Peter van den Berg

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since May 27, 2012
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He's been a furniture maker, mold maker, composites specialist, quality inspector, master of boats. Roughly during the last 30 years he's been meddling with castable refractories and mass heaters. Built a dozen in different guises but never got it as far as to do it professionaly. He loves to try out new ideas, tested those by using a gas analizer.
Lived in The Hague, Netherlands all his life.
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Recent posts by Peter van den Berg

Gábor Kertész wrote:On another  note: I have a concrete foundation for the RMH. Is it enough if I place firebricks only under the core? Should I cover the entire foundation with some sort of bricks, or can the bell floor be the concrete?

The floor of the bell could be concrete, tiles or any other non-combustible material. Temperature will stay fairly low, especially when the entire core is elevated above the floor. That can be done by using a steel pedestal or simple frame that is supported by the walls of the bell itself.

Gábor Kertész wrote:Remembering Peter's earlier advice about the floor levels I've got the feeling that this could distract gas flow between the first bell (floor mostly filled by the core base) and the bench bell.

Yes, you are right, the above mentioned steel frame would solve that problem quite elegantly. The space under the core would be completely empty this way.

Gábor Kertész wrote:And yet another: I need to bridge the bell above the oven. I can by a 1000x250x60 (mm) brick bridge element rated to 700 °C. It would sit about 60 cm-s below the top of the riser and 2 cm-s above the oven. Would that suffice?

I would call that lintel overkill, in the Mallorca build we used two steel rods of 10x10 mm to support the bell wall. In fact, all three openings (firebox, bell to bench and exhaust box) were bridged this way. Of course, those steel bars were all done with superwool all around in order to provide for expansion.
4 days ago

Randy Butler wrote:For the last two summers, we have been living with a small (Little Moe All Nighter) woodstove - in the cellar.
No where near big enough to heat the upstairs, but with 13" thick ICF cellar walls, nearly all the heat hangs under the wooden floor.
Not exactly a "radiant heat" floor, but it did make the place very livable. So heating the unoccupied area is not a "make or break" deal.

OK, understood.

Randy Butler wrote:The open space in the main living space is largely kitchen and living room.  And the bell will emerge between the two.
BUT - I need to keep the height below 4 feet, or the brickwork will eliminate the line of sight between the areas (wife says "no").
So while I can make a wider bell, or add a bench seat for more radiant exposure, I am limited on elevation.

OK, the WAF (Wife Acceptance Factor) is a very important one, in some cases The Most Important One.

Randy Butler wrote:On the 7 vs 8 inch device, can I not run an RMH at less than full load capacity?
Is it less efficient, or just more costly to build the larger size?
I have plenty of K26 Insulating Fire Brick, and so far, nearly all the red brick has been free for the taking.
What is the disadvantage of having a larger heater?

The batchrocket is burning as fast as it can, even with half a load.
It's not about money or efficiency, it's about the existent chimney.
The larger heater is as good as the slightly smaller one. The larger heater won't run well on a 6.75" chimney, as I told you before. 'Nuf said.

All that said, here's the proposal. A large column in the basement/cellar, containing the batchrocket core of whatever size. All hot gases rising up to the main floor, nothing down to the cellar's floor. The walls of the column heavily insulated, in order to minimize heat dissipation downstairs. A wide bell in the living, partly not higher than the WAF allows, partly bench, but large enough to have sufficient Internal Surface Area together, according to the bell sizing table. Exhaust gases are to be extracted from the living floor level.

How to build it is quite another story, though.
5 days ago

Randy Butler wrote:The site for the heater is an old cottage, 1.5 story, very little insulation, about 1200sf and open to cathedral ceiling.
I have no plans for winter usage (pipes are drained and the place is unoccupied during the cold spell), but I
do want to extend from mere summer use to three seasons.

The 7-incher would be perfectly adequate, you won't be there in the depth of winter anyway. The cathedral ceiling won't hinder at all, most heat is emitted as radiation which leaves the heater's walls at right angles. Only hot air will rise to the ceiling, people tend to forget that.

Randy Butler wrote:The design has the firebox in the cellar with the bell extending through the floor and up into the center of the open space living area.
It allows me to have the actual firebox at a very comfortable loading level, and keep the bark and wood chips mess out of the travelled path.
I will be burning 80 to 90% red spruce - that's what grows all around us, so I figured that an 8 (which seems to have anecdotal evidence to be
the best behaved and most forgiving) would be my best bet.

Please, don't build the firebox in the cellar. The firebox will get hot as well, that part of the bell would emit heat in an unoccupied space. The heater being in the center of the licing space is almost ideal, though.

I'd strongly recommend to build a footing in the cellar, ending just level with the floor above. This way, the entire bell will be in the living quarters and the fire is also visable there. Have the core elevated from the floor, about 2 feet would be very nice. You said there's a cathedral ceiling, so in practrice, there won't be height restrictions.

Red spruce will be a very potent fuel in a batchrocket. Good to know: every wood species has got more or less the same energy content, by unit of weight, that is. Coniferous species contains resins which has a higher heating value than the wood itself. Which means species with a high resin content have about 5% higher heating value as compared to oak, again per unit of weight, of course.

And last but not least: the batchrocket development has been done exclusively using soft wood. You will be surprised how much heat is generated with such humble fuel in a batchrocket.
6 days ago
I'd think Glenn is right here, a 6.75" square chimney would likely drive a 7" batchrocket quite well. Officially I should say something like "within reasonable tolerances" but "close enough" will do as well.
And yes, the whole of the batchrocket system scales up very fast. So a seven-incher is already a very capable heater.
1 week ago

Gábor Kertész wrote:That actually was my main question. I will keep the two chamber on the same floor then.
If I have to make a call however between an undersized chimney, an oversized ISA and a somewhat undersized CSA in the bench, would I still be best to go with the tighter bench? Or should I keep looking for another solution?

I would look for another solution. Why would you incorporate compromises in building such a heater, while the published design parameters are known to work? Almost guaranteed, I dare to add.

Gábor Kertész wrote:I'd also like to understand the physics of placing the oven. Would it be too cold further away from the firebox? Would it's top be too hot, being closer to the top of the bell, while it's floor not enough? I understand that for one the firebox itself is very hot, and two that in the bell the hotter gases are higher up.

On top of the firebox would be the best location, in my opinion. The top of the oven could be insulated and the hot floor could be damped somewhat to place an oven plate on spacers. By the way, the firebox is a very capable oven in itself, provided the fuel is all down to coals.

Gábor Kertész wrote:Also: are 4 cm thick firebricks sufficient for the firebox, and can I cast a clay bonded perlite riser for it? I'm a bit more hesitant to try my hand at cutting firebricks than at casting. I would also use as little rock wool (or alternative, as with the 5-minute riser) as possible.

Please use sturdier bricks for the firebox, able to withstand manual abuse. The riser however, could be built using 3  or 4 cm firebricks. When cast, preferably bought material, no home DIY recipes. And no rock wool in those quarters, this isn't up to the job in a mass heater.
1 week ago
Randy, I am not good at all with math at that level. What I do know however: in practise the chimney size need to be about the same as the riser. In some cases people used the same idea as you stipulated and installed an undersized chimney, but such combinations are rarely succesful. What I love about physics is their reliability, one could really build on. So there might be some flaw in the reasoning above, otherwise more unequal instances would be working right out of the box but apparently they don't.

I'd think there should be one more parameter in the equasion, namely gas velocity. Volume is much larger in the riser, about four times, but so is speed. Moreover, chimney temperature is much lower than with normal wood stoves so the chimney draw would be lower as well. In order to persuade the afterburner function to work, there should be a minimal underpressure in the chimney. Most of the time, the start underpressure is between 5 and 10 Pascal, depending on wheather conditions and left-over temperature in the mass heater.

Does this sound plausible?
1 week ago

Gábor Kertész wrote:Should the 1:5 rule be adhered to in general however? If I lower the bench with about 10 cm from the level of the first bell I get that ratio, but I also exceed the recommended total ISA with at least 0,5 m2.

Not sure I got the right picture here, so the following might be beside the point.
In order to get a correct stream pattern, please keep the floor of the bench at the same level as the floor of the main bell. And in case the exhaust opening is  somewhere on the perimeter of the main bell, in the bench there will be a stream of warm gases entering the top part of the bench and another one that is coming out of it along the floor level. Any level difference would disrupt the free flow of gases.

The 1 to 5 rule is one that is meant to help by designing a single bell, just to provide for enough space around the heater's core. The recommended ISA is what it is: a recommendation. Adhering to that ISA means the heater will work, almost guaranteed. With an ideal chimney of sufficient height one would have some tolerance in the ISA numbers, though. Both rules are tools to make building a well-behaving heater possible for DIY people.
1 week ago
In my opinion, there are a couple of things, three in fact that you might have missed.

The white oven will function much better when placed directly on top of the firebox, much easier to install as well. The top of the firebox will get awfully hot, you'll need a raised pizza stone or something like that. Moreover: the firebox itself can be used as a black oven when the fuel is all to coals. And with a steel grid on legs placed over it to hold the casserole or römertopf or whatever.

The 1:5 rule of thumb for a stratification chamber shouldn't be applied in horizontal direction. In order to let it function properly, the connection to the main bell should be as wide as the whole of the bench' inside. By building it like that, the bench is acting as an integral part of the bell construction. In order to better understand how the bell is filled with heat, turn your drawing upside down an imagine that it's filled with water. The places where the water comes first will be the hottest, where it comes last the coolest. It's easy to see that the seat of the bench will be (much) warmer than the sides.

According to the above example, the top of the main bell will be the hottest. But... in your drawing that is the part that isn't double skinned. Buiulding it like that, the whole of the bell's top will crack very badly, I fully expect it will do that within the first month of being in use. The steel T-profile and superwool solution in the Mallorca build is much more succesful. By the way: this construction is used many, many times now with commendable results, I might add.
1 week ago
I've read your first post on this topic Thomas, and had to let it sink in. Horizontal seams with superwool, OK, gravity will take care of that very reliably. But I have my doubts about the vertical seams though. Not that mortar is doing a better job there, brick wandering is really a pain in the proverbial behind. In a couple of applications I am using simple steel wire to keep bricks and so on together. The wire will expand quicker than the bricks, but while cooling off it contracts and will keep the bricks in place.

Now you've mentioned a steel tension frame, that's what I'd call the closing stone. My latest development model was more or less built this way,  most of it is contained in a steel box.

I've never seen 1/8" superwool, the thinnest have been 1/4", years ago and no longer on the market now. But everything is different everywhere, so if it's available why not? There's a material called ceramic paper, is this similar to this superwool?
1 week ago

Gábor Kertész wrote:I also just looked it up: the yurt that we bought was (is) until now heated with a 3,5 kW output split air conditioner. That would indeed be in the range of an 8" batch with 2 fires per day.
Can 2 fires follow each other if the mass is sufficient, or do they have to be apart in time? (I have since found the term "double fire" on Peter's site, which I presume is exactly this.)

Better to have two fires apart in time, like one in the morning and one at night. Of course you can do two loads back-to back but the chimney temperature and thereby the losses will go up quite a bit.

Gábor Kertész wrote:Edit| For the wood load above: there was a decimal error, I calculated about 0,054 cubic meters. Stills comes out to just above 4 cords per winter (averaging 20 °C delta T). It seems a tad too much, but that could be my lack of experience speaking.

One full cord is about 3.5 m³, so according to my calculation, that would be two cords and some more.

All this is assuming the minisplit of 3.5 kW, being the number which all is calculated against, is running day and night, all winter long. We both know that won't be the case, this minisplit was chosen so it would be sufficient for the extreme depth of winter. I'm inclined to think that you won't get up to two cords or 7 m³, but the proof of the pudding is in the eating.
2 weeks ago