rebecca m.

Oops, sorry.  Metalbestos is a brand name, I guess, for an insulated, double wall, stainless steel chimney pipe.  I have seen reference to a similar product being used as a heat riser, but haven't heard much of an evaluation of its performance.  In the meantime, I've decided to go with a brick heat riser, so never mind...  Thanks for getting back to me though.    I'll begin putting the stove together today!  I can't wait to see what mistakes I'll make!

I'm considering using 8" metalbestos double wall chimney pipe for the heat riser in an 8" system and wonder if anyone has experience with using this material and/or comments on the pros and cons of using it as opposed to a brick heat riser.  I'm also wondering about using a smaller, 25 gallon barrel instead of a 55gal drum.  It seems like this is common in many of the videos/photos posted and I'd like to know what are the main considerations here.  I'm building the system inside a greenhouse with the main goal being to heat a cob bench for bottom heat for the flats, more so than radiant heat from the barrel itself.  Because I'm not so interested in the heat coming off the barrel, what size/configuration of barrel/heat riser would work best?  Thanks for any input and my apologies for any repeat questions.

I have the same question and am still not clear about using the galvanized heater duct.  I've been gathering materials for building a rocket mass heater and was able to get most of the 8" horizontal flue pipe (heater duct) from a recycle center, and did end up buying a few pieces new.  Some of the recycled pieces look different- more mottled and flaky looking than others and the new stuff is much less so, but it's all galvanized...  Am I hearing that older galv. material has more zinc and is thus more toxic?  Just to be clear, most of the stuff from the recycle center looks exactly like what they sell at Home Depot.  How many different kinds of 8"duct are there out there?
I was thinking maybe the the first lengths coming off of the stove might need to be regular stovepipe and then switch to the duct material farther out where it's not so hot, but it sounds like it might be possible to use all duct material if it gets burned off first?  I'm feeling worried about using the wrong material and causing harm... 

Thanks for the succinct answer, Paul.  I posted the the question about the maximum length of horizontal flue pipe because I have yet to see examples of any quite that long, although it's been alluded to that "more than 30'" works fine.  I suppose I'll give it a go with 40'.

In answer to the questions from Ronie, i'm planning to follow all the specs for an 8" system described in the Rocket Mass Heater book.  I've also been pouring through the available video/photos/description I've found on the net. It seems clear there will have to be some playing around with the height of the vertical chimney at the end.  I know there are lots of variables, and do intend to stick to the tried and true for the most part.  That is also why I thought to ask a question on this forum. 
"The person" does have experience burning wood, but never using a rocket stove. 
It hadn't occurred to me to put a whirlybird at the end.  Is that a suggestion?

Thanks for all the great info !  I'm new to this forum, so please forgive any repeat questions, as I haven't yet read everything there is to read...

I'm currently designing a rocket mass heater for a propagation greenhouse here in NW Washington.  The greenhouse is 24'x 12' and the intended purpose is to provide bottom heat for starting flats of vegetable and flower seedlings in late winter/early spring.  The greenhouse is not an earth sheltered design as in the video, but is a solar greenhouse with insulated north wall and dbl. wall poly for glazing on the rest.  I am off grid and had considered setting up a system with hot water tubing on the benches, with wood as the primary heat and propane as a back-up, but once I learned of the rocket mass heater idea, decided to go that route.  And now that I've discovered this forum, I'd like to get as much input as possible before making too many mistakes in the design.

I have lots of questions, but the one most pressing at the moment is about how long I can run the horizontal stovepipe through the thermal mass and still expect to get a draw and/or take advantage of any heat.  I originally thought to run one straight pipe the length of the greenhouse, ~20' or so, but am now wondering about turning the corner and running it back the length to exit near the stove, for a length of about 40'.  Is that too  long?  I haven't seen any examples of RMHs with a run quite so long... 

Any ideas about the maximum length of pipe would be much appreciated!  And any other ideas about using a RMH in this context...