A rocket cookstove with heat risers

The goal: design a rocket cookstove that is also a mass heater and an oven, and is as clean and as efficient (or very nearly) as a typical RMH.

I like the walker cookstove, except that it uses too much wood for my liking. I think that this is because it lacks an insulated heat riser. The problem is, a heat riser uses up a bunch of heat really fast, by radiating it out into the room, so it seems like having a separate heat riser

Idea number 1: make four heat risers and connect them together in series, with the tops of the barrels removed and a stone slab placed on top. This would hopefully create a differentially heated surface like that of the walker, but with greater efficiency.

Idea number 2: why bother with the extra heat risers? Just put in one, bring the exhaust up under the slab, and spread the heat that way, á la walker.

My worry with both of these ideas is that too much of the heat will be extracted by the heat riser, and the rest of the stone surface as well as the Russian-style oven-bed combo that this concept calls for will not be heated properly.

Idea number 3: one heat riser, in the middle of the slab. Exhaust goes straight to the mass. My worry with this one is that perhaps there won’t be enough heat differential to easily cook whatever you want at whatever temperature, especially if you are using multiple pans.

I think you have some misconception about heat risers... when properly built and insulated, they will not lose much heat at all. The common barrel around the heat riser does radiate a lot of heat by design, but that is not a requirement. I am pretty sure that multiple heat risers would not work well.

What gives you the impression that the Walker riserless core as used in his cook stoves uses a lot of wood? As far as I know, it is an efficient design on par with other RMH cores, and if it uses a lot of wood it would be giving out a lot of heat. The Walker core has a highly insulated horizontal secondary combustion chamber and depends on a good chimney for draft, but would not burn up extra wood. Peter van den Berg has developed cores with horizontal "risers" and with instrumentation has shown them to be on the same level as other RMH designs.

I believe any kind of stone slab directly exposed to a heat riser's outlet would suffer differential expansion and crack rather quickly (with the possible exception of soapstone, whose properties I do not know well enough.) I think steel or cast iron or the Walker-recommended glass cooktop material would work much better. If the cooktop area is sufficient, I believe you would get a good range of temperatures for several pots.

I think OP is confusing the terminology a bit.  Heat Riser, as commonly used, is commonly considered the vertical column just after the burn chamber where gases rise up and burn off hydrocarbons.  In the typical RMH, this riser is surrounded by the barrel, which in itself can be considered a bell of sorts, where immediate radiant heat is made available.  I was going to do a barrel surrounded by bricks set in a staggered pattern so the space of about one third of a brick would expose the barrel, tempering the rate of radiant exchange while adding some mass to it for longer term radiance.  

I don't think the Walker Stove really uses that much more wood than other RMH type stoves, but it does have a small firebox and needs to be fed often.  But then again, there are always trade-offs to any design.  What Matt does have is an excellent stovetop for cooking on.  Even if depending on it for a primary heat source might get cumbersome.  

I would like to have a BatchBox which has a small cooktop and an oven, and also heat a rather largish bench.  Have to be careful not to ask the core not to do too many tricks at once though, or the builder may find himself disappointed.

My advise is to look at how cooktops are constructed out of steel, and fabricate one like that, or go with Matt Walker's ceramic glass cooktop, if you can source the material.  Personally, I'm having a hard time with that.

Good luck.

The thing is, traditional masonry heaters in Russia and China have stove tops, ovens, and heated beds, all from one fire. If they can do it without the efficiency of the rocket, why can’t we, with that advantage? In fact, I am so unreasonable that I require that whatever design with these components that I implement to do all that on less than a cord of wood a winter, that I don’t ever need to replace parts, and that I will have a choice between just using the stovetop, using the stovetop and the oven, and using those two and heating a bed at the same time. If we can make a design that polished, the Russian stove-builders may start building RMH’s. If an RMH can’t do at least as much at least as conveniently as a traditional Russian stove, they are unlikely to show interest.

Rocket mass heaters have been used for both cooking and space heating since their beginning. Matt Walker's earliest RMH videos were him modifying a standard J tube wood feed and burn tunnel to include a hot plate on top and a small woodstove door in front to insert bread to bake and that worked, and it still had the thermal mass laid out as a bed/bench.

His more recent cooking stove/oven combo has a highly insulated core so that the heat reaches the glass cook top without losing heat to the brick surround. It then goes down and past an (I think black) oven, and then I believe he has a bypass to either direct the remaining heat into a large bell for space heating, or bypass that and go directly out the chimney to avoid heating in warmer months.

As far as convincing builders to switch from traditional masonry heaters to RMHs, I don't see why they would. Currently they build a $10K+ device which is at least half labor cost. Switching to a $50-$1000 RMH (depending on the parts you use) which takes less labor and is a novelty relative to existing masonry stoves, seems like a lose-lose for the builder who wants billable hours.

For the owner-builder, a RMH is by far a major win-win. Far less costly, quicker to build, and just as efficient. Ianto Evans came up with the original RMH design with the goal of making an incredibly affordable heater that uses very little wood. When I visited in 2016, he said the used barrel in the library named Myrtle was the same one he installed in the '90s. It was made with recycled red bricks and recycled vent ducting, and was totally free. It's not optimized for daily cooking but Matt's design aims for that hybrid option. Matt's still radiates heat off the cook top, but it radiates up rather than out like a round metal barrel. That's where (as a heater only) it loses some efficiency in heating.

So the idea I have from this, is that if I have an insulated heat riser instead of a radiant barrel, it should be fairly simple to design in a stovetop that gives off plenty of heat. Good point about the professionals. But I think that a comprise can be reached. Using their own materials and design style, but hooking it up to an RMH core might be the best bet.