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rocket mass heater slopiness

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Hi I'm attempting my first rocket mass heater based off the 3rd edition of ianto Evans book. We we're unable to do the brickwork in a 7 inch by 7 inch square or 6 by 8 rectangle without breaking lots of fire brick. We got close (6 and 1/2 by 9 in) and decided to roll with it and it works. The other questionable thing was the mortar, maybe I used too much sand, it was hard to apply and didn't seem to seal the gaps very well between the bricks although I stuffed as much in as I could. For the insulation container we cuth lath and attached with bailing wire, but this expanded buy an inch and a half once filled with perlite and vermiculite thus leaving approximately .75 in space between barrel and insulation. also its a bit wonky and has to touch the barrel in some places. however when fired the smoke still came down like it should, smoking out the bottom of the barrel. should i rebuild, partial rebuild or go forth? thanks
Rocket Scientist
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Location: Upstate NY, zone 5
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Hi Daniel, welcome to Permies. Sorry none of us with information seem to have noticed your post when you made it... I hope you were able to get your heater running well.

Do you still have just a test build outside? (If you have smoke coming out from the bottom of the barrel I would hope it is outside ) I presume you are working on a nominal 8" RMH. If you built the feed and burn tunnel 6 1/2" wide by 9" high, that should work fine. You can let an inch or two of ash stay on the burn tunnel floor without problems and maybe improve the function. The heat riser is better square or best round, but if it burns clean you are probably okay. You can make the riser perfectly 7 1/2" or so square by setting the bricks in each course pinwheel fashion, alternating direction so each course anchors the ones above and below. The small step in the sides going from burn tunnel width to riser width will not hurt.

The insulating jacket expanding so close to the barrel sides is likely to be a problem once you try to install the core in a complete system. You want a couple of inches clear all around if you can get it. I would remove the lath and perlite, and turn the lath around so the straighter lines run horizontal instead of vertical. That along with more frequent wire ties should keep it from expanding so much when you fill it with perlite.

What kind of mortar were you making? Clay and sand? You need to use very fine uniform sand for this mortar, and mix it very thin and wet so it can make a thin layer when applied. The bricks need to be wetted before setting, too, or they will suck the water out of the mortar too fast and make it impossible to adjust. Clay-sand mortar is just to fill all the gaps, not to glue the bricks together. The good part of this is that it is easy to disassemble and clean the bricks and reuse them.

If you have any other questions, please ask, and we will answer faster.
What are you doing? You are supposed to be reading this tiny ad!
A rocket mass heater heats your home with one tenth the wood of a conventional wood stove
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