Cookstove + Masonry Heater Hybrid

@SolidRock, it’s an honor to welcome you to the forums. For those who don’t know Eric, he is one of North America’s most accomplished masonry heater builders.

Indeed, I am planning on a slightly larger than normal, for a cookstove, firebox and importantly for the main combustion route to lead into a significant heat riser which I am hoping will produce the kind of thrust that we see in rocket mass heaters. I should have some sections done within a week so that the inner workings will be more understandable and open to critique and response.

Max,
I look forward to this. I will have to look at getting a link up on my website for this forum so that others can see it, learn and contribute.

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The Heat n Eat (alator)! But in all seriousness, i’m really looking forward to seeing your design come to life in April. It reminds me of the first masonry heater I ever saw, at Granja Valle Pintado back in 2011 with a small water tank up above for hot water heating and a lounge-y heated bench… I became infatuated with the idea of hot cob!

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Hey, hey! Welcome Jim! Another great heater builder joins our ranks!

Indeed, once you get interested in masonry heaters, especially from first-hand experience, that interest tends to continue strong.

I love the “Heat n Eat (alator)”! Just now realizing I can’t change the poll after it has been live 5 minutes.

Yes, looking forward to further developing the design. Stay tuned!

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Hi all! I wanted to post some drawings before the week was out. Here are two sections that I have begun to develop for the inner workings of the stove. It still needs a good bit of refinement and a couple more sections to fully describe its function.

This is my first shot at describing the combustion chamber and the two possible pathways that gases can travel as they exit. It still needs some effort to make it “cozy” for the fire…

And here is a first sketch of the main heat exchange battery. The oven and hot water coils still need to be detailed in this section…

Stay tuned for progress next week!

How’s the design coming along?

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.

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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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