I plan on using a 55 gal steel drum as the outer container. Would 2 or 3 layers of cement board, overlapping edges, strapped together tight with plumbing strap or the strap used in shipping where you use a special tool to tighten it work as the burn chamber? Even if it would work I still don't know how I would attach the bottom of the J-tube.

The heat will destroy cement board. Some clay products can take the heat. Concrete products are unsuitable.

Cement board fails under normal fire temps, it would die a very quick death in a rocket stove.

Your idea may make a good mold to pack a fireclay around, though. It is strong enough to not collapse and would survive the initial firing to dry out the fireclay.

But bricks are easier.

Don't!!!

Dan : One more Don't !

Have you been to Rocketstoves.com to Download your PDF Copy of the brand new 3rd edition of Rocket Mass Heaters (?) This is "The Book",
with ~100,000~ RMHs Built word wide, most were made following 'The Book', and 95% of all the 1st time builds (that worked )followed 'The Book'

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rising tide floats all boats ! For the Good of the Crafts! Big AL!

In this video, Matt Walker shows how to use cement board to make a form for a rocket stove core. You build a box from plywood, put in 4" of a mix of clay, perlite, fireplace mortar and fiberglass fibers, then set in the burn tunnel form you've made from the backer board, then fill around it with more of the same mixture. You end up with a highly functional burning chamber, the bottom half of the "J" for an 8" rocket stove system.

The clay mixture will be able to take the heat, but isn't terribly strong. The plan is to encase this core in cob. The backer board will burn out eventually. You also need to build an insulated heat riser for best performance. (Matt has a video for this as well!)

What about using cement board for the sides of bells?

- joe

You could use cement board as part of a bell, but it does not seem useful for the job. A bell needs mass, which cement board has very little of. You want to use brick or other heat resistant thick masonry for the bells. A bell around/over the heat riser cannot be cement board as the heat in parts of that will be much too intense.

By bell, you mean the outside that conducts the heat to the outside? I was going to use a 55 gallon steel drum. As far as the video, I don't really have access to fire clay, other than buying it on ebay, which the shipping is as much as the actual cost. I was thinking along the lines of concrete and vermiculite and rebar.

Dan : Portland cement based Concrete will not stand up to the temperatures of a rocket mass heater !

PLEASE ! Consider going to Rocketstoves.com to Download your PDF copy of the 3rd Edition of Rocket Mass Heaters.

With over 100,000 RMHs build world wide, this is 'The Book' around which the majority have been made. Over95% of all
the 1st time builds (that worked) were made following 'The Book '!

This book contains the information in easy to follow print and illustrations giving you the exact knowledge to come here
knowing that you will be using the same language to describe the Size(s), Shape(s), and Orientation of the RMHs parts,
both to the whole rocket And each other!

It will also save you time money and frustration in the making of your 1st rocket ! Get "The Book'' and continue to follow
these pages ! If there is a rocket in your future your Fellow members will help you build it ! for the Craft! Big AL

I have the Evans and Jackson book on my Kindle but it doesn't work on some pages. (I think its the 1st or 2nd edition). I was able to print out the pages that specifies the dimensions of the inner and outer chamber and burn chamber depending on how you want the stove to perform. I should probably hunt for a steel barrel that would work as an inner chamber. I think the tank in my water heater would be too big as an inner chamber. I have a Mig welder but it will only work up to 1/4" steel.

Dan H. Stand by will send you a P.M. Big AL