less than
perfect word choice.

I prefer to not scrape clean the Burn Tunnel, leaving behind a 1/2'' to 3/4 inch layer of fly ash within the burn tunnel as a further
protection from the Freakishly high temperatures of the combustion chamber and actually make the Burn tunnel slightly oversize
to protect and conserve the Fly ash within the combustion zone and the bricks of the combustion zone Also ! I hope this helps and
is timely, For the good of the Craft !

penlight needs to be 1.5-2.0'', vermiculite 2x that and Rock wool 2xs That, how much room have you got ?! Big AL

Think like Fire, Flow like a Gas, Don't be a Marshmallow ! As always, questions/comments are solicited and Welcome Big AL !

Damn Auto correct!!!
In my post above that should be perlite 1.5''-2'', vermiculite 2X that and Rock wool 2Xs That ! - How much room have you got !

Thanks for the reply(ies) Big Al! Sorry its taken me a few days to respond, i've been stricken with the flu.

Are you saying that the cylinder surrounding the burn tunnel i should have 1.5-2" of perilite, 4" of vermiculite and 8" of rock wool? That takes up approx 28" of diameter? Thats bigger than a 55 gallon drum.

or do you mean insulation from the bottom of the cylinder to the top?

Willie Shannon : Sorry for the confusion We are using slightly different terms for similar parts, hence my near constant plea For 'The Book ' I was writing about the
Heat Riser that starts where the horizontal Burn Tunnel leaves off, wrapped around the outside of the fire brick* should be some additional insulation the Perlite
mixed with just enough clay slip to allow you to make dirty snowballs out of it! is effective at 2'', Vermiculite will need twice that amount and Rock wool twice that!


Question 1) Answer the Heat riser should be 3 Xs the depth of the Feed tube, and 2 Xs the length of the Burn Tunnel !

Question 2) Ans. use perlite if you can get it then vermiculite then- - -

Question#) Ans. I would make your Burn tunnel 7'' by 7.5'' and not clean it as often as some other Folks

Perlite can be tamped down successfully if in a container with stiff sides, otherwise its a little like herding cats !
Yes depending on what you are building on you will need to insulate under and around your Rocket burner !

Hope this finds you better Big AL !

I said burn tunnel, I meant heat riser! I'm glad you followed my line of thought!

Ok all clear!