Cheap(ish) prefab mass heater with Shorty core

Last month (April) I concluded the development of an octagonal stove, steel on the outside, concrete bricks on the inside. About 70 cm (2.3') wide, 190 cm (6.23') high, with a 130 mm (5.12") Shorty core in it.

The steel parts can be easily transported, concrete bricks are dimensionally stable and cheap to obtain. The intention is that it can be built in a day with a few people, everything can be prepared in advance.
It took a while before this project yielded anything visible, firstly I had to talk with a steel company that could laser cut and bend the parts.



Most of this thread will be translated from Dutch to English by Google. With some editing by me, transferring it all to the past tense.

I copied the principle of a metal shell with the mass on the inside from the pönttöuuni, Finnish for "pot stove". A fairly common and relatively cheap solution to build a (small) mass stove. Unfortunately, the manufacturers of those rolled and folded parts are all in Finland and as far as I know they do not export because the stuff is very thin (0.7 mm) (0,00276"), has an awkward shape and is therefore also vulnerable.



If you don't look too closely, they actually look like a classic Swedish tile stove, the kakelugn. Only without tiles but with a kind of luxury oil drum around the mass. Internally they are actually quite conservative things, the combustion quality is actually only moderate in my opinion. But they have developed special materials for this type of stove.



What I'm going to try is to produce a flatpack, much like Ikea, from that octagonal biscuit tin, the concrete bricks can be purchased locally. As you can see in the first picture, there is a steel container inside that can hold the Shorty core. I wanted to assemble that core from ceramic oven plates, with insulation around it so that there are no problems with expansion. Since the core spews the hottest gases forward instead of upward, only a limited number of firebricks are needed directly above the firebox to protect it. The top does not have to handle high temperatures so there is no need to apply stone-like material there (I hope). In addition, a simple stove fan can remove the heat produced at the top. Most of the work will be in the specific parts of the door and air supply.

There are no channels in the thing, only mass along the perimeter, which makes stacking or laying bricks relatively easy. The total mass is in the order of magnitude of about 800 kg (1760 lbs). I also drew another version, which is wider, about 1 meter (3.28'), it can hold a larger core and the weight of that comes to about 1200 kg (2650 lbs). The finish can be anything, from rolling with black fireproof paint (works quite well!) to a RAL color powder coating of choice.

If it doesn't work out, I'll have used up my budget for a year and have been happy experimenting, right?

The 8-sided pepper shaker project is now really on track, the core has been drawn in and most of the materials have arrived.

On Friday 21 March I picked up the steel in Maasdijk, it looks good. Of course I did put a few parts together to see if it all fitted. The side panels fit and come out well, the pop rivets can be squeezed in reasonably well. Unfortunately there is a mistake in the lids, the holes do not come out in the right place. That means drilling holes from the outside, that can still be overcome.





That same weekend I degreased the 12 panels plus 4 half lids and painted them with a light grey heat-resistant paint. Fortunately the weather was nice, I laid out everything in the garden, a few hours later it was dry and the stuff could go back inside.





To choose the material for the core was difficult, all options were quite expensive. But... I still had two bags of Licofest refractory castable lying around, bought for the stoves for Ukraine project that didn't go ahead when the financier pulled out. Although the concrete is already three years old, there were only a few small lumps in it and I was able to make everything with it except for a large casting. The molds are boxes of leftover triplex material with blocks of hard foam glued in them.



And a picture of the result:



Plus some smaller pieces, the last large mold is ready, the one sack of castable has been ordered and will hopefully arrive on Monday.



Worked on the steel of the door and frame on 30th of March, welding the folowing day.



The concrete paving bricks have also been ordered and would arrive during the week, the clay for the mortar was still in the pipeline (I hope). The plan is that I could start building on Monday April 7, Ivar from Numansdorp is coming to help.

For now I see it as a proof of concept, in my workshop the midget J-tube was coupled to the chimney but it could be removed without any problem. There is a good chance that someone wants to build the 8-sided pepper shaker in series as a (partial) source of income. And for others it is a showpiece of what is possible in a combination with the Shorty core.
In contrast to all those experimental, collected scrap examples in rusty barrels, this is a real stove that also looks nice.

Ten years ago I could not have built this, my knowledge of the materials and processes weren't up to the same level as today.
The process has moved on, the door frame is ready and the core too. Today I cut a large hole in the front panel for the door and fitted a large part of the casing.

It looks beautiful, I think. Especially when you consider how simple the construction really is.

The 7th of April construction did start, but the 144 concrete paving bricks that I ordered from Hornbach didn't get delivered, yet. There was an option to pick them up at a competing DIY store, but that would have involved a lot of lugging and I would have had to drive at least three times, maybe four. Now I did pay €90 to have it delivered to my door and the promised delivery time of 5 working days has not been met.
Bummer!

In the meantime, the project has progressed further. The base has been set up, neatly level and plumb. Together with the help from last Monday, the mass in the first floor has been built up. The mortar is clay plaster from Oskam, which is considered pleasant to work with. So, nice clay.



Working around the exhaust was a bit of a fiddly job, but it worked out wonderfully, without a large wet saw but with only a small angle grinder.



The slats that are around it keep everything neatly in shape while the bricks are going in, otherwise the next layer of steel would definitely not fit on it. The next photo shows the first floor with all the bricks in and the scaffolding removed. Now you can also clearly see that the design is focused on the size of these bricks, it all just fits in.



Then the rear panel was put on and the steel box with welded-on door frame, and the front panel with a large hole was cut out. The entire core is now in place, the only thing missing here is the right side panel. All panels are riveted together, works wonderfully well with the 1.5 mm material thickness.



And another picture from the other side.



The last panel on the right has also been put in, we didn't get any further on Monday. At that moment I was slowly collapsing, I became dizzy and nauseous. That's what you get when you keep working for 17 days in a row without a break. While I am not the youngest anymore, even though it doesn't feel like it. I slept a lot for three days, at that moment I thought I could start again, very slowly, the folowing Monday.

After a week of sleeping a lot, shuffling around the house with a walker and later a walking stick, I got back on my feet again. On Monday the 14th Ivar was able to come and help again and together we built in the second floor, around the core.



Plus the side panels of the third floor were popped into place. The following two days I continued on my own.



On Wednesday afternoon the entire lining was up to top level, including the outlet of the bypass.



It is an abyss of 190 cm (6.23") deep, it is still really unbelievable that the hot gases would go all the way down.

On Wednesday evening the lid was also on and the pipes were connected, it was operational for the first time!



With the bypass open, starting up was no problem, despite all the water that was still in the clay mortar and the bricks.



With two stove fans on top it even became very spectacular.



Those concrete bricks can absorb and transport heat quite quickly, once dry and started from cold, the heat started to be felt on the outside after 35 minutes. But there was also something that bothered me a bit: when the bypass was closed and the fire was burning at full blast, the chimney temperature remained somewhere around 100 ºC (212 ºF). Even after three fillings, it remained that way. Apparently, the whole thing could extract so much heat that the final temperature would not rise any further. To find out whether that was due to the bare top plate, I stacked 18 bricks on top.



That didn't seem to have much effect, the graph from Saturday evening still showed the same behavior.



The clear steps in the blue line are a result of closing the bypass in two steps. After that, the exhaust gas temperature remained at an even lower level, somewhere between 70 and 80 ºC (158 and 176 ºF). After this load of pine planks, I topped it up with a bundle of beech planks, expecting that the temperature would increase.



Not so, towards the end it dropped back to 80 ºC (176 ºF) again. The only effect of the bricks on top is that the whole thing stays warm a bit longer. But the quality of combustion is somewhat better when the draught is stronger, so I wanted to do something about it.
The plan then was to lift the lid in the following week and removing one ring of bricks. Then replace it with a ceiling of the concrete clinker bricks. Because the maximum heat output is limited in this core, the bell was already made 10% smaller. By placing a brick ceiling inside, the bell did become a little smaller and the mass increased a little bit.

Two possibilities: a single-walled ceiling of 105 mm (4.13") thickness limited the internal surface of the bell by 16% compared to the table on the batchrocket site. With a double-thick ceiling of 185 mm (7.28") thickness, the mass would increase a little more and the internal surface would decrease even more, by 19% compared to the table. The biggest expected advantage is a somewhat higher final temperature, combined with a larger storage capacity. Months ago I had already estimated the expected decrease of the internal surface area at 15%. Let's first see what that yields, these remain interesting times!

The last full week in April I have been busy removing the bricks from the lid, prying the lid off the sealant, removing the top ring of bricks. The goal was to create a brick ceiling inside the metal shell.



Then placing a ring of 8 bricks on flat, 8 cm (3.15") high.



Using a set of steel T-profiles 40x40x5 mm and strips of ceramic felt, make a support, the brickss on their side, 105 mm (4.13") high this time.



And knitting the whole thing closed with small pieces of brick and clay mortar in the corners.



The picture shows the lid loosely on top, on a knitted glass strip. That lid is quite tight, no signs that it was leaking there.



In the meantime she has been run a few times, the top was still getting quite warm but about the same temperature as the walls. The heater fan still comes on, but only after an hour instead of 5 minutes after lighting as before.



The flue gas temperature to the chimney is very stable, after closing the bypass the thermometer remained at a level of 100 to 105 ºC (212 to 221 ºF).



There were still a few very small adjustments I could make, but there was no rush and I could just leave it like that.
At the time everything was cleaned up and she was showing off like this, I am quite proud of her.

Do you have any temperature readings inside the bell?

I am curious if the temperatures are high enough to allow for a modification to add a white oven.

Peter-Having all the tools at hand, this intrigues me a great deal,  ( Casting hi-frequency vibrator plate-  Heavy bending break-lazer- large wet saw- CAD background and all the materials  I presume this was considered a success?

Warning- there is more than one question!

1) Can we cast anywhere near the same cost as fire brick itself?   Using larger bulk quantities of the castable material.

2) just an example, if I firebrick weights 2 Kg or 4.5 pounds,  Will it take roughly the same weight of castable material to achieve the same volume of area? This I guess would answer question #1

3) In this upright "mass"  was there a ISA formula that was followed for this sized shorty core?  

4) The use of the steel exterior cladding I can see have many benefits on the building ease,  does it serve as a quicker heat dissipater?

The stove looks great!

Alex Ronan wrote:Do you have any temperature readings inside the bell?
I am curious if the temperatures are high enough to allow for a modification to add a white oven.

No temperature readings inside, sorry. A white oven can't be added just because there's very little space left. Having said that, the firebox itself is a very potent oven by itself. Once there's only glowing charcoal left, a stainless steel grid could be place above the coals and there's your oven!

Very cool. I was thinking of designing a similar thing. I was considering the idea of using 85 gallon "salvage drums" which have an inner diameter of 26 inches (66cm). (Repainted with a high temp wood stove paint)

I hadn't gotten far enough to solve a method for connecting them together once stacked, but there might be a kind of band that could join them.

I decided I would go a different route in order to add some water heating into the design, so more likely would be two cylinders side by side (perhaps 55gal drums), with one just being an upward channel for the firebox and insulated riser. The cylinder next to it having a downward smoke channel, heat exchanger, and water reservoir

Scott Weinberg wrote:Peter-Having all the tools at hand, this intrigues me a great deal,  ( Casting hi-frequency vibrator plate-  Heavy bending break-lazer- large wet saw- CAD background and all the materials  I presume this was considered a success?

Yes, I would call this a resounding success. The laser cutting and brake bending were done by a specialized, though small, company.

Scott Weinberg wrote:Warning- there is more than one question! 1) Can we cast anywhere near the same cost as fire brick itself?   Using larger bulk quantities of the castable material.

Assuming you are able to produce the moulds and build the vibrating table yourself, it's about the same cost, volume-wise. Mark that there is only refractory used for the core and the immediate area above it at the front. All the rest, being 90% of the mass, is ordinary, plain vanilla, building quality concrete.

Scott Weinberg wrote:2) just an example, if I firebrick weights 2 Kg or 4.5 pounds,  Will it take roughly the same weight of castable material to achieve the same volume of area? This I guess would answer question #1

Most castable refractories are somewhat lighter as compared to dense firebricks. In case you are talking about the majority of the mass: I also made plans for specialized bricks, with a trapezium-shaped footprint. Any specialized prefab concrete plant could make those, opening up the possibility to stack most of the mass completely dry. Because of mould costs, only meaningful when bought in bulk.

Scott Weinberg wrote:3) In this upright "mass"  was there a ISA formula that was followed for this sized shorty core?

Yes there was, assuming a good chimney and so on. The cross section area of the chimney in square meters, multiplied by 255 will result in an ISA that's awfully close to ideal. Normally, this number would be 300. This particular core will take it's time to consume the fuel, not racing through it. Resulting in a slightly lower maximum power in any given time. With other words, it's tuned down a bit.

Scott Weinberg wrote:4) The use of the steel exterior cladding I can see have many benefits on the building ease,  does it serve as a quicker heat dissipater?

As soon as the heat is coming through the bricks, it will be dissipated in the room. The steel itself isn't the limiting factor, it's the mass itself.

Scott Weinberg wrote:The stove looks great!

I totally agree, I think she's really cute, thank you.

After firing a few times in as many days, I still didn't feel like this was the final configuration. It took too long to my taste before the bypass could be closed. Those concrete paving bricks can absorb a lot of heat!
So: on the morning of May 3rd, I lifted the lid again. On the inside of the lid, a lot of condensation moist and rust stains, not good. After walking back and forth between the drawings on my computer monitor and the stove itself, I decided to remove the brick ceiling again, plus the ring of 8 bricks that had been placed there earlier. I did that late Saturday afternoon, it was a bit disappointing, it was all much more just a solid mass than I had thought it would be.

And then to build the whole thing back in on Sunday morning, with the ceiling placed 8 cm (3.15") lower. This made the bell a bit smaller and hopefully it would took a bit less time to warm up.
This was the beginning.



Everything went in quite easily, after all, this was not a renovation but a re-renovation instead.



Unfortunately I didn't have enough 1/2" superwool left to do the closing with, so everything was stuffed all around in clay/sand mortar to make it as airtight as I could. With pieces and fragments of leftover refractory bricks and kiln plates in it wherever there was ample room to fit it in.



As you can see, after this re-renovation there was now a space of +/- 10 cm (4") left, which space I filled in with another layer without mortar this time, using the same clinker bricks.



The accumulating mass increased by another 18 bricks compared to the situation without a brick ceiling at all. The total weight wasn't really clear at the time, the concrete of the bricks still contained quite a lot of water according to my construction moisture meter. I now have 10 bricks on their short side on the workbench and I check the moisture content every now and then. That's slowly decreasing, time will tell to what extent the bricks want to dry out. The first firings were very promising, no smoke leakage at all and the exhaust gas temperature is now increasing a bit quicker again. I didn't replace the lid at the time, I planned to make a ring of 10 mm (0.394") holes in it. Completely in the style of a pepper shaker with sprinkle holes. 8-)
The goal is to provide holes for the water vapor to escape. The lid should remain on in order to keep the correct shape at all times.

After a tiring but successful annual meeting of the AFPMA, the association of French mass stove builders, see https://permies.com/t/281596/Mass-bench-fed-modified-sidewinder, I am starting to recover a bit. The octagonal pepper shaker doesn't seem to be getting any drier now, so today the lid was put back on. To prevent condensation on the inside of the lid, a ring of 8 mm (0.315") holes was drilled in it, 14 in total. I am assuming this is sufficient to allow any moisture to escape. The whole thing now really looks like an oversized pepper shaker. 8-)



The lid is loose, although it is a tight fit so it won't be blown off. I'm not going to use pop rivets anymore, it's not worth the effort and it (the lid) will no longer be gas-tight. Today I also stoked it up again with an estimated 5 kg (11 lbs) of softwood, in two batches. Everything gets really hot, of course less so at the bottom. The hottest above the door, up to 115 ºC (239 ºF). Just within the possibilities of regular powder coating, which is cured at 200 ºC (392 ºF). Combustion was very calm and even, only at the last moment could the bypass be closed completely.

A few weeks ago I already tried how warm starting went with the bypass closed. Not bad, I must say. It starts up calmly and as far as I can see there is not a whisp of smoke to be seen. The temperature inside the workshop was raised from 20 ºC (68 ºF) to 24 ºC (75 ºF) in about four hours today. Firing was over in an hour and a half, the outside was already starting to get quite warm at that point in time.
My workshop is of course part of a passive house so heating up is quite easy. According to the last full test run, started from cold, the efficiency is 92% on average, just as high as a large batch rocket mass heater.
In all, I'd think it's safe to call it a succes!

I won't do any more test runs or configuration changes, summer is already upon us now.

Peter van den Berg wrote:  totally agree, I think she's really cute, thank you

So are you going to start selling this flat pack sheet metal kit? Is it already for sale?

Also, have you experimented with storing heat in water?

Peter van den Berg wrote:. The octagonal pepper shaker doesn't seem to be getting any drier now, so today the lid was put back on. .

You may consider one of those blower fans that are commonly used to dry carpet after a flood. The impeller design allows them to work with high static pressure. Letting it force air though should dry it much faster and reduce possible corrosion

Brandon Hands wrote:So are you going to start selling this flat pack sheet metal kit? Is it already for sale?

It won't be me, going to sell this flat pack kit. And it isn't for sale at the moment by a third party. It is meant as an open source design, anybody who want to, can take the design and make a business out of it. I did this just to satisfy my own curiosity, could this be done in a reasonable way and with cheap mass? Up til now, it seems to be affirmative.

Brandon Hands wrote:Also, have you experimented with storing heat in water?

No, I have not, my interest lies in the combustion side of things, almost exclusively. In order to make it possible to measure the results, I had to look around for a way to cool down the exhaust gases. A bell system turned out to be the simplest and most effective way. For quite some time, I used 55 gallon barrels to this end. The question kept coming in my head: what is the possibility to make a real mass heater out of this? The barrels proved to be too small for this goal, so I did my own design, just for the fun of it.

Brandon Hands wrote:You may consider one of those blower fans that are commonly used to dry carpet after a flood. The impeller design allows them to work with high static pressure. Letting it force air though should dry it much faster and reduce possible corrosion

Or simply, fire the heater repeatedly. The inside of the steel panels aren't rust protected in this prototype. But for a business building this type it would be a good thing to have it all powder coated, also the inside.

Brandon Hands wrote:Very cool. I was thinking of designing a similar thing. I was considering the idea of using 85 gallon "salvage drums" which have an inner diameter of 26 inches (66cm). (Repainted with a high temp wood stove paint)
I hadn't gotten far enough to solve a method for connecting them together once stacked, but there might be a kind of band that could join them.

The salvage drums are open top types, with a separate lid and clamp. The bottom rims are nuch smaller, there's a good chance two of those will fit in a standard clamp. For this goal, you have to cut out the bottoms of the barrels. Be careful not to cut the bottom out completely, leave a ring of around one inch. Doing it this way, the stability of the round shape will be maintanted.

In order to discuss this further, it would be best to start a new topic about your ideas.

Peter van den Berg wrote:
Or simply, fire the heater repeatedly. The inside of the steel panels aren't rust protected in this prototype. But for a business building this type it would be a good thing to have it all powder coated, also the inside.

I was just thinking to reduce your workload. The fan can run 24/7

Peter van den Berg wrote:

Brandon Hands wrote:So are you going to start selling this flat pack sheet metal kit? Is it already for sale?

It won't be me, going to sell this flat pack kit. And it isn't for sale at the moment by a third party. It is meant as an open source design, anybody who want to, can take the design and make a business out of it. I did this just to satisfy my own curiosity, could this be done in a reasonable way and with cheap mass? Up til now, it seems to be affirmative.

Brandon Hands wrote:Also, have you experimented with storing heat in water?

.

You may consider putting some of the basic drawings into a PDF, and posting it online with a licensing statement similar to LGPL where it allows other to profit from the work while it's also open source

Brandon Hands wrote:

So are you going to start selling this flat pack sheet metal kit? Is it already for sale?

 

I will gladly help with your flat pack design as Peter designed it.

But, I will not design and depict every detail for every person. But this is a fairly simply explanation.

1) With Peters basic design as has been depicted here, there are just a few simple controlling factors. NO need to change these basic controlling numbers.

   A)   8 sides mean the corners are 22.5 degrees   NO changes here,  8 sides in a circle is simply 22.5 degrees on the corners

    B) it has to be bigger than the depth of your shorty + the gap between the brick and shorty depth + your brick thickness.   Please note: some have various shapes and sizes, I for example have over 1000 bricks that are 4" square, by 11" long, but many others are completely different. And as Peter has stated, I think he had some concrete blocks used.  This is where I am not going to get involved in describing what you should have, as I simply don't know what you have.  All that being said,  once this is known, the entire size can be then dictated. A quick floor layout for anyone `quickly `would tell the measurements.

   C) While I am not sure what thickness of metal Peter used, by only needing a 22.5 degree corner bend, lets us use a bit thicker metal in our brakes. than what one could get by with if bending a 90 corner.   I would offer this service but with shipping and above mentioned variability. it would simply take to much time for me to sort it out.

    D) by keeping uniform side section widths,  it becomes a simple clamp   -drill 1/8" hole    - stainless rivet assembly.  Having cleco's greatly helps but not required  (these are aircraft assembly removable rivets)

    E) Again the top and bottom are predictable in shape and size thus are simple to design and make.

If this has moved to a new subject forum title, just let me know here directly.

Peter V, Tom ( dragon tech) , and Glen in Colorado have seen what I have done in the past   If what I have done becomes something that is sold, it will soon end. This is simply my public offering of open source measurements.  If I have time, I will post a few photos in the next few days, showing what was stated above.

Just an example as to the use and design of the scaffold used to keep brick in alignment from bottom to top using a level of course for vertical and frame for maintaining size.

This frame can be stated and shown ( described in detail both imperial and metric ) in minutes with your KNOWN size.  NO rocket science here, You (the builder) tell me how long you want each side, and I can tell you how long each frame piece of wood you need.  With each piece being exactly the same. Note: They do not need angle ends as shown.