Shorty Core + Oven Single Skin Bell Build

Mar 16, 2026 10:50:06

Years of practice, study, and asking lots of questions have finally led to my first indoor RHM build!
She’s a single skin, top-venting bell with bypass powered by a 6 inch Shorty core.
For the core build, see: https://permies.com/t/368044/Shorty-Core-Oven-Build-Test

The goal is to heat this centralized location in the house. The living room was an addition to the house, so there was an old window hole that I re-opened to pass heat through. A cozy L shaped seating area will be added against the long wall, which is why the firebox faces out of the narrow end.

Sizing
The internal surface area (ISA) is sized for a standard 6 inch batchbox at 8064 sq inches or 56 sq ft (I originally had different core plans.) So technically it’s oversized since Shorty wants 15% less ISA, but knowing I would have a bypass, I proceeded with this size. Exterior is 56 inches long AND high, 32 inches wide.

Materials
It's around 1800 lbs of bricks, another 1000 in concrete, and 200 in misc mortar other materials. Probably around 3500 lbs including the core. The first step was adding jack-posts and sistering floor joists in this corner of the basement to make extra sure the weight would not be an issue. A single jack-post is rated for over 10,000 lbs, so I feel very confident with support.

The “pad” below is build with air channels. The idea is that the stove will heat the wall heat shields and the air behind them will rise. The air channels provide a source for cool air that will warm, rise, and convect out into the room. Its made of Allura concrete board. Also helps spread out the weight.

My use of concrete block in the walls may cause some controversy. Concrete blocks goes up faster than brick. They’re much cheaper. And this low in the bell does not expose them to any damaging heat. I parge coated the inside of the block layers with refractory mortar, which was likely unnecessary. As you’ll see, bell air temps at the bottom do not get much over 100 dF. All this saved me time, money, and having to over-order bricks.

The clay bricks themselves are “jumbo” from Pacific Clay via Home Depot. We really liked the brown color and the ability to get 2 small pallets delivered greatly reduced handling and hassle.

The chimney system is all Supervent brand from Menards. Double wall black stove pipe converts to stainless steel double wall insulated Class A stove pipe at the ceiling transition. Exterior chimney (not pictured) follows all the height and support rules.

All the insulation is ceramic fiber alternative from Lynn Manufacturing. This is an important point because this material does not have the health risks of standard ceramic fiber. It costs only a bit more. And I have a whole lot of it involved in gasketing the core, surrounding the oven, covering the bell roof, and stuffing around all the ceiling bricks (not pictured.) I considered adding some on the back wall that takes the highest heat from the core exhaust, but with 21 inches of space to dissipate and a stainless steel T blocking the path, I felt safe. If I see any cracking I can always add this later.

8 thermocouples are built in. Chimney, oven, high bell air, low bell air, ceiling mass, and 3 more down the wall mass. I like data. Also why I bought a cheap thermal imager for my phone.

This was NOT a cheap build. I invested heavily in good materials to get it right the first time (hopefully.) I wanted it to look good and work well as a one time investment to lower my especially expensive rural electric co-op bill that usually costs $300 a month in winter (electric-only heat.) Not sure on the return on investment timeline but the furnace did not run at all on burn day so that's a good sign!

First burn
Yes, it’s mid March. I’m WAY behind the heating season. Partly because of another project and partly because I’m slow at new things. The bell alone took me 5 weeks (I’d never laid a brick before) while the core took me even longer (I’d never welded before.) But with a sudden cold front on the way, I rushed to finish the thing just in time to test it against a blizzard. Conditions were not ideal with 50mph wind gusts. But taking so long to build gave the bell time to dry, and building the chimney with proper materials to the specified height all meant I was able to fight through the wind with only a couple smoke-back moments. Otherwise, no smoke leaks! Hint: if you open a window on the windward side of your house, the increased air pressure blowing in counteracts the wind trying to blow down the chimney. Under normal conditions, start-up should be much easier.

Conclusion
After one burn, I’m sold! 3500 lbs of mass heated to an average of 120 degrees in a short burn with so little wood is a no-brainer. And the infrared heat it puts off is oddly satisfying to feel, even from across a room. The oven reads relatively low on air temp but the oven floor temp is closer to 500. There will be some learning curve and maybe a wire rack or some pizza stone to get used to cooking with it. Over the summer I will build seating and also plan to finish the exposed concrete block with lime wash or cob. There will also be some decorative trim here and there so the ends of the radiation shield are hidden. I’ll do a follow up post after I get more work done. Many thanks to all the builders out there who donate their time to answering questions, especially Peter van den Berg and Thomas Rubio!!!

Mar 16, 2026 11:24:08

Sweet! Great job, Matt, with excellent documentation!
It's a lot of labor, but it's so satisfying to see each layer of bricks rise.
And as I'm so fond of saying, "Once you heat with brick, you'll never go back."

Mar 17, 2026 07:18:20

Second burn was an absolute dream! Perfect draft, easy start-up. I pushed her harder with more wood load and had no issues.
Back wall bricks in the core exhaust hot-spot made it to 185 dF on the exterior. Long wall bricks 140 dF.
Pretty soon I’ll have the confidence to just load it, light it, and walk away.

Mar 17, 2026 07:38:12

Very well done.

Mar 17, 2026 08:08:16

So Matt, you have discovered the miracle of Masonry heat!
Isn't it just awesome!
Careful, you do not run yourself out of the house by adding that extra load.
Midwinter is easiest: one fire in the morning and one fire in the evening.
Now that spring temps are here, I find one fire is not enough, but two full loads will have all the windows open.
Now I am burning two half loads; we still have windows open, but just for the cool, fresh air.
Just enough to keep my heat reservoir filled but not overflowing.

Shorty is just a wonderful addition to the awesome first-generation Batchboxes.
Thanks should go to Lasse Holmes from Homer, Alaska, for the original Batchbox design, and of course, Peter Berg for all his work in refining, testing, building, and freely sharing his results with the world!

Mar 17, 2026 08:16:52

thomas rubino wrote:
Shorty is just a wonderful addition to the awesome first-generation Batchboxes.

Indeed! And the "no thermal runaway" with Shorty is an excellent new feature. On the second burn I still didn't have a full firebox just yet (in the spirit of taking it slow and letting materials dry out.) But as the burn went on, I wanted to add one more log for the cold evening ahead. Shorty graciously took it in stride and calmly consumed it.

Last time I tried that in my DSR2 things got real scary real fast as it flew into runaway mode. Shorty has truly tamed the power of flame.

Mar 17, 2026 11:42:32

quality build , very high standard achieved there and looks a very handsome result , congratulations

Mar 18, 2026 17:20:53

Superb work. I especially like your judicious use of a few blue collar blocks, coating with refractory as needed.

Mar 19, 2026 08:31:38

The build is so clean and the documentation is so clear.
Thank you for this amazing contribution!

I think the ability to use conventional bricks in the top half of the build is notable, is this due to how the shorty core works?
The concrete block with refractory parge is also great, using cheap/conventional materials expands the possible use cases of rocket mass heaters.
I will be looking into that ceramic fiber alternative, it's very exciting to have new materials to consider.

That the shorty core used a metal frame seems both limiting and expansive.
It makes it harder to diy on the one hand but it could make it easier to standardize/commercialize.

Mar 19, 2026 08:56:28

Hi William;
I agree Matt did a fine job.
Yes, the bell-top temperatures with a Shorty Core stay well below 800°F, making the use of clay bricks possible.

Yes, using concrete blocks is fine at the lower level, and it cuts back on the overall cost.
In my opinion, bricks look better, but it's all in the eye of the beholder, and in their wallet!
A stone or tile facing would improve the look.

Ommm... Morgan Super wool is a non-ceramic fiber alternative.
Morgan also makes non-ceramic boards as well.

Shorty or any Batch core can sit on a brick plinth.
The metal frame is simple to construct and speeds installation, but there are other options.

Mar 19, 2026 21:25:30

thomas rubino wrote:Ommm... Morgan Super wool is a non-ceramic fiber alternative.
Morgan also makes non-ceramic boards as well

Woops, my bad!
It seems that Lynn Manufacturing sells Morgans super wool plus, so it's not a new material after all.

Mar 19, 2026 21:32:45

This photo makes it look like the riser exhaust is pointed almost directly at the by pass opening.
Is that actually the case?
You mentioned a hot spot on the back of the bell, maybe this has something to do with that?
Also, can you please tell us about the chimney bypass damper?

Mar 22, 2026 02:54:46

Matt Todd wrote:Conclusion
After one burn, I’m sold! 3500 lbs of mass heated to an average of 120 degrees in a short burn with so little wood is a no-brainer. And the infrared heat it puts off is oddly satisfying to feel, even from across a room.

Congrats, Matt! You've seen and felt what a mass heater could do in a room. Also the second burn went swimmingly well as you reported. The wall behind the internal chimney assembly might experience too high thermal stress, keep an eye on it. The Shorty core seems to perform like she should, in my opinion she will stay this way for many years to come.

Welcome in the wonderful world of this oddball system!

Mar 23, 2026 07:30:17

William Bronson wrote:This photo makes it look like the riser exhaust is pointed almost directly at the by pass opening.
Is that actually the case?
You mentioned a hot spot on the back of the bell, maybe this has something to do with that?
Also, can you please tell us about the chimney bypass damper?

The riser exhaust is indeed pointing at the bypass. Granted, the bypass opening itself is slightly higher than the core exhaust. I almost used standard single wall stove pipe but I had the stainless T on hand and felt it was safer to use stainless here directly in the 700+dF exhaust path.

The hotspot on the back of the bell is simply because the riser exhaust is pointing that direction. The inner chimney/bypass T is taking some of that exhaust blast, but it's off-center in the bell so some of the back wall is still taking a direct hit. Even so, the bricks in the exhaust path are only reaching 320dF on the inside and 240dF on the outside so I'm holding hope that they will be alright. The cleanout door makes a good inspection point that will allow me to keep an eye on that back wall for any brick degradation.

The bypass design is copied directly from Thomas Rubio. A standard cast iron chimney damper with a scrap of ceramic board cemented to it. Plus an extended handle to stick up high enough to reach above the bell ceiling. I suppose I didn't photograph this very well. Here's a shot looking up from the cleanout door with they bypass open. You can just barely see the two layers: cast iron inner with the ceramic board stuck to the outside. A 90 degree turn of the handle is all it takes to operate.

Mar 23, 2026 07:45:05

thomas rubino wrote:
In my opinion, bricks look better, but it's all in the eye of the beholder, and in their wallet!
A stone or tile facing would improve the look.

Beige lime wash for now. Just close enough to my wall paint color. Maybe something even more decorative in the future.
I designed the bell with the concrete block size in mind and only had to cut ONE SINGLE BLOCK (to accommodate the cleanout door.)
That made for a very quick and sturdy base for me to then fumble with brick laying. Mistakes were made in brick laying but it all came together in the end.

I can't tell yall how my times I went back and looked at previous build posts to visualize how this could all work!

Mar 29, 2026 09:32:36

How does this do in the room the heater is in verses the room on the other side of the window? Does one room have higher temp than the other or does it even out well?

Mar 30, 2026 09:49:10

Jackson Bradley wrote:How does this do in the room the heater is in verses the room on the other side of the window? Does one room have higher temp than the other or does it even out well?

The dining room where it sits gets most of the heat. The approximately 2x4 foot opening into the living room does get some, but we haven’t re-arranged furniture yet to get things fully open to receive heat.

These heaters work best in line-of-sight. So if I’m sitting on the couch in the living room, I can feel those long infrared beams hitting the back of my head/shoulders through the hole.

Time is a factor as well. At first you are only feeling the heat beaming directly from the bricks, but by the next morning the heat has had time to soak into all the surfaces and you feel a more all-around sense of warmth. Since I didn’t finish the build until mid-March, there hasn’t been much chance to burn on consecutive days to really feel what it can do. I’m pretty impressed so far though!

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