Permies Design Introduction & Questions

The heater will be located in the basement, and the main bell will extend upward into the floor space above, ending at about 15 inches above finished floor height. I’ve already completed the heat‑load calculations, and a 7.5" system should let me run one full burn during the shoulder seasons and two full burns per day in mid‑winter, which fits my heating plan well.

Right now I’m considering either a Sidewinder or a DSRv2 core. I’m leaning toward the Sidewinder because of the compact footprint, but I’d appreciate input from anyone who has built both.

I’m pretty much locked in on using a double‑masonry bell, with the flue vent exiting high in the basement. I’m also kicking around the idea of a raised core to move the heater away from the concrete floor and make fire starts easier for me.
I’d really appreciate any advice, examples, or lessons learned that could help me refine this design before I start building.

Questions:
• Which core type gives the smallest footprint at 7.5"?
• Can a Sidewinder exit into the bell from any of the four directions — left, right, front, or back?
• Is there a repository of SketchUp (.skp) heater designs? I found some of Peter van den Berg's designs already.  Ideally something using imperial firebrick sizes (9×4.5×2.5") or splits.

I’m excited to get this design underway and would really appreciate any advice, examples, or models that could help me refine the layout.

Thanks in advance for your help!  Your gracious sharing and help, makes big DIY projects like this possible for the rest of us.

Hi Jim
In my opinion, the newest variant, Shorty core, has the smallest footprint.
The DSR variants may be slightly smaller, but I am told they are a more difficult build.
Keep in mind I have never built a DSR core.

Now about sidewinders.
They only exit left or right, or they would not be a sidewinder.
A first-gen Batchbox or a Shorty core can be built as a sidewinder; a DSR, I think, cannot.  (I could be wrong)
I haven't built a Sidewinder myself, but many have, with no complaints.
Here is what Lasse Holmes told me: he designed the very first Batchboxes and built the very first Sidewinder variant.
The Sidewinder was a good design. He had one in his cabin in Homer, Alaska, for over two years.
However, he said it was a fussy starter; once he determined how it liked to be started, he had no trouble with it... But his wife and daughter had nothing but trouble starting it. Eventually, he replaced it with a conventional Batchbox design and all were happier.

I have a 6" Shorty core with a single-skin bell
A 6" first -gen with a double-skin bell.
And a hybred 7" first gen with double barrels and a single-skin bell.
They all work perfectly, I could not be happier with them.

Jim St Laurent wrote:The heater will be located in the basement, and the main bell will extend upward into the floor space above, ending at about 15 inches above finished floor height. I’ve already completed the heat‑load calculations, and a 7.5" system should let me run one full burn during the shoulder seasons and two full burns per day in mid‑winter, which fits my heating plan well.

Hi Jim, a heater in the basement will heat the basement well. And sticking through the floor above it, it will heat that space somewhat. All depending on the size of the part that is in the space above, obviously. Please, build the heater in the place where you live, that is the best option. In case that is on the first floor, build a large footing and place the heater in your living space.

Jim St Laurent wrote:Right now I’m considering either a Sidewinder or a DSRv2 core. I’m leaning toward the Sidewinder because of the compact footprint, but I’d appreciate input from anyone who has built both.

The whole of the batchrocket family consists of four main designs, being: the first generation with a high riser, the DSR2, DSR3 and the Shorty. All those designs can theoretically be built as sidewinders, where the riser or afterburner area is placed beside the firebox instead of behind it. That said, the first gen and Shorty are built multiple times as sidewinders now, I don't know of any DSR2 like that and just of one DSR3 built as sidewinder. The smallest footprint comes with the DSR3, no doubt, because there isn't a separate afterburner space behind or beside it, but on top instead. At the same time, the DSR3 is a real pig to build as a first one and get it right at the same time, I won't recommend that approach.
Looking for a well-behaving heater that isn't a hell of a job to build? The Shorty core is what you want, in straight or sidewinder version.

Jim St Laurent wrote:I’m pretty much locked in on using a double‑masonry bell, with the flue vent exiting high in the basement. I’m also kicking around the idea of a raised core to move the heater away from the concrete floor and make fire starts easier for me.
I’d really appreciate any advice, examples, or lessons learned that could help me refine this design before I start building.

Raising the core is always a good decision, it'll make the day to day life with such a heater appliance much more relaxed.

Jim St Laurent wrote:Questions:
• Which core type gives the smallest footprint at 7.5"?

Usable ones and ease of build and use? Shorty core as sidewinder version, no doubt about it.

Jim St Laurent wrote:• Can a Sidewinder exit into the bell from any of the four directions — left, right, front, or back?

A sidewinder Shorty core can exit to the left or the right, or... up! The straight version can be exiting front or back, or up. A first gen is always exiting up only.

Jim St Laurent wrote:• Is there a repository of SketchUp (.skp) heater designs? I found some of Peter van den Berg's designs already.  Ideally something using imperial firebrick sizes (9×4.5×2.5") or splits.

Most of the batchrocket designs are to be found on the website batchrocket. eu.

The site is in multiple languages, although the Shorty description is in Dutch only, so far.  Google Translate is able to solve the problem of a passable translation, though.

If you know what core and size you want to build, I am able to draw you a design using the correct brick size and layer by layer, for a reasonable fee.

Peter van den Berg wrote:
The whole of the batchrocket family consists of four main designs, being: the first generation with a high riser, the DSR2, DSR3 and the Shorty. All those designs can theoretically be built as sidewinders, where the riser or afterburner area is placed beside the firebox instead of behind it. That said, the first gen and Shorty are built multiple times as sidewinders now, I don't know of any DSR2 like that and just of one DSR3 built as sidewinder. The smallest footprint comes with the DSR3, no doubt, because there isn't a separate afterburner space behind or beside it, but on top instead. At the same time, the DSR3 is a real pig to build as a first one and get it right at the same time, I won't recommend that approach.

I’d like to build a DSR3 soon, especially for the chance to see the double vortex form.
I wonder what is meant when people say it’s the most complex one to build. Is it perhaps due to the difficulty of sourcing the round pipe? Or are there other reasons?

Gentlemen, thank you both for your expertise and the quick responses. I’d like to be fully transparent about my situation and overall plan.

I’ve wanted to build a rocket mass heater for a long time, and I’m finally in a position to start learning. My masonry experience is limited — I’ve only assisted in building a block‑wall foundation — but that same foundation is still available and ready for a future permanent RMH installation.  

Right now, I’m held back by my lack of masonry skills.

Peter, I understand your plead to build completely in the living space and eventually will.   But this training build will allow me to develop the skill set to build a follow up system in the future.  I have 3 season space with a cement slab that I can practice core dry layout and learn how to lay running bond brick using a sand/clay mix.  Goal here is to be able to build a structurally sound single wythe brick wall, only then can I move onto the basement heater build.   This real first heater will enable me to further develop this skill set and end up with a clean visual appearance.

This basement build is a masonry learning system, where I can use the existing tiled 8×12 flues shown in the attached floorplan.png This heater would run for two years or longer, built with sand/clay mortar so it can be easily deconstructed and the materials reused.

The 27″ × 33″ floor grate shown in the floorplan is the area I’d like to use to extend the bell’s thermal mass up into the first‑floor living space. This opening can be reframed larger if needed and could support a bell height of roughly 7 feet into the living space above, plus the 5‑foot basement core height below.   The basement height is influenced by a 32- or 40-inch block foundation with an insulated floor under the core.

A 7½ inch core delivers about 42 kWh per load, which calls for a bell ISA in the range of 120–150 sq ft.  
Using the ISA table below, I’m trying to understand how to get the most heat high up into the living space Bell surface area.  

ISA for a 12‑ft Tall Square Bell (5 ft basement + 7 ft living space)
| **Side (in)** | **Side (ft)** | **ISA @ 12 ft  tall |
| ------------------ | ------------------- |------------------------ |
| 24″sq             | 2.0                    | 84.0                      |
| 32″sq             | 2.6667             | 113.78                  |
| 40″sq             | 3.3333             | 147.78                  |
| 48″sq             | 4.0                   | 176.0                    |

Questions:  
1. Would an oversized bell store more heat higher up into the living space? either more ISA or more thermal mass in the top section

2. Will insulating the core where appropriate help?

3. Would a clever bell cover design with more internal surface area, that won't hold fly ash be an improvement?
    Something like power semiconductor heatsinks but with brick ends
    Looking like this        ____          ____         ____         ____              
                                              |____|     |____|     |____|     |____|  

Based on our discussions, I’m leaning toward an up‑venting Shorty core paired with a 40‑inch square bell and a insulated block wall foundation.

I can accept some heat loss into the basement, knowing that part of it will return as gentle warmth through the flooring above. My main goal is to retain as much of the core’s output as possible up into the living space.

I’d appreciate any ideas or improvements you might suggest.

Thank you again for pointing me toward the Shorty core — choosing a core was a daunting step, and now it feels settled.

Best regards, and a huge thank you to both of you, Peter and Thomas for taking the time to respond.
Jim

P.S. Peter, I’ll PM you about discussing the custom plans.

Welcome to the rabbit hole, Jim. On insulating the core: I did that around my batch-box build here in Finland and the difference was night and day. The fire runs hotter and cleaner, and the exhaust still arrives at the bell plenty hot, so the mass charges fully and gives the heat back later in the day when you actually want it. And yes, in a tall bell the hottest gas pools near the top, so that upper masonry does the most work warming your living space - your instinct on question one is right.

For anyone interested, here is one of Thomas' shorty cores.

Jim St Laurent wrote:A 7½ inch core delivers about 42 kWh per load, which calls for a bell ISA in the range of 120–150 sq ft.  
Using the ISA table below, I’m trying to understand how to get the most heat high up into the living space Bell surface area.  

ISA for a 12‑ft Tall Square Bell (5 ft basement + 7 ft living space)
| **Side (in)** | **Side (ft)** | **ISA @ 12 ft  tall |
| ------------------ | ------------------- |------------------------ |
| 24″sq             | 2.0                    | 84.0                      |
| 32″sq             | 2.6667             | 113.78                  |
| 40″sq             | 3.3333             | 147.78                  |
| 48″sq             | 4.0                   | 176.0                    |

Something isn't right here, according to the table on the batchrocket website ISA for a 7.5" system should be between 77.5 sq in. and 101 sq in. So, 84 sq in. would be about right but... a Shorty core is less powerful, tuned down a bit as it were, as compared to the first generation batchrockets. As such, the ISA should be reduced by 15%. Failing that, there's a good chance the core won't get up to working temperature easily, if at all. Mind you, all internal wall and ceiling of the bell is part of the calculation. So, if you are able to do that, use the rule of thumb for calculating the bell and calculate the system size chimney in m² first, multiply that by 255 and you've got the correct ISA in metrics for a Shorty core.

Jim St Laurent wrote:Questions:  
1. Would an oversized bell store more heat higher up into the living space? either more ISA or more thermal mass in the top section

The top venting Shorty core will spew the hot gases up in the bell, thereby driving the cooler gases out through the exhaust low in the bell. This was always the behaviour of the 1st gen batchrockets and is still the case by this particular variant of the Shorty core.

Jim St Laurent wrote:2. Will insulating the core where appropriate help?

Yes, it will, absolutely, especially when starting cold.

Jim St Laurent wrote:3. Would a clever bell cover design with more internal surface area, that won't hold fly ash be an improvement?

No, any obstacles aren't helping, as experience showed, rather the contrary. This effect isn't well understood as yet, I am unable to tell you why.

Jim St Laurent wrote:Based on our discussions, I’m leaning toward an up‑venting Shorty core paired with a 40‑inch square bell and a insulated block wall foundation.

Provided the calculation is correct, of course. By the way, all Shorty core bells are now built with a bypass to warm up the chimney and getting the core up to working temperature in short order. There might be a possibility that the 7.5" system can't be coupled to a large enough bell. Just scale the core up a bit further, these cores are able to run happily on half a load as well. I seem to remember that an 8" system is better to avoid cutting (a lot of) bricks.

Jim St Laurent wrote:I can accept some heat loss into the basement, knowing that part of it will return as gentle warmth through the flooring above. My main goal is to retain as much of the core’s output as possible up into the living space.

Hot air will rise and might go through the  floor above, that's true. But... a brick bell isn't emitting just a bit of its power as convection, most of it's heat is radiation. Nevertheless, with a warm space underneath the effect can be felt upstairs. The bell in the basement will emit quite a lot of heat, since the core is in there, despite the insulation around the core, as experience tells us.

Hi there, just wondering if the rmh 8dvd set would work in UK dvd players? Thanks in advance. Tec