Cookstove + Masonry Heater Hybrid

Hi all. Here’s an update of what I’ve been working on…

The first is just a section summary graphic so that the sections can be understood in the context of the overall project:

I developed a first go at the flow pattern at the level of the bench, which also includes the ash collection area.

I was surprised at how complicated the entrance into and out of the bench ended up being and am hoping I can simplify it. It is a constant goal of mine to keep things as simple as they can be.

I started thinking that the firebox design on the right, with the inclined exit, would result in better organization of the fuel, fire, and its byproducts. The down side is that it complicates the construction some.

I would love to receive feedback on any of these ideas/drawings. It really provides energy to the project to know that people are interested and there is a dialog happening. What do you think of how this is developing @Canyon, @mheat, @Tallgrass, @SolidRock?

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Max, the bench is looking quite long for the size of the firebox. I doubt it would balance out in the Austrian heater calculation program. I would suggest cutting the bench length in half, mainly due to the size of the small firebox.
The angled firebox back is not needed.

Looks awesome brother! Interesting fireboxes. Are you considering a port or is the firebox exit full riser size?

Thank you for the feedback.
How would the Austrian heater calculation program account for an even smaller fierbox like the one linked below easily powering a bench with a lineal run of 18’ (6 m)?

I am thinking the Annual Meeting is a good place to test things out and when it’s been cold, the heated bench has been appreciated.

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Looking good Max! Where is combustion air being fed? Through the door? The classic-ish design of the rocket stove horizontal “burn tunnel” and 90 degree entrance to the riser was the original point of mixing to induce the most turbulence and combustion. I wonder if the 45 degree angled exit into the riser smooths out the transition and it if wouldn’t be worth annunciating the initial throat to more of a point like Peterbergs tripwire brick to really enhance mixing in the 45 degree angled run.

Also, on a selfish note, whats the software you are using for these structural design prints, looks very clean and welcoming way of presenting a design to a building department.

@Tallgrass, thanks for your reply!

Well, I imagine it potentially coming in from three different locations… either from one of them exclusively or some combination of the three:

I have just always been so intrigued with the efficiency of the cigarette-style end-burn we have witnessed in an original rocket mass heater. This is an effort to allow for more fuel at a time with a glass door for viewing but maintain some of the synergy and symmetry offered by the original design….

This is a good point. One of my personal goals for this project was to take the time to review what had been developed and documented in the other forums with regards to batch box development. Your response forced me to begin that review process of the “p-channel” and the “tripwire”. I will see if I can continue design development to document that area that you and @Canyon are talking about.

Nothing selfish about asking a question in a public forum! You might be surprised to know that I am doing the design drawings in SketchUp. It is known for its 3D modeling features but I have found myself using it to draft in a more traditional way. I find sometimes when I am 3D modeling that I get lost in the specifics of each unit in a brick-by-brick approach instead of being able to design more globally. I use the companion program LayOut which is part of the Pro suite to add our logo plate, labels, dimensions, etc.

This is exciting. While I can’t speak to any of the technical specs, I think the concept and the aesthetics are coming along nicely. I like the added bench. The one masonry heater I have seen in operation had an air intake, I believe, right inside and below the door, so that it kept the firebox door from getting dirty. I don’t know how much that adds to the design complexity :slight_smile:

Air intake and door glass air washes are ripe for another topic! :slight_smile:

I spent some time this last week detailing the oven and the hot water heat exchanger…

I am going to update the original post with the full drawing set, where we are at now. I think that the original concept has mostly been flushed out at this point but there are still many details to continue working on.

One example is the switch from firebrick shiners to splits to accommodate the flow around the oven. I had imagined this from the start but seeing it drawn leaves me feeling that the portion of the wall where the bypass damper is mounted appears weak.

I also know that I need to draw a section from the front exploring the second route of the combustion gases across the cooktop.

This is a creative process and it is full of the excitement as well as the uncertainty of any such endeavor.

Here is a summary of the hardware that Firespeaking supplies that you would need to build your own version of this project:

The one item that we are limited in stock on is the cooktop. We currently have 10 in stock so if you are following this project and interested, now is a good time to make your order.

Here are two additional drawings…

The cookstove formation seems like the weakest part of the whole design. @Canyon , you had asked about the “port”… I’ve sketched it on the right. It’s currently roughly 6 w x 6 3/4 h. Thoughts on recommended size?

One of the biggest dilemmas I’m meditating on is that their is a confluence between the longer flow through the vertical masonry heat exchanger and the cookstove flow as they join towards the bench that I feel could be problematic in that it might cause a short circuit of the longer path and disrupt or weaken the more “rocket” type combustion we are trying to create when not in cooktop mode… Any thoughts out there? I will be thinking about it.

Greetings everyone,

I am sponsoring materials for this project and will participate in the MHA workshop, helping Max to build this hybrid heater. My long term goal is to install this heater design in my mountain home that is currently in early construction close to the NC-TN border.

The home will be a 32’ x 24’ timberframe with loft (~1000 heated sqft) on top of a 10’ high, prefabricated concrete basement. The walk-out basement takes care of the slope and will be built out with bath and kitchenette to serve as the “basecamp” for the timber construction cut from trees on the property. Since the green timbers have to dry as assembled before the structure can be enclosed with SIPs, there will be another heating season as an opportunity to modify and improve the design of the hybrid heater on my site, which is about 45 minutes away from the location of the annual MHA workshop.

In other words, if we get the heater running properly this April, great. In this case, the materials I purchased will go in storage and the heater will be permanently installed later in my timberframe on top of a masonry column extending from the basement. If we need more time to work some bugs out, I will rebuild the heater after the workshop in my basement with clay mortar for more testing and refinement.
Third build in the timberframe will then hopefully be the charm.

Highest priority for my needs is the proper switching between instant heating of the cooktop and long term storage in the masonry mass. My goal is two fires a day, ~12 hours apart. One for a man’s breakfast, the other one for dinner. (Summer cooking will take place on an induction range powered by the home’s PV system. Electricity will also heat the water tank above the masonry heater during the warm and sunny months).

Capturing the last few percent of efficiency is a lesser concern for me since we are inundated with hardwood in this part of the country. In previous years, I was not able to burn even the wood just falling down on a similarly sized property with a conventional woodstove in a skimpily insulated manufactured home.

The cleanliness of the glass in the fire door is also pretty low on my list of concerns. Radiation from a nice, hot fire will take care of any build-up quickly. We are not going to have a fire smoldering for 4+ hours, like in a conventional stove.

I am very excited about this project that will extend the range of core technologies utilized in my home to well over 1000 years. As an engineer, I have never been a fan of new vs old debates but instead looked at what gets the job done in the most effective way. (The pinnacle of efficient home heating is still the Roman hypocaust system where you essentially live inside an oven.)

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We may want to look into separating the task of “rocket” turbulence generation and potential secondary air induction from how the flow progresses afterwards. I.e. focus on a firebox design that reaches the desired chemical combustion efficiency and then direct the resulting hot gases under the cooktop or not.

PS: I just came across the Walker riser-less design that Max was involved in. This looks compact enough to fit under the cooktop with enough room for flow diversion. Short path would then be firebox-cooktop-chimney and long path firebox-oven-masonry mass-chimney.

If we run into issues with the confluence between short and long path in any layout that we choose to build, I would be amenable to an additional damper that blocks the path not being used at the confluence.

In the above drawing we have three vertical channels in the back of the heater. Left goes up, middle down, and right up into chimney. We could arrange the fire box and the left channel in a way to duplicate Peter Van Den Berg’s batch rocket core with vertical slit port and secondary air introduction. (images below used and adapted from cited source)

There is a good amount of data for this design and it would be a low risk approach for achieving good combustion. This would accommodate the mass heater function of the hybrid nicely.

To facilitate heating of the cooktop we could tap the flow in the riser and direct the gasses under the cooktop and into the right vertical channel leading to the chimney.

The way I understand the minimum length for the riser is that after that distance the flow cross section expands dramatically into a 50 gal drum or masonry bell. If we maintain the cross sectional area of the riser as we circulate under the cooktop, then we should not experience any problems due to the effective riser length being too short.

What do you think?

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Hi @alpine! Welcome to the forum and thank you so much for your support for this project through your purchase of the hardware that we will be using in our workshop build and now through your attention and contributions in the design process…

I will follow up further with some drawings but will try to give you some initial assessments:

  1. I am hoping that the downward funnel that is presented in my drawings is an improvement on the design you reference. I am hoping that it will both provide for more focused self-organization as the fuel burns as well as for a more directed cigarette burn at the leading edge of the fire, with these two factors feeding on each other. It is an effort to recapture some of the original J-configuration of the RMH as it evolved to the BBR, while trying to maintain the door and larger fuel load.

  2. I like your idea about focusing on one optimized combustion path but… the fact that I am a chef/cook heavily influences my design work. One thing I have learned in our work with the sidewinder (which I think Firespeaking is inadequately credited for) is that removing the direct relationship of radiant energy between the flames and coals and the cooktop can create a significant reduction in heat to the cooktop. Designing the flow patterns in a masonry heater with optimized combustion can be quite an academic experience which sometimes loses touch with this reality. That said, I will try to respond with some sketches to further study this subject.

  3. I am also thinking that we will need to introduce one more bypass damper / valve, especially in this prototype to study the natural flow tendencies.

More soon…

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I have cooked quite a bit on wood-fired stoves myself while living in Bavaria and yes, they all had direct flames plus radiation from the fire/coals hitting the underside of the cooktop.

So lets keep that feature and hope that we can, in the heater mode, get enough energy stored in the mass without overheating the room via the inevitable heat transfer from the cooktop.

During test firing, we should definitely try to suck the fire away from the cooktop by switching to air from above like you sketched here:

If this works by closing all other air sources and simply lifting up one of the cook plates, then we can find more elegant ways to reverse the flow under the cooktop.

I propose trying the following for the cookstove mode:

Using small, dry wood sticks, a fire will be lit from the top in the firebox and supplied with air either though the intake on the door or from below through the ash grate. The flames and hot gases will then shoot upwards and hit the larger round insert of the cooktop. This is how most of the traditional wood-fired cookstoves work.

Instead of then lazily routing the flue gases under the cooktop over its entire width to the chimney, we should try creating a choke point between the two plate inserts to form a vortex under the second, smaller insert. The goal is to achieve secondary combustion by drawing additional air into the chokepoint, similar to the P channel on Peter’s BBR design. This would not only clean up the flue gases but also increase the temperature of the plate insert where the cookware is placed for fast heating.

The challenge for achieving secondary combustion is that the cooktop will remove a lot of heat energy from the gases and it may not be possible to maintain ignition temperature. If that is the case, a catalytic combustor could be employed in a later design iteration.

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I just realized that the majority of Peter van den Berg’s work is documented in detail here. - Introduction is more of a high level summary.

There are two of Peter’s designs that in combination could fulfill our goals of a quick heating cookstove combined with a long duration mass heater.

The BBR (top left) has been used extensively in mass heaters. The DSR (top right) has satisfied the direct heating needs of many cookstoves.

How about combing the two designs in our hybrid (bottom center)? A shared firebox that can either blow the flame vortex into a riser, which then continues into the mass heater or -by changing the dampers- create a vertical flame vortex under the cooktop.

The Double Shoebox Rocket has been operated successfully by Peter without secondary air injection. We would have to figure out how to route the secondary air to the rear venturi port leading to the riser in a way that the metallic air channel does not get burned out when the stove is in the cooktop mode.

@alpine, thank you for your recent contributions. The drawing set including the shoebox hybrid rocket is very interesting. In general, this design background has informed this project’s design work and it is very helpful to see it in this summary. As mentioned previously, I am interested to see how creating more of a funnel in the firebox shape can contribute to the fuel and fire self-organizing itself better and I continue to be unsure about the effects of isolating the firebox from the cooktop when in cooking mode.

If that is a drawing from @peterberg10’s Batchrocket Site, can you please provide a specific reference/link to the original by editing your recent post so that it is properly attributed - thank you!
And just a quick plug for the dampers and cooktops that Firespeaking has available to make these designs possible.

For anyone wanting access to the raw SketchUp file at this point, here it is:
220405 Cookstove and Masonry Heater Hybrid with Sections.skp (2.1 MB)
It’s worth reminding that this is still very much a design in progress. I am digging into a brick-by-brick approach now to continue planning and looking at the design…

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Here is the latest drawing set in preparation for the build at WildAcres:
220410 Cookstove and Masonry Heater Hybrid.pdf (1.2 MB)

The most significant work is on the cookstove detail…

I also simplified the horizontal flow in order to be able to define the project as a more compact module and then be able to add the extensive heated bench:

I wanted to include a reference to @mheat’s excellent article on thermosiphon plumbing for a masonry heater also as a reference:

You can also see an example of passive thermosiphoning to heat how water from our portfolio:

I recommend placing the heat exchanger a little higher in the system to improve combustion efficiency.

This might be the last post to this thread before we build the prototype.

Success! Here’s a photo by @Sam of what we built last week. It performed better than expected. Stay tuned for more project reports.

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