Rocket Heater for tiny house

hey, I need some help planning a small rocket heater for a tiny house in a garden-colony. The house is 12,73m² (137sqft).

I d like to build a cast core (box like shape) with a round heat riser placed on top of it.

This is what I have available:

Steel barrel 100 liter: 45cm(17,7 inch) diameter, height = 81cm (31,9 inch)

a few meters of 10cm/4inch stainless steel pipe and 4 90°bends

100l perlite

dug-up clay

Sand

lots of sand-lime-bricks

some alumium-tape (package says "heat-resitent")

some pieces of used wooden board

used mineral-wool-insulation


heat riser: I´d like to use a 10cm stainless-steel pipe placed into 20-25 cm (8-10 inches) stainless-steel pipe, space filled with clay and perlite. Is it possible to use a temporary plastic tube for the outside of the heat riser and remove it? I have some pieces of wide plastic tube/pipe.
How do I make the transition from a square burning-chamber into a round heat riser? How do I keep the same CSA?
How tall should it be?
If i make the cast core 10 by 10 cm, the base already is 20cm (8 inch) high


From which material do I make the cast-core? I read about clay-slip and perlite. In a consictency so that it can be hand-formed but will crumble under pressure. Does it make any sense to put rock-wool fibres into it?


For the cast core I plan to build a sacrificial form from scrap wood. What dimensions do I use? To keep the same CSA of the 10cm/4 inch stovepipe, that would be 8,86 by 8,86 cm (3,5 inch) for the feed tube and burning chamber. Can I make that bigger and cover it with a brick to reduce air-flow (competing chimney effect?)

What width of the walls (clay + perlite) of the cast core do I need to all sides?

I d like to have it 10cm by 10 cm. Feed tunnel and base for the heat riser 20 cm deep. Burn tunnel lenght 20 cm. So the whole lenght of the tunnel at the bottom would be 40 cm (16 inch). Would that work?


restrictions:

no outside chimney possible

has to be able to be dismanteld and set up again (the houses walls and floor are not finished yet) in a few months

no or no big mass

no welding possible atm




I need some help, sorting some questions out and I need to order my thoughts here and see, how I could build that.

thank you
tobias

What do you mean by "no outside chimney"? You can't let the exhaust go into the house interior. We need some clarification on this.

A 4" RMH system is possible but at the lower limit of what has been found to work, and needs to be designed and built exactly right or it will fail.

For core sizing, a square channel has about the same flow capacity as a round one of the same diameter, so you would have a 10cm x 10cm feed tube and burn tunnel with your 10cm diameter riser.

For the riser, a metal liner, even stainless steel, will corrode and fail (a 4" system may last longer than a larger system because it will not be able to get as hot). I would use the 4" pipe as the inner form, with some sticking out of the top after packing the perlite-clay, and carefully pull and twist the pipe out of the riser once it is firm enough. If you can find a cardboard tube of the right size and burn it out, that would probably be easiest. You want a metal tube for the outside of the riser, but it can be anything of 20-25cm diameter. Old air duct will do. It will not be subject to heat degradation the way a liner would.

A wooden sacrificial feed tube/burn tunnel form is a good idea. I have done a few of them and they worked well. Your internal proportions sound okay. The recommended proportions are 1:2:4 feed tube to burn tunnel to riser, measuring either along centerlines or along outside edges. As you have a barrel 81cm tall, I would start from that, leaving 4-5cm space at the top, and saying the barrel sits 8cm above the burn tunnel floor. This gives a riser of up to 84cm, call it 80 for simplicity, burn tunnel 40cm max, and feed tube 20cm max. This might be too stretched out for a 10cm system, so shortening the burn tunnel would probably be good. A feed tube 15-20cm high from burn tunnel floor, burn tunnel 20cm total, and riser 80cm high from floor should work. You might try a loose section at the top of the riser so you can experiment with heights from say 60-80cm. I would set up the barrel and exhaust from it so that you can cut the barrel down a bit or set it lower if desired. You will have to experiment some because parameters for a 4" J-tube are not well known, and just scaling from larger sizes may not work well.

Finally, I would consider a different option from the barrel in a tiny space. A small masonry bell (hollow brick box) with the riser inside it may well be safer and easier to fit into the space. If the riser casing is 20cm diameter, a bell interior of 40cm and exterior of 60-70cm should be workable. You could build most of this except the top with ordinary bricks and clay mortar that will come apart easily to rebuild. For a temporary 10cm system with its small heating rate, I would even consider a cast-in-place cob top of say 20-30cm thick. I would want something more durable for the permanent top.

Here is a wooden inner form for a 6" rocket cooking stove I built. You can see the taper I built in to transition to the round riser.

John said he also has a small place and is building a small rocket stove here: https://permies.com/forums/posts/list/40/53224

Thanks FStyles, here's a link to my first compact rocket also
https://permies.com/t/52476/rocket-stoves/compact-RMH-hybrid-cook-top
Or check my YouTube below \/

thank you so far ....

concerning chimney: I meant no vertical outside chimney. Just an exhaust covered by metal-grating. I have kinda outlet in the wall in a height of 1,80m, but i could set it higher to have an internal chimney of 2,5m.

I d like to use a barrel as bell. I want the heat to shed quickly. As far as I understand, I want the gasses from the heatriser to cool quickly in the barrel to create kinda "pumping-effect" to help with draft. Maybe I d make a structure around the barrel from adobe bricks later, with ventillation openings. Not sure.
Atm I need heat source soon. I want to do cob-plaster, so it s important to heat the house to dry it.

I m not sure if it might be doable to add a second bell.

The house is build from 11,5cm sand-lime-building blocks and roof covering is from fibre-concrete-sheets.

Will perlite-clay be ok for the cast core? How wide must the walls be? 2 inches? 4 inches?

The higher in the wall you can set your exhaust outlet, the better. You do realize that whenever the wind is blowing against that outlet, you will get smokeback, right? Is your wind 100% constant direction during the heating season?

If you want heat pretty much only when you are tending the fire, that's your choice. Have you operated a fire of this size to understand how often it will need feeding?

The pump effect of the cooling barrel may indeed be necessary for draft if you do not want to have a vertical chimney.

Perlite-clay in a good insulating mix is rather soft, and will break down quickly in the feed tube and burn tunnel where it is subject to abrasion. In those areas, I would use something stronger, like 1 or 2 cm of mostly clay-sand. Refractory cement would be better but is expensive. I would make around 10cm of perlite-clay around the core aside from the riser inside the barrel.

ok... it might be, that i am mentally a bit fixed on the 4" system. i first planned a portable heater. so now i have 4" pipes.

could i make the cast core 6" with the 4" exhaust pipe? when i close the feed tube to 4"? and change to 6" later, when i ve got the parts? i do not plan to have mass now, but probably later.

we do not live in that house permanently. it s more for summer, weekend and afternoons.

the wind comes from the other direction. the exhaust is on the backside of the house, which is somewhat shelterd from wind.

i tested it with just the pipes. i made a j-tube from the 4" pipes and put the directly into the internal chimney. it had good draft even in stormy weather. i think, with a proper insultated burning chamber and a bell, it might work well.


(sorry, my thoughts are not very ordered yet)

If I got one thing from the 4 tape set it is that you want at least the same volume throughout the system, and you probably need that to be the same volume as in the burn chamber. So you can have more volume in say the barrel, but I don't see that being good in the burn chamber. But it should be possible to throttle it back with some quality bricks, or tiles.

The other thing you say is you you want the heat coming out fast, which makes sense if your episodic use is such that an RMH would only get up to heat when you leave. What I got from the tapes was that Rumford might be your best bet for a nice warm fire that switches on and off during occupancy. You can stack one of those, it might not be all that much work compared to even a pared back RMH.

hey tam,

thanks for the reply. a rumford might look nice, but it would not burn smokeless and require more wood.

i m thinking about adding a mass. but before i could do that i would need to finish the floor and the walls. and that would require some heating for cob-plaster and maybe earthen floor to dry

It's perfectly reasonable to build a basic core of an RMH to use while finishing construction, and then add the mass afterward. It will work better if you plan for the mass from the start, of course.

You do need to size the core and exhaust together or they are not likely to work well. One thing you can do with a bell system is to make a good sized steel access panel in one side, and possibly a steel cooktop, either of which would act as a fast radiator while the remaining masonry stores heat. A steel panel could also reduce the overall size of the unit, if that is critical. Changing one wall from 4-8" thick to 1/8" makes a big difference in a small space.

Could this work:

i was soooooo thinking about that but with rammed earth walls.
the point is, i would have to rethink from J-Tube to batch. and i would have to research if that would work with clay and perlite. or something other that i could get without too much trouble

which would probably work better, since I have 4 inch pipes laying around