Rocket Mass Heater Pebble-Style Build

This thread will document my build of a pebble-style rocket mass heater based off the “free heat” 2021 rocket mass heater 3d plans with some modifications similar to the fisher price RMH as I’m building it on a wood floor.  Also using David’s Huang's build (https://theartisthomestead.com/rocket-mass-heaters-increase-your-wood-burning-efficiencies-50-to-90-percent/) as inspiration but the finished product likely wont be nearly as pretty.

This is my first rocket mass heater build and I’m still figuring things out so feel free to give suggestions or criticisms.

I plan on using duraboard underneath the core for insulation instead of the firebricks that they used when they were building on concrete.

I‘ll be building it right next to a blaze king wood stove. I plan on leaving the wood stove in place and hooking up the exhaust pipe from the RMH heater to my pre-existing blaze king exhaust pipe with a tee-connection. The current exhaust pipe that I will be connecting to is 8 inch so I will probably go with an 8 inch exhaust for the RMH instead of using the 6 inch juice box connection.

I built the frame out of 2x4’s. Next step will be using 1x4’s for the bottom and siding of the box.

Materials already sourced:
Lumber - local lumber yard
55 gallon steel drum: Azure Standard


Materials yet to be sourced:
Firebrick - Possibly will purchase from dragon tech if I can’t find a local landscape supplier.
Super Wool - Probably will purchase from dragon tech.
Duraboard - Still trying to figure out where to source this. Seems like Unifrax is one of the companies that makes it.
Stovepipe - Hopefully can snag something used from Facebook marketplace or Craigslist but may have to buy new if nothing shows up soon.
Pea gravel/stones/sand - Likely will purchase from local landscaping company.

Looking forward to seeing how your build progresses!

The wooden box for the rocket mass heater is complete. I'll put bricks underneath the bottom to lift it off the wood floor.

I've decided to follow the walker stoves superhot J eight inch core plan for my core(https://walkerstoves.com/product/superhot-j-eight-inch-core-cut-plan/). Once I get the ceramic fiber board and superwool I'll build the core and test it outside. Already picked up some firebrick for the wood feed portion.

Picked up some used barrels locally, was going to get some from Azure Standard but every time it got close to delivery that month they would switch from in stock to out of stock. The blue barrel used to have honey in it, the outside is a bit rusted. The white one had paint in it. I need to clean them out and burn the paint off. Will have to figure out which one I want to cut up to fit around the core and which one I want to put on top.

Found some cheap 8 inch stove pipe tees, one will be a clean-out spot and the other will connect to my current chimney. Still working on sourcing the rest of the stove pipe.

Also picked up some flattish rocks. Will likely mostly use flat rocks with cob in between for the thermal mass instead of pea gravel.

Slowly but surely, making some progress.

You might be happy to use ceramic fibre products in your home but, just be aware there are potential  heath risks especially in an 8” J tube where you will be regularly disturbing the ceramic board when cleaning out ash.
If you do use it then consider coating it with something to stop it dusting into the air.

That's a good point. I plan on using a respirator whenever opening the system. You're talking about coating the inside of the core? What would you coat it with that can withstand the high temps?

The problem is a J tube is pretty much open all the time, I have tried a few different coatings like Fondue cement, Water Glass and Zircon.
None of them are brilliant, the zircon being the best but it only lasted one season before flaking off in some small areas.

Interesting. Doing some touch up paint/coating with zircon once a year doesn't seem too bad. Thanks for sharing that.

The build is complete. As previously mentioned, I followed Walker Stoves Superhot 8 inch core plans for the core. I lined the bottom of the box with perlite clay for insulation and filled around the core with sand. Added some extra brick to lengthen the wood feed and used super wool for a five minute riser. For the thermal mass I covered the stovepipe with cob, rocks, and brick to form a cobglomerate. I used a cement board for a heatshield. Will likely add a stone veneer to it later.

It starts really easy and has a strong draw. The vertical stovepipe is pretty close to the barrel. The top brick layer in the thermal mass has gotten up to around 80 degrees and seems to warm up better now that the cob has dried out some but doesn't really get hot to the touch. Will be interesting to see how much wood I need to use to keep the house warm this winter.

Hi Rex.
I am new to this forum.  Thank you for this contribution.  I am in the research stage and planning a RMH build this spring. The "tee" exhaust to the previously existing chimney and woodstove has me intrigued. I would like to maintain my existing wood stove for now, do the build of the RMH, and later switch the old woodstove to a cookstove - maybe a Walker.  

How is it going so far?  Is your RMH exhaust working out?  Have you fired up the Blaze King, and if so, have you had any complications with the exhaust/chimney combined?

Thanks again, in advance.

The RMH exhaust is working well. I haven't used a RMH anywhere else to compare but the draw seems quite strong. The vertical exhaust being very close to the barrel likely helps a lot with that.

I haven't fired up the blaze king since building the RMH but I don't expect there to be any issues with it. The horizontal pipe has a gradual upward slope until it connects with the "tee", so it's unlikely to act like a competing chimney when running the blaze king. If I were to continue using the blaze king, I would probably have the chimney cleaned out every year to remove any creosote build up, which I shouldn't have to worry about if just using the RMH.

Rex Holliday wrote:The build is complete.

How did paint removal off the barrel go just filling it with rubbish?

Coydon Wallham wrote:

Rex Holliday wrote:The build is complete.

How did paint removal off the barrel go just filling it with rubbish?

I had to do a few burns to get all of the paint off. I wrapped the barrel in wet clay-soaked newspaper like in this video: https://www.youtube.com/watch?v=MpCu7aG3fgI And I burned cardboard and sticks in the barrel. I did not make a pocket rocket but that would probably be the most efficient way to burn all the paint off.

Rex Holliday wrote:I had to do a few burns to get all of the paint off. I wrapped the barrel in wet clay-soaked newspaper like in this video: https://www.youtube.com/watch?v=MpCu7aG3fgI And I burned cardboard and sticks in the barrel. I did not make a pocket rocket but that would probably be the most efficient way to burn all the paint off.

I tried various versions of the pockety rocket to get it to work best, but even then couldn't get more than the bottom half to burn off, even after the bottom got hot enough to warp. I'm suspecting the clay slip paper is the most important part.

I think it is easier to sand off the paint with a wire brush attachment in a grinder.

I got the bottom half with the pocket rocket, the top 1/3 with a plain barrel fire, and finished off the rest with a wire brush on a drill. I'd say wrestling with a power tool like that is not 'easier' than lighting a fire and watching it burn, but there are a number of trade offs. To me I'd go with what ended up being the best way to contain all the gick. Where are all the paint flecks going with a wire brush? I'm thinking all things considered, the clay slip coating done well would make the paint melt evenly all around and give the easier way to capture it as it comes off. However, if much of it evaporates and goes 'away', is it adding significantly to environmental problems?

I think that whatever method you use will produce some nasty side effects whether burning paint into the atmosphere or firing it into the ground ( i should just put down an old blanket) but I have done a fair few now and find a good 4” grinder with a wire wheel whizzer gets a clean job done in less that one hour.
I have a video but I have already posted it several times so I will just the link.
https://youtu.be/LbZTkuiMBRU?si=ylQt-UzznyGDcDeI