The RMH Builders guide build-pic heavy

Hey guys, Eric Hammond here.  When building my house I've always designed it to have a rocket mass heater.  I had purchased the Ianto book, and frankly found it quite poor on information and not a valuable asset.  After posting here on a discussion about shippable rocket cores, someone had pointed me to Ernie and Erica's book(sorry I can't remember your name and I'm too lazy to look it up).  This book was perfect for me.  I read it all winter and now that its July, I'm worried as hell about winter!  I read the book cover to cover and encourage you to do the same.  Please purchase the book The Rocket Mass Heater Builders Guide  Read its entirety and then proceed to build.

This will be a step by step build from how I perceived the instructions in the book to go, plus all of my mistakes.  I am in SW Missouri so prices may be different if your area.

First was sourcing the fire brick and building a mock-up to see how it was going to sit in the house.  I'm going to build a bench that will become the kitchen table/dining area.  It's located in the SE corner of my shop house, It will be built on 4 inches of fiber reinforced concrete.

The book did not list a fire brick count.  I counted bricks from the pictures listed to get the amount I needed to purchase.  I think it was close, but not quite accurate.  By my count in the book I needed 56 full size bricks and 28 half bricks/splits.  I purchased 58 full size fire brick but I think I really need 32 splits.  I haven't finished my heat riser yet, but I think it's going to come out short.  I purchased the firebrick from ACME brick in Springfield MO on East Battlefield.  Both the full size brick and splits were $1.49 a piece.  This equates to $138.52 with tax. The correct total of bricks ends up being 56 full size and 34 splits, two to cover up the burn tunnel after completion.  You might consider two extra full size bricks to practice cutting!

Here's the mock-up in the area where the RMH will be:



I'm going to stop here and see if the images show up, and then I will continue!

They had refractory thin set at Acme brick, but it was 77 dollars for 50lbs and that seemed a bit steep.  In the book, they mention clay slip made from fireclay as a great alternative to regular mortar, especially if your using new firebrick that the dimensions are uniform.  There's a place called L+R in Nixa on main street that supplies clay for all the pottery shops in SW Missouri. I went in there like a moron and said " I need fire clay whatever that means and 25 lbs of it"  They straightened me out and fixed me up.  They sold me 25 pounds of something called Hawthorn 35. It's a Missouri based fireclay.  Cost with tax was $9.89 for 25 lbs.  I've never made clay slip.  My sister a potter, recommended 2 gallons of water for the 25 lbs of clay.  I used a paddle wheel in a 5 gallon bucket. It turned out awesome.





This made about 2.5 gallons of clay slip.  I was worried at first this wouldn't be enough, but I now think it will be

It's suggested in the book to insulate underneath the burn chamber, regardless of what surface the heater is to be built upon.   I believe this is to keep the burn chamber as hot as possible for the most efficient burn.  They recommend clay stabilized perlite.  I sourced the perlite from the local MFA(feed store for farmers).  They sold me 4 cubic feet for $20.  Came in a large bag



Looks like this inside



I added a bit to a 5 gallon bucket and mixed in the fireclay slip to play with some of the materials I'm just not familiar with.  It makes a really cool substance.  If you squeeze it together it holds shape, but also breaks apart easily.  I mixed enough slip to make it brownish.

With a little help from the wife I laid out right where the base layer/pad needed to be.  The barrel, needs to be 18 inches from the wall if you intend to use a heat shield on the wall.  Helpful tip: the barrel sits on this base layer pad, so if you measure 18+ inches from the edge of the bricks, you will have the correct clearance for the barrel.  I had trouble deciding how I was going to lay down my insulating layer of perlite, I decided to place a layer of random found bricks around my base layer and house the base layer inside of this layer, on top of the insulation.



Puppy help

Eric, your photos are either private, or not visible for people not logged in, in google. Could you fix that please?

I was afraid of that.....it seemed too easy.  They showed up on my phone and computer just fine.  I tweaked google a bit and added all the photos to an album that is permies, did that seem to make a difference?  I will hold off posting anymore until I can get this resolved.

Please let me know!

It appears with google photos, you must click on each of your photos, click share, then click link, once it provides a link THEN you can copy the images address.  They should be working now correct?  If so I will move on

It's worth noting at this point, they STRONGLY recommend laying out all the components for the heater and making your final placement based on where the roof penetration is.  I did not do this.  This is a steel building and my area to come out of the roof can be anywhere in about a 4 foot radius, so the positioning of the roof penetration really isn't critical.  

I had trouble deciding the width of the bench.  I think I'm going to go for 30 inches.  I laid down blue masking tape, the outside edge marks 30 inches to the wall.  I think I will build right up to the wall with no air gap.  Seems like its been done both ways.



As a random fact, in the next picture you can sort of see where we tested various colors of concrete stains before choosing the final color.  Knowing the bench was going to go over it and cover it up, we tested this area.  The concrete stain is soy based called soycrete.  Works really good.

At this point liking how it mocked up in relation to the bench


I went ahead and traced the outside of the red bricks with a pencil, keep a sharpener handy(pocket knife) because concrete and brick eat through a pencil.  I then carefully removed the inside firebricks, If I shifted a red one, I could line it up with my outside line.  I then used the pencil to trace the inside.  Once both edges were confirmed I glued each brick down with a few dabs of silicone.  This silicone is the ultimate in silicone technology.  Its for making oil pan gaskets on 7.3 diesels, when it dries you don't even need bolts its so strong....but I suspect any silicone would do.   After all the bricks were glued to the concrete with silicone, I went ahead and caulked the seams to the floor and between each brick to prevent perlite from escaping around the house.







The puppies never stop "helping"

The recommended depth for the clay stabilized perlite under the burn chamber was 1-2 inches.  I went ahead and marked the bricks at 2 inches all the way around with a yellow paint pen so I could tell how much I was putting in.  I recommend the stainless steel ruler.  It works great for measuring things less then 14 inches and easier then a tape measure.

I made the clay stabilized perlite a half a 5 gallon bucket at a time. I know crocs went out of style, but they are super comfy, please don't judge.







I went a little overkill.  I actually went about 2.5 inches. This equated to approximately 2.5 full buckets worth.  I still have A LOT of perlite left in my 4 cu ft bag

The thing about insulation is that it must not be compacted to work.  Air is the insulation, its the tiny air pockets that allow it to work.  If you placed an R-19 insulation made for a 6 inch wall cavity into a 2x4 wall and squeezed it together, it would not be an R-19 anymore, perhaps even less then an R-13 made for a 2x4 wall.  So in our case, I do not want to compact the perlite.  I placed each block carefully into place, of course the heights do not match up.  Using a dead blow hammer and a level I tapped each block flush with each other and leveled the pad while paying special attention to not compress the material too much, and to keep the gaps between blocks as small as possible.  This is a critical step.  If the foundation is not level, everything built upon it will be of poor quality





I grouted around the entire outside of the firebricks to the red bricks to add support from the outside.  I used a type N mortar found at home improvement stores.  I had a open bag special $3 dollars for 80 pounds.



I had a small gap because my fire bricks were apparently not the correct size I reckon so I filled it with perlite and mortared over the top.

To make the bricks dimension-ably stable from the inside, it's recommended to place sand in between the cracks.  This step took a lot of time, I was very particular to get everything packed in great.







All the little grains of sand are different shapes and sizes, so this was taking FOREVER, like over 3 minutes, so I had a wild idea to just brush the type N mortar into it with the dry powder and then I was going to take a water spray bottle, wet the surface and lock all the bricks together.  This didn't work at all. For some reason, even though it was a small consistency, it was lighter and hard to get to penetrate the cracks.



I gave up on that, vacuumed it out with a shop vac and went back to sand.

After I got it done, it DID make for a really firm, really stable building surface that was well insulated from the ground.



It's worth saying that this point, the 4 outside bricks, two on each side, are not really talked about in the book, you could place these incorrectly if you weren't careful.  They are used to support the lower 55 gallon drum.  So if you mock up your bricks without mortar first, you can estimate the middle point of the heat riser and pull out a radius from that center point to figure where the barrel will sit.  The bricks must fall on that arc path to catch the rim of the barrel.

Looking so good so far!

Excellent Job so far Eric, Keep up the good work!

The next layer was mocked up without mortar.  The brick closest to me is the only one that has a set dimension.  I want it to cover up that gap I had that I mortared over.  I covered it by maybe a 1/4 inch.  Moving the back brick I could change overall burn tunnel length.  The bricks were centered on the pad left and right and the burn tunnel width was set to 7.5 inches wide by 24 inches long.



Once I found their final place, I again traced the inside and outside with a pencil, then numbered them clockwise, so I wouldn't forget where they go.

Each brick was then removed from its place.  I used two buckets to lay the bricks. One bucket, half full of water with a sponge and putty knife in it and the other bucket full of my clay slip.  I had to take the paddle wheel to the clay slip each day to keep it mixed well.  The spot where each brick is intended to lay, I moistened with water from the sponge.  BE CAREFUL not to wipe off your pencil mark....I did this only once.  Per the book each brick was then dipped in the bucket of water.

These next photos show the process, but they were for a different level of bricks so it shows the clay slip being applied to the wrong edge of the brick.



Then dipped in the mortar/clay slip to be laid into place.





I laid A LOT of the bricks in the assembly this way before I changed my method.  This worked just OK, as I got further along I found it easier and better, to use a paint brush(the one I used for the sand) and brush the clay slip onto the surface of the brick I was about to lay and the area where the brick was going to mate to.

In the first layer each brick was laid with slip, tapped together with my dead blow hammer and leveled and plumbed.  If you wait a few minutes the bricks pull enough moisture out of the clay slip that it becomes firm and smooths out easily with your finger.  I smoothed each gap.  Using my scrubby sponge and putty knife, I remove the excess clay while I work to keep it all tiddy.





First layer is complete!

NOW THIS PART IS SUPER IMPORTANT  I did not do this or even think about it until I got to the point of needing to make the first/lower barrel. At some point the lower barrel has to be cut around each of these layers of bricks to fit over the burn tunnel.  Get a piece of paper and measure outside to outside of this layer and height write it down, so that you will easily have your dimensions to cut your barrel with later.  I wish I would have thought of this, but I didn't and now I will have to do math!  Do this for each layer and you will thank yourself!

The next layer went just as smoothly.  Mocked everything up, dipped, slipped and set into place, paying special attention that the inside edges form a smooth, uniform edge. Disregard where the outside edges fall.  Again, after this layer is finished, measure outside to outside where the lower barrel will sit.

Hi Eric;  Looking good.
A few questions ; what is your plan for heating your mass? I know it will be your kitchen table. Are you using piping or a bell design ?  Is this an 8" or 6" system ?  What is your plan on a riser? To use split brick ,insulated brick , full brick ? Are you going with a standard square  riser  or building octagonal?  Have you heard about using ceramic fiber blanket as your riser ? If not investigate the (Five minute riser by pinhead) on the donkey pro boards.  Its quite the invention . My build this summer will use that as a riser and hopefully ceramic fiber boards as my core.  I also will replace my fireclay perlite riser in the greenhouse /studio with a five minute riser.

thomas rubino wrote:Hi Eric;  Looking good.
A few questions ; what is your plan for heating your mass? I know it will be your kitchen table. Are you using piping or a bell design ?  Is this an 8" or 6" system ?  What is your plan on a riser? To use split brick ,insulated brick , full brick ? Are you going with a standard square  riser  or building octagonal?  Have you heard about using ceramic fiber blanket as your riser ? If not investigate the (Five minute riser by pinhead) on the donkey pro boards.  Its quite the invention . My build this summer will use that as a riser and hopefully ceramic fiber boards as my core.  I also will replace my fireclay perlite riser in the greenhouse /studio with a five minute riser.

Hey Thomas, good questions.  This is an 8 inch system,  and I intend to use piping inside of the mass.  As far as filling the mass, I'm open for options there, but I'm kind of on a time constraint( I say that but its only July and I have a bit of time before winter hits) I was considering something easy like pea gravel although I know this isn't the best material.  If I could mix the pea gravel with a thicker clay and rid it of most of the air pockets do you think It would make a better mass?  Any suggestions on the fill for the mass?

As far as the heat riser, it is a standard square,  split firebricks that will be wrapped with ceramic fiber blanket.  This is my first build so I'm doing everything by the book(literally Ernie and Erica's book) with very little experimentation.

I am about to purchase the ceramic fire blanket, It's a much better deal if I buy 25 feet, but I don't need that much.   The one I'm looking at is 1 inch thick, 24 inches long and 25 feet long.  Its about 99 dollars.  I estimate I need about 8 feet, so I could but a roll that's 12.5 feet but its about 60-70 dollars.  Would you be interested in splitting a roll to save cost and I send you my left overs?

This is the one I'm looking at:

Fire blanket on Amazon

Hi Eric;  About your mass.  Pea gravel would be my last choice. Are you encasing this mass with something (ie brick, rock, )?  If so then your job of filling the mass just got easier.  If you build a bench entirely out of cob then your mixture must be just so...  If you encase your mass, lets say with clay brick, now the cob only needs to be mud. Use large rocks placed carefully around and eventually over your already cob covered flue pipe and fill everything around them with a clay type mud, no need for sand, no worries if some plain old dirt gets mixed in, you just want no air pockets. When I built the mass in our studio I had slate , lots of slate,  So I made a Cob and slate lasagna ...layer of slate layer of cob...  I actually mixed cob with sand to do this not realizing it was serious overkill on work.  The brick holds in and hides everything as well as adding more mass to hold heat. Remember rock has a better heat retention than cob, so more rock less cob means less mixing of cob more holding of heat !