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rocket mass heaters

First attempt at test core for 6" RMH

Posts: 105

Location: Coastal Maine

posted 7 months ago

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After browsing (aka lurking) here for years and asking oddball questions, I have finally attempted my first RMH core. Knowing that I would get something wrong, I poured a flat concrete surface outside to build on. I used the general measurements of a 6" batch box RMH, altered slightly to meet the dimensions of the standard bricks in my stash.

Interior is 8" wide by 13+ high, split fire brick lined and K-26 IFB on face for the surrounding. I started with 30" deep, knowing it was deeper than standard but trying to use the wood I already had cut. While it did burn, it certainly didn't "rocket."

I cut the firebox length back to 21", using the same 1x3 secondary air channel.
Again, it burns well, with smoke at the startup and clear exhaust after it gets going. Still no audible "rocketing."
I'm wondering if I've pushed the secondary air riser too close to the port, or maybe too high?
System riser is IFB, octagonal (close to 6" diameter) and 54" high. I did not add the wedge at the base, yet.

There is no chimney attached since I'm only testing the core at this point.
When I install the finished unit in the house, the existing dual flue brick chimney is 30' with plenty of draft.

IR thermometer exceeds its 1000 degree F limit when pointed into the middle of the firebox.

Suggestions, please?

Thanks,
Randy

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thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 7 months ago

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Hi Randy;
Excellent move to build a test core ahead of time, more first-time builders should do so.

So, bad news about hearing the burn.
J-Tube cores rocket (or roar like a dragon)
Batchboxes do not, at least no more than a box stove does.

The secondary tube should run flush with the floor bricks and butt directly up to the port entrance.
The riser stub should sit 6" tall from the floor (tube)

30" was stretching quite a bit, 21-24" should do it better.

Consider buying remote temp-sending units and placing one midway up the riser and another at the top of the bell.
They are not very expensive and it is nice to see how much your stove creates during full burn, how hot the top of your bell gets, and how long it stays warm inside.

Your mock-up looks great!

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 7 months ago

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Hey Randy
I was looking at your photo.
Is that your secondary tube I see lying on the lawn?
If it is, then it is designed wrong.
The stub should be within an inch of the port opening not several inches back.

20210107_141247-1.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 7 months ago

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Thank you Thomas, and yes that is my secondary air tube.
BUT, the stub comes from a truck exhaust pipe, so the top of it is bent and sits right close to the port.
I was wondering - maybe it is too close?

Randy Butler

Posts: 105

Location: Coastal Maine

posted 7 months ago

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The stub for my secondary is unconventional, but it is stainless!

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Peter van den Berg

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Location: +52° 1' 47.40", +4° 22' 57.80"

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posted 7 months ago

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The core will produce an audible roar when all of it is hard firebrick. The soft ISB of the riser is damping the sound. I've done an open system a couple of years ago, hard firebricks all over, brick bell over it as well. It wasn't a large house and one could hear the roar everywhere, including the loo.

In your case, depth of the firebox could be 21" to 22" maximum. Riser 6" square, no need for a wedge at the base. When you built the riser as not 6" square because of the brick size, that's a mistake. In the picture the riser looks as wide as the firebox, that won't work either.
As long as the firebox is open, leave out the secondary channel altogether. Assuming everything is to specs, it should work. The secondary channel is way off specs, you should adhere to the designs that work, not just something you have lying around. Tell me what size of bricks you have there, possibly you'll need an adjusted base number to get things right.

regards, Peter

Peter van den Berg

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posted 7 months ago

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Randy Butler wrote:
System riser is IFB, octagonal (close to 6" diameter) and 54" high. I did not add the wedge at the base, yet.

How close to 6"? That 54" is far too high, 44" is sufficient, higher doesn't add anything.

I would suggest that you build a very simple core, bricks on edge, firebox and simple square riser. It won't produce a good roar because of the IFB though. Use some clay and fine sand to mortar the bricks together, this design is very susceptible to air leakage. You'll get a correct idea what the thing is capable of this way.

regards, Peter

Benjamin Dinkel

Apprentice Rocket Scientist

Posts: 362

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posted 7 months ago

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thomas rubino wrote:
Consider buying remote temp-sending units and placing one midway up the riser and another at the top of the bell.
They are not very expensive and it is nice to see how much your stove creates during full burn, how hot the top of your bell gets, and how long it stays warm inside.

Hey Thomas, those temp-sending units sound very interesting. Can you give me a link to an example?

A good starting point to all RMH research: https://permies.com/w/rmh-resources and here https://www.permies.com/t/41635/Top-Questions-Rocket-Mass-Heaters
A good starting point for permies.com questions: https://permies.com/w/how-permies-works

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 7 months ago

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Hi Ben;
Unfortunately, they are not wireless.
Here is the riser sensor https://www.amazon.com/dp/B0BYSCNY2W?ref=ppx_yo2ov_dt_b_fed_asin_title
And here are the lower temp probes and the temperature unit https://www.amazon.com/dp/B018QHQSB8?ref=ppx_yo2ov_dt_b_fed_asin_title

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 7 months ago

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Peter and Thomas -

Thank you both so much. Now to elaborate on the questions.

The riser is really close to 6" octagonal, although a bit skewed in a couple places.
This is intended to be a trial, knowing I need to be more precise in the final construction.
And the riser looks really large because I put a second layer of IFB all around.
I ran the height to excess, only because I got no roar. Very easy fix.

I'm not sure if it's an "across the pond" consideration, but I have read a lot about building the core dry-laid, allowing for easy repairs.
I'd love to hear discussions on the clay slip vs dry-laid opinions.

The standards I see for the 6" system indicate roughly 7 SI Total Air Intake with 1.4 SI of that in the secondary.
On Matt Walker's design, he noted a better burn with secondary being just over half of Total Air Intake.
Could that be due to his "riserless" build?

And I do plan on a door, I just haven't built it yet, so I attempted to simulate it with the IFB.

This is such a great place for education and information exchange. Thanks to the contributors and administrators for all their efforts!

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thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 7 months ago

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Hey Randy;
I stand corrected about burning a batch with no door and having it roar.
All of my builds have had a final door before lighting it off.
Once you have a door it will sound like a box stove wide open.
J-Tubes always roar.

Clay slip versus dry stacking.

Both of my current batches are built dry stacked with new bricks.
Both have a steel tension frame to keep the bricks snug but not tight.
They are entering the third season with no issues of air leakage or bricks cracking.
The steel frame is kept finger-tight and is periodically checked during winter.

My newest build "The Shorty Core" has a brand new approach to sealing a core.
It utilizes 1/8" of superwool gasket on every joint.
It also has a tension frame to keep things snug.

During test firing, the gasket paper performed exactly as expected, sealing the bricks yet allowing thermal expansion with no cracking.
Only time will tell if this is the "new best way" to build.
This core is being moved indoors next week and long-term testing will commence.
Results will be shared on the performance of "Shorty Core" as well as the durability of using the superwool gasket to seal things up.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Benjamin Dinkel

Apprentice Rocket Scientist

Posts: 362

Location: Province of Granada, Andalucía, Spain

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posted 7 months ago

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Thanks Thomas, and sorry Randy for sidetracking the thread.
One more question and I hope I can leave the attention completely back to your exciting first build.

Do I understand the thermo elements correct, that you put them in place and plaster/cob over them and when you want to measure you connect the wire to the meter?

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 7 months ago

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Yes, correct.
The riser probe is inserted through a riser brick with only a small portion exposed, the wire is fed out through the bell and accessible to monitor.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 7 months ago

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Hi Benjamin -

no worries on the thread diversion. Of course all you really did was to beat me to the question! So, Thank You!

Scott Weinberg

Posts: 316

Location: North East Iowa, USA

posted 7 months ago

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Measuring what is happening inside the stove, I feel gives a great deal of satisfaction for the dollar spent. In other words it does not cost much to place the probes when building,

Just knowing how things internally change, depending on what you do, can be research rewarding. This probably will not make your stove better, but can make you better at using your stove.

In my case, if we do more than 3 burns in a day, the question always comes up, When we know- my exit flue temp starts getting higher as the mass of the bell gets hotter. Which only stands to reason. This at least allows us to size up if additional burns make sense.

Simple math tells us, with the stove preforming well, having a exit temp of 175-200 is perhaps ideal for my stove. and when approaching 300, my mass is adsorbing less % of the produced heat. ( temperature spread between produced heat and mass is less) The stove works equally well across the board between these temp spreads. Knowing these temps, Just helps us to decide, when good is good enough.

That might be as clear as mud. And might be more fun than anything else. Never hurts to know!

Scott

https://permies.com/t/241976/Batch-Rocket-single-bell-build

Randy Butler

Posts: 105

Location: Coastal Maine

posted 6 months ago

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Any more thoughts or discussion on dry laid FB, dry laid with Superwool or Ceramic Paper, or clay slip for the firebox core?

Thanks for the insight,
Randy

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 6 months ago

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Hey Randy;
I can say that dry-stacked FB works.
I have two batches entering their third season with no issues at all.
I am using a tension frame on them.
Clay slip works, it eventually cracks and needs to be reapplied.
My new Shorty Core is the first that I know of to use an 1/8" Superwool gasket between bricks, it also uses a tension frame.
I will let you know next summer how well it worked over its first season.
I can say that it worked great throughout the test firing and curing process that we did this spring.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 6 months ago

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Good Morning Thomas

Are you running the FB laid on edge or on face?

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

3386

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posted 6 months ago

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On edge, with 2.5" strips of SW gasket on all sides with Shorty Core.
On edge, using only new bricks with no mortar on the dry stacks.
Tension frames are only hand snugged not tightened with a wrench.
I only stack bricks flat on surrounds or bells

It turns out, that although Shorty might be the first build using SW gasket on all joints.
There is a builder in Iowa that used the same gasket material at several locations in his build.
He reported to me this morning that the SW gasket has indeed performed perfectly over an entire burning season.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Jeremy VanGelder

gardener

Posts: 1324

Location: Proebstel, Washington, USDA Zone 6B

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posted 6 months ago

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thomas rubino wrote:Tension frames are only hand snugged not tightened with a wrench.
I only stack bricks flat on surrounds or bells

That's good to know. For some reason I always assumed that tension frames were welded. Loosely bolting them sounds a lot easier.

Tradition is not the worship of ashes, but the preservation of fire.

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 6 months ago

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Hey Jeremy;
There are angle iron containment boxes completely welded together, the bricks sit inside.
The door is hinged off the frame.

A tension frame uses welded pieces of angle as brackets at the rear of a core. All-thread rods connect from the brackets and through the front door backing plate.
The door is hinged from the backing plate, and the all-thread maintains a light snugness to the bricks.

20240509_172155.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 4 months ago

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Good Day Thomas (and all the other RMHers). And happy thanksgiving to all.

I can't find a ready source of thin Superwool here in Maine, but the local refractory outfit did have some "ceramic paper" with comparable temps.

Seemed to be a good option, so I bought a 1x2 foot chunk and plan to cut into strips (brick width) to test.

I went with the 1/8" thick, although they had 1/4" and 1/16" as well.

Now my question - do I lay the "gasket" strip along the horizontal seams only, or should I wrap top edge and both ends of each brick to ensure a tight seal?

Thanks!
Randy

thomas rubino

rocket scientist

Posts: 6521

Location: latitude 47 N.W. montana zone 6A

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posted 4 months ago

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Hi Randy;
I used the gasket on every joint horizontal and vertical.
Most times I made them separate. I bent a few vertical pieces but it seemed easier to use separate ones.
1/8" is the thickness I used.

Look on Amazon, you'll find Superwool gasket paper there.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Randy Butler

Posts: 105

Location: Coastal Maine

posted 4 months ago

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What I find online for the prices seems a tad steep.
I paid 6 bucks for a piece 1/4x12x24. Much of what I saw in eBay was closer to $20/SF.
Now I'm no rocket scientist (oh, does rocket mass heatering count?), but my math isn't terrible.
Maybe I should post my local shop contact info?

Scott Weinberg

Posts: 316

Location: North East Iowa, USA

posted 4 months ago

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Randy Butler wrote:What I find online for the prices seems a tad steep.
I paid 6 bucks for a piece 1/4x12x24. Much of what I saw in eBay was closer to $20/SF.
Now I'm no rocket scientist (oh, does rocket mass heatering count?), but my math isn't terrible.
Maybe I should post my local shop contact info?

I would suggest to look a bit further, such as this quote, which maybe a black Friday thing

https://www.amazon.com/dp/B0CY82N7V5/ref=sspa_dk_detail_6?pd_rd_i=B0CY82N7V5&pd_rd_w=EoP4I&content-id=amzn1.sym.7446a9d1-25fe-4460-b135-a60336bad2c9&pf_rd_p=7446a9d1-25fe-4460-b135-a60336bad2c9&pf_rd_r=TV59ZZY54ZQHXV7HPV1Q&pd_rd_wg=GsXui&pd_rd_r=9ac98283-feb8-45db-9296-d8f96b9eb038&s=kitchen&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw&th=1

but I have bought from these folks before and was pleasantly surprised how nice the material was.

Sure would use 1/8" for drystacking your brick core, vs .25" you can get creative on temporary holding of strips on bricks during your build. Gravity takes over on all horizontal strips. You don't want a wobbly build.

https://permies.com/t/241976/Batch-Rocket-single-bell-build

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