3D Plans - Pebble Style Rocket Mass Heater

As per the email request for feedback.  Great images and descriptions, but what I was really looking for that I didn't see was a Bill of Materials.  It's always a shame when you buy too much, or worse, too little of materials.  Also any sourcing suggestions or links would be appreciated; things like fire brick and stove pipe are expensive and getting them from a big hardware store may not be the best option.

Paul, you asked for feedback, so I wrote notes of the questions that came to mind whilst I read through. If you are a proper DIYer these are probably obvious ...

Where does the exhaust go? does it have to be a certain height? Does it have to bend before gonig out of the wall? Do you use double skinned tube?
Steel flat bar - how thick? Does it matter?
What is an orange fire brick? I live in the UK and I am not sure we have such a thing ... if we do, it has another name.
Ditto white fire brick
Any particular kind of sand to get or to not get?
Does the size of pebbles matter?
Can this be built on a wooden floor?
What is the black stuff between the bricks in the diagrams? Is it cob?
The white bricks in the diagram look like wood - it didn't make sense, but took me a while to realise ...
Step 3 D: Does the diagram show an overall 48" or 48" plus at least 3"?
Are the airflow holes both sides of the box? Ah, yours was built against a wall, so I presume there doesn't need to be ... can you build it in the middle of the room?
How are the metal bars attached? do you drill a hole in each end and screw them to the wood?
Is there a good/bad pebble size?

Thanks! I would love to build an RMH!!!

Read and checked all the links. Under other resources "F. Join the Bootcamp team at Wheaton Labs" the link that is there goes to Rocket Oven bundle. Please double check this link.

Overall I like reading about this rocket mass heater. I feel that if you had a master rocket mass heater builder using this pdf as slides in a presentation it would great! I like how the layout and use of picture help to explain how a RMH is built.

Andres asked for some feedback so here it is.  Unfortunately I'm a mechanical engineer so technical drawings and instruction are my thing ;)

Overall it does a good job conveying the info.  I love the intro image.  Ok, now for all the nitpicks:

Intro section: Maybe change to "This is one 3D plan out of a set of six. To see more..."
Intro section and maybe elsewhere:  I think it's normally spelled "3D", not 3d
Page 3: is it really cutters or should it be tin snips? Were pop rivets / rivet gun needed?
Page 7: showing how the metal bars attach might be helpful
Page 11/12: are those the same thickness white bricks?  
Page 12: lower dimension isn't quite lined up with what it's trying to show so it's a bit confusing
Page 14-16: it's really unclear how the top of the burn tunnel is created.  Maybe a cut through view would help?
Page 18: If the pebbles are 2" deep they shouldn't come up to the top of the 2x4s.
Page 18:  Should it say "...leaking into the J-tube..."?
Page 19: Might be worth saying the pipe hole is an oval since it's exiting the barrel off center.   Would have reeeeaaaaaally been nice to get a measurement for that barrel cut-out.  Wrap some paper around the barrel and mark the cuts and then add dimensions.  Maybe next time.
Page 19:  Is that the top of the barrel, bottom or any old chunk?  Flange shouldn't be shown in the image until page 20.  White dimension line should have a line showing what is 16 inches.  The barrel or the funky cut?
Page 20: How do you add the flange?  Bolts, pop rivets? Fasteners sticking out on the inside or outside?
Page 21: How does the pipe drop down to the pebbles?  Two pipe elbows or some other magic?  Tape the joints?  How do you keep the exhaust pipe from dropping lower later?
Page 23: Typo maybe? "To learn more about RMH see the RMH Risers Ebook"
Page 24: Looks like the top of the barrel is missing?
Page 24: Is the jog in the exhaust at the upper right required or a site built adjustment?  Seems like if straight is best, that is how it should be shown.
Page 25: Metal mesh should be on the parts list and maybe a 1" hole saw or spade bit on the tool list
Page 25: Metal bars seem to magically connect to the middle of the 2x4s.  Some detail there would be nice and/or move them up or down to the actual location
Page 26: Seems like the metal bar detail should be moved back onto page 25
Page 27: Upper right pic is confusing.  Seems like extra bricks have been added?
Page 28: Seems like words got jiggled/duplicated between C and D

Mike Haasl wrote:Page 18: If the pebbles are 2" deep they shouldn't come up to the top of the 2x4s.

Looks to me like they do come up to the top of the 2x4s, but shouldn't they be mounded 1/2" over them...?

I don't have it in front of me any more but I thought the 2x4s were attached vertically so the 4" (actually 3.5") dimension is vertical.

Sorry, I was sleep-typing last night...

page 17: photo could use a caption that the left part is a jig to help cut the riser bricks shown on the right...
page 19: what is that crazy art thing on the end of the 8 inch duct going into the manifold and how is it done?!?

Mike Haasl wrote:Page 19:  Is that the top of the barrel, bottom or any old chunk?  Flange shouldn't be shown in the image until page 20.  White dimension line should have a line showing what is 16 inches.  The barrel or the funky cut?

Also, is it 16 inches measured from the manifold top with the flange as in the diagram, or without as it would be following the procedural steps given in these instruction (different order from how they were done at the jamboree).

Page 4, 19 & 20: Do the manifold and flange all come from the original barrel? Is there a trick to sizing and attaching the flange to get an airtight seal (overlaps, crimps, ...)?

Pages 19-22: instructions and photos for steps 3B and 3C are jumbled. The picture of Isaac cutting the manifold on 20 goes with the instructions on 19, then comes the first picture from 19, then the first picture from 22, then the second picture from 19, and finally the second picture from 22 is almost an exact duplicate of the second from 20. The second from 20 was actually before the first from 19, since Isaac did step 3C before 3B, probably the source of this very confusing paragraph...

Hopefully not too much duplication with folks' comments above:

Overall design: is this missing a vertical firebrick at the outside corner of the burn tunnel and riser? Seems like this goes right up against the orange brick that makes up the retaining wall. I don't remember this from the Solarium build, but I'm pretty sure the builds that I worked on this fall had white firebrick for the feed tube, burn tunnel, and riser base.

4: I would add a sidebar about making cob, or if there's a good commercial replacement. The previous comment about a bill of materials would resolve this; x lbs clay, y lbs sand, 2 bales straw, stable fill, etc.
4: 1.5 - 2 barrels are required, a half for the manifold and a full barrel for the radiator.
5: I think the vertical lines are a bit out of place, the step labels make it pretty clear to me.  I would like a heading on this page - something like "An overview of the build"
19: Another angle on the manifold cuts would be helpful.
19+: I would use "barrel" when talking about materials, but once we make a cut or are talking about it as part of the build, refer to it only as the Manifold or Radiator.
19-20: I would swap these pages. One page dedicated to cutting the manifold and installing the flange, then a second for installing it on the jtube.
22: Highlight the flappy-dos in the top image.

Thanks so much to everyone whose provided feedback! We’ve made a pass to include some of the suggestions and we’ll keep working to make this plans better in future iterations.

Im trying a new compression algorithm that reduces the file size drastically  but retains quality so the new .pdf only weighs 13mgbs, any feedback on this would be lovely too.

Thanks again permies community!

Is there a thread to discuss this design at a conceptual level? I'd like to know how the decision on length of duct was arrived at...

Coydon Wallham wrote:Is there a thread to discuss this design at a conceptual level? I'd like to know how the decision on length of duct was arrived at...

The size of the bench was decided and that led to the length of the duct.

paul wheaton wrote:The size of the bench was decided and that led to the length of the duct.

How has the efficiency been? Seems like a short run that would send lots of heat out of the exhaust. Does the juice box mitigate that enough? How much longer could the duct be while using the juice box design?

Wow! These plans with lists and measurements are GREAT! It's so much easier to build something with exact dimensions like what's laid out in this resource. THANK YOU!

Coydon Wallham wrote:

paul wheaton wrote:The size of the bench was decided and that led to the length of the duct.

How has the efficiency been? Seems like a short run that would send lots of heat out of the exhaust. Does the juice box mitigate that enough? How much longer could the duct be while using the juice box design?

If we are talking about a six inch system using an 8 inch duct - it can be very long.  

If we are talking about an eight inch system, the total length of duct could be about 60% longer (guessing).

Hi all, I am making my own 3d model using the instructions given in order to fill in the gaps in the bill of materials and to help me understand the project better and I’m noticing some inconsistencies.

First, there is a brick missing from the riser end of the burn tunnel (unless the burn tunnel is supposed to feed the 8” duct and bypass the riser).

Second, there is a layer missing from the feed end (three bricks 4.5” wide mounted vertically is only 13.5”, you would need another layer 2.5” wide to add up to 16”).

Third, 125” overall length would mean that a 10 foot long 2x4” isn’t long enough, even with the boards attached at the ends.

Lastly, the spacing of the feeding port is not adding up, I’m getting around 6.25” from the back edge and about a foot from either of the sides, measuring from the inside of the 2x4s.

I’m assuming that the bricks are clay fireplace bricks for the base and insulated refractory bricks for the burn tunnel, but links to commercially available products for all the parts would clear up any confusion.

I’ve never seen a rocket mass heater in person, but I’m interested in building one outside in my detached garage, I’d like to have one in my basement eventually, I’m not sure my floors could handle the weight of having one in my living room.

Thanks for doing all this work, the 3d model looks fantastic! I plan to post my model when I finish it.

If a pea gravel mass creates lots of air space, would a mix of pea gravel and sand offer a noticeable improvement in performance?

paul wheaton wrote:If we are talking about a six inch system using an 8 inch duct - it can be very long.  

If we are talking about an eight inch system, the total length of duct could be about 60% longer (guessing).

I made a new thread for my follow up questions...

Nick Neufeld wrote:If a pea gravel mass creates lots of air space, would a mix of pea gravel and sand offer a noticeable improvement in performance?

What I've gathered is that the air needs to flow through the pebbles to distribute the heat. Seems to me sand between the pebbles would just block airflow and prevent most of the heat from being drawn from the ducts. The first version they tried like this had pure sand in the box and it was too insulative.

Before I would buy there are things I need to know:

footprint size
can I cook on the top of the barrel
is a batch box version available and if not, how long will this burn with a fully loaded feed tube
will one feeding warm the rocks or will I need to feed it for more than 30 minutes once it's burning well (assuming, say, a 42 fahrenheit room/starting temp of rock/bench, and hard, dry wood)

thank you

Robert Kish wrote:First, there is a brick missing from the riser end of the burn tunnel (unless the burn tunnel is supposed to feed the 8” duct and bypass the riser).

Second, there is a layer missing from the feed end (three bricks 4.5” wide mounted vertically is only 13.5”, you would need another layer 2.5” wide to add up to 16”).

I was just going to make my own comments and found that Robert had already posted them, but I notice that no one has addressed these issues.
Will there be a version 2 of this plan that addresses some if not most of the feed back given here?

Also, anybody have a source for the fire bricks? All I have found locally is the non-insulated fire brick that is 1 1/4" thick. I can't find any that are 2 1/2" thick.

Tim Osborn wrote:Also, anybody have a source for the fire bricks? All I have found locally is the non-insulated fire brick that is 1 1/4" thick. I can't find any that are 2 1/2" thick.

I'm not near any major metropolitan centers, but found a landscape company in a nearby town that sells all sorts of bricks and stone. They had a pallet of each size of regular firebrick. They charge the same for both sizes (~$3+ US a year ago). The local big box stores also sell the 1 1/4" bricks in a bundle of 4 or 5 for about the same price (these actually came down in price from last year when I checked a month ago). They are marketed to wood stove owners as replacements for what those burn chambers are lined with.

Page 14: Robert and Tim have mentioned problems with the measurements here already, but another particular I'm noticing is that the measurements given have the feed tube 8.75" from the wall side of the frame and 10" from the room side, but the diagrams clearly show the feed sitting closer to the room side of the frame.

Also, there is a clip in the movie where Isaac is telling the camera that the feed tube is too close to the frame on the short side and he is planning to cut the frame and bump out the cross piece and/or stick a sheet of metal in there. I did not see this mentioned again. Did he decide it was okay as is or was something done there?

In previous movie about pebble style rmh, to keep the sides from spreading you were using plumber's hanger strap instead of the steel flat bars that are called for in the plans. I'm just wondering if the hanger strap was found to be not strong enough?

Thank you for this gift.

Andrés Bernal wrote:Thanks so much to everyone whose provided feedback! We’ve made a pass to include some of the suggestions and we’ll keep working to make this plans better in future iterations.

Im trying a new compression algorithm that reduces the file size drastically  but retains quality so the new .pdf only weighs 13mgbs, any feedback on this would be lovely too.

Thanks again permies community!

Hi Andres,
Is the updated version available somewhere?

Tim Osborn wrote:
Hi Andres,
Is the updated version available somewhere?

The latest version is 6a and you can find it attached to this thread or if you got it in the daylish download it again.

Thanks for all the new feedback received! We’re analyzing it and including more stuff. Atm I’m finishing up the last details for the rest of the Free Heat Plans and will make another update to this one soonish, when those are wrapped up.

Andrés Bernal wrote:

Tim Osborn wrote:
Hi Andres,
Is the updated version available somewhere?

The latest version is 6a and you can find it attached to this thread or if you got it in the daylish download it again.

Thanks for all the new feedback received! We’re analyzing it and including more stuff. Atm I’m finishing up the last details for the rest of the Free Heat Plans and will make another update to this one soonish as soon as those are wrapped up.

Thanks for all your hard work on these! Really appreciate it.

This is a really cool artifact, I look forward to the new iteration and perhaps more such plans in the future...

Barbara Kochan wrote:Before I would buy there are things I need to know:

footprint size
can I cook on the top of the barrel
is a batch box version available and if not, how long will this burn with a fully loaded feed tube
will one feeding warm the rocks or will I need to feed it for more than 30 minutes once it's burning well (assuming, say, a 42 fahrenheit room/starting temp of rock/bench, and hard, dry wood)

thank you

Hi Barbara, thanks for your interest. The footprint is 38” x 125” and there’s not a batch box available at the moment.

Stephen, from the bootcamp, has been staying at the solarium and he kindly helped me out with these questions. Here are his answers:

Can I cook on the top of the barrel?
I've never tried this. However, I know that the barrel can become -extremely- hot. If you have a rack sitting on top of the barrel so that your food doesn't touch it, you would be able to dehydrate it easily.
It's also easier to reach exceptionally-high temperatures if you use thinly-sliced wood. There's more surface area compared to mass, which makes a "skinny stick" burn quickly and generate more heat than large logs (I refer to this more, below).

--> How long will this burn with a fully loaded feed tube?
If you use large logs (4 to 6 inches in diameter, for example) , it will burn longer. For the Solarium RMH, I think it would burn close to 35 - 45 minutes.
If you use thinner pieces of wood, it will burn quickly in comparison. I would think a full burn chamber of "skinny sticks" would be reduced to embers in 20 minutes or even a little less time than that.

--> Will one feeding warm the rocks or will I need to feed it for more than 30 minutes once it's burning well (assuming, say, a 42 fahrenheit room/starting temp of rock/bench, and hard, dry wood)?
Based on my observations, you will feel warmth in the rocks after about an hour. You will feel it in less time than that if you've used the RMH the previous day, and definitely if you've use it several days in a row.
When going to bed at night, I typically feed the fire for 45 minutes to an hour prior to going to bed.  The mass still feels warm  in the morning, and you can sit on it to warm up if you feel a chill.

Hope you find this helpful :)

Andrés Bernal wrote:

Unable to access. I'm not going to build a rocket heater so I'm not upset.

Ellen Johnson wrote:

Andrés Bernal wrote:

Unable to access. I'm not going to build a rocket heater so I'm not upset.

Ellen, did you do something that would give you access?

Hey friends,

Hoping someone can help answer a couple Q's about these plans, and also hoping that I'm posting in the right place!

We're first-time builders aiming to install this RMH in our new home (post & beam / tongue & groove) and definitely don't wanna eeeeeff anything up.

Question 1 is, could the bench be essentially doubled in length and function just as well? We are hoping to run a longer bench through a bedroom and into a living room. And, it would actually run under an "empty" wall of tongue & groove separating the two in order to do so... hopefully no issues there?  

And 2: Could the wood feed conceivably be shifted 90 degrees? Ie, be installed to the side of the barrel, rather than the "butt" end?  Or might such a joint there cause issues with draw or else-thing?

We'd also be interested in consulting with a seasoned RMH'er in person in/around southern New Mexico just for some peace of mind, if such a person exists! Or maybe this can be done remotely with good communication and the sharing of drawings? Definitely open.

Thank you kindly for sharing your thoughts! These plans are great and we appreciate them and you.

Brittney Gee wrote:Question 1 is, could the bench be essentially doubled in length and function just as well? We are hoping to run a longer bench through a bedroom and into a living room. And, it would actually run under an "empty" wall of tongue & groove separating the two in order to do so... hopefully no issues there?  

And 2: Could the wood feed conceivably be shifted 90 degrees? Ie, be installed to the side of the barrel, rather than the "butt" end?  Or might such a joint there cause issues with draw or else-thing?

Are you specifically looking for a lightweight and/or temporary RMH build? If not, it's likely like you would benefit from a cob style design over this pebble style.

I'd suggest looking over Ernie and Erica's book on RMHs (the rocket mass heater builders guide) before settling on a design. My understanding of the principles spelled out there is that you would be able to to build something to meet your criteria on both points.

If you go with a variation of this pebble style but include a longer bench, I would suggest using an 8" exhaust pipe that is flush with the horizontal ducting. The 6" vertical exhaust going down into the 8" duct for stratification effect used on this iteration seems to limit the draw through the system. Systems this size without the "juice box" design reliably work with much longer runs of ducting through the mass. Specifics on how to calculate those numbers for varied situations are in the E&E RMH book.