Rocket Masonry Cook Stove & Home Design Planning

Hello,
I am new to this forum. My husband and I hope to build a home in Alaska in the next couple of years (Zone 4a/4b). I have been doing what researching I can, and a Rocket Mass Heater seems like the way to go. We have never built or even seen one in person, but I am basically planning the whole house around the RMH. As such it seems important to get a little more info on what they are like to live with and what practical expectations should be. I have been most impressed by Matt Walker's Full Cook Stove and Continental Cook Stove designs and they are basically the starting point for what I'm thinking (or maybe we will just use his plans as is).  In my mind simple is better, so I would LOVE it if this unit could be the only source of heat, cooking, baking (baking and roasting options would be ideal), and hot water, along with a warm place to sit/lay...so long as its truly simpler. We are definitely okay with going outside of the status quo on our living situation and lifestyle choices. But want those choices to add to the simplicity and joy in life, not take from it.

All that said, I am trying to learn what is practical to expect this heater to accomplish, and what it will require from us on the daily. We do not yet have children but hope to in the near future, so I have to build in some margin in the time inputs required from me to keep the home warm and everyone fed. (The plan is for me to be able to stay home, my husband will work full time outside of the home). Since I have no experience I'm hoping those who have lived with these stoves could give some input.

As a reference, I am guessing we will start with something like a ~600sqft cabin and then add on down the road expanding to ~1,000sqft give or take. I'd like the initial build to basically be the main living area for the long term plan, with a sleeping loft above. Then as resources allow adding on a bedroom or two on the main level and potentially expanding the loft as well  (this will depend on how many munchkins make their way into the picture). Matt says his stoves can heat up to 1200sqft, so i'm hoping its not impractical to have this be the only heat source, even in AK. I am a little concerned about getting cooked out before any addition is put on...?

SO if you have a Rocket Mass Cook stove, I'd love to learn the following from you:

Configuration: of the stove & the building its in

Cooking:  Is there anyone who uses an RM Cook Stove to cook exclusively? How many meals per day do you cook on it? What is the heat up time or total time required to prepare most of your meals? Does it ever get too hot in the house for cooking? Obviously our cooking needs do not change with the seasons, so we will need to heat the stove to cook regardless of temperature. Will bypasses be enough to accommodate this? High temperatures in this area are generally in the 70's in the summer months. We would also probably include an outdoor cooking area because we like to be outside, but I'm not sure how I feel about that being the only option in the warmer months.

Baking: Do you run a white or a black oven and why? What do you use it for? What is its proximity to the riser and what temps does it run? Are your bake times longer than in a conventional oven? How much? Does anyone have 2 ovens in their stove (one for higher baking temps and one for lower roasting temps)?

Water: Is anyone heating their water with their stove as well? What is your configuration? Does it decrease the heat up times or cook temperatures for the stove and oven?

Bench: How big is your bench? Is it a flue pipe or a bell? If the stove is already heating a stove top, oven, and water, what length bench is practical? I know this will depend on the size of the system (6", 8" etc). I am not sure what size system Matt is running in his cook stoves... but would you guess it would have to be increased?

Overall : do you guys think its more practical and efficient to have multiple stoves to do all this, and just manage multiple fires as needed? Or is doing it all in one stove a manageable solution? My simple is better approach loves the idea of all in one, but if it means that cooking is slowed down enough to be impractical, then having the water have its own stove or a conventional water heater would be worth it.

I hope my ideas and questions make sense. Feel free to ask any questions necessary to clarify. Any and all experience and input is VERY much appreciated!

Hi Eloise, welcome to Permies!

I'm using a RMH for 100% heating a fairly insulated stick-frame 900 sq. ft. house with 450 sp. ft. add-on, but actively heating the main (open floor plan) living space of some 680 sq. ft. While my RMH is the best woodstove on the planet (for my use) and has cut firewood usage from 3.5 cords (for a steel box stove) to between 1 and 1.3 cords (RMH) per winter, it is still a chore gathering and processing fire wood on a continual, year after year, for the past 20+ years, basis when one has other things one would rather be doing:o)

I'll pass on some personal opinions, and you're welcome to take or reject anything useful or not so useful etc. Depending on just where in AK you're building your house, you may want to consider building a house (or incorporating some of the ideas) where the house both heats and cools itself, collects, stores and filters its potable water, provides its own electricity, and grows food for you too. And you can still have a Matt Walker masonry cook stove (for auxiliary heating plus cooking) and/or an electric (solar powered) induction cook-top.

When viewing the following video, keep in mind that these dwellings can easily, actually more easily, be built from traditional materials. I.e. instead of labor intensive beating of earth into auto tires, build the walls using steel reinforce MCUs (concrete blocks) filled with concrete, and forget all those cans and bottles too, unless you have a source that will walk itself to the build site. Do keep the four foot wall thickness (thermal mass), perimeter insulation, and moisture barrier - obviously.

TEDX talk by Michael Reynolds:

   

Just realized, that TedX video is from 12 years ago. In it Reynolds mentions that Earthships are built into the ground. However, the latest models of the Earthships are built on grade, and then have the dirt etc. berm pushed up to and formed around the building and perimeter insulation as the building goes up. Here's an example and tour of a late model Earthship, the  "Encounter" model:

Thanks for your response, Byron!

What is your RMH configuration? Do you have a steel barrel? Cob bench? What size is your system (as in the flue pipes- 6", 8")? You mentioned that you primarily heat the 680sqft open living space. In your experience would your system heat the whole house if need be? I also noticed your are in TN. what type of cold temperatures do you have in the winter months? Certainly gathering wood can be a chore, but I expect we will be doing it anyway. For us, heat is too important to be dependent on a utility company.

Thanks also for the video. I have heard of the earth ship concept but hadn't had it broken down into the individual facets. We are definitely pursuing being as self sufficient as is practical. We will most likely do a wood construction (could be round log, timber frame, or stick framed depending on what lumber is available). A garden & greenhouse is a must as store bought produce in AK leaves things to be desired. The water and power will depend on the land we end up with (we have not purchased yet, and probably won't for a year-ish). Depending on it's distance from existing electricity and our power needs it may end up cheaper (and arguably more desirable) to invest in a self sufficient off-grid system. In reality that is a part of why I am trying to get a better grip on the stove first. In my mind having self sufficient heat is most important given the climate. Understanding what else the stove can provide will give me a baseline of what needs will have to be met another way. If it looks like we don't have practical independent solutions for the electricity issue, then I know we need to pursue obtaining land closer to the existing grid.

Thanks again for your response, Byron.

Properly constructed and configured, an Earthship will maintain a 70° nominal living space temperature all year round, summer through winter, in temperature extremes from summer's high 90° F, to winter's below zeros, by way of the building's thermal mass / thermodynamics / passive solar. That one thing is what caught my attention. Zero utility bills, with the exception of gas (LP/Propane) on demand hot water to back-up the solar hot water on cloudy days - as per Mike Reynolds designs (see https://earthshipbiotecture.com for more details.

About the Rocket Mass Heater, my stove is essentially Ernie & Erica's Cabin-8 but modified to use a Peter van den Berg batch-box combustion unit; see Cabin-8 link below. The whole nine yards, bench and all, is 9.5 feet in length. No cob used, I chose to use brick instead (see photo below). The trusty steel drum is painted with 1200° (F.) stove black for good looks. During an average winter, on the coldest days, overnight temperatures at my place will tend to drop into the low teens, rarely but sometimes near or below zero. Daily highs usually go above freezing to as high as the 40s.

The stove is typically fired each morning from 4 to 6 hours straight, bringing the immediate room temperature up (from 62° to 65° F) to between 72° and 75° F. The mass heats up and provides a nice 24 hour flywheel into the following morning. The overnight drop in room temperature could be made less by better wall insulation (currently R11). I do have R38 in the ceiling. I imagine, that in AK, code will dictate insulation R-values. For mass heaters, more insulation is better and I would shoot for at least R30 in exterior walls and R50 for the ceiling, and an air-lock entry doors arrangement.

The 8" system size RMH will easily heat my entire house and addition, with the use of a couple of fans pushing air at floor level from the furthest rooms towards the stove.

Ernie & Erica's Cabin-8 RMH photos and plans:

https://permies.com/t/40984/Rocket-Mass-Heater-Plans-Cabin#319710

Hi Eloise, this is my first time posting on this great forum but I thought I might add my two cents. I have absolutely no experience with RMH's, but that being said I'm gearing up to build one this summer here in Kenai where I live fulltime, and possibly on my off grid property in Nikiski, if time and money allow. I own the plans for the Walker Tiny Cooktop and have thoughts about buying the plans for the Continental for the additional oven. The reason for my posting is that I wanted to know approximately where in our great state you are wanting to settle in? Why I ask is that I have found it difficult, but not impossible, to find certain supplies for building a RMH here on the Kenai Peninsula. Clay bricks, as far as I can tell at this point are nearly impossible to locate locally, even Los-Anchorage.  I plan on going with the "Bell" route with both of my builds. Besides the difficulty of sourcing building materials another issue is the wood that you'll have available for fuel. Everyone around here wants to sell you beetle kill, of which is crap in my opinion, but hey, it's fuel right? I've been burning with seasoned cottonwood for the last two seasons in my Englander wood stove in my Bushop AKA Skoolie Shop, I'm a huge fan of cottonwood. It's readily available and I swear people around here hate it so much they'll pay YOU to take it off their property! Birch can be found but it's difficult to source unless you are using your own trees of course, but I like the look of my birch so I don't tend to use it much. I also have a very large amount of alder but I haven't had the opportunity to use it yet, and I hope to save it for smoking foodstuffs. If you need any help locating materials for your build definitely hit me up, I'd be more than happy to help! If you're close enough to the Kenai Peninsula and want to help build a RMH for learning purposes I'll take all the help I can get.

Byron - Thanks for the specs on the RMH. That is all really helpful information.

Jacob- Thanks for taking time to put in your 2 cents- its super helpful. We are in the process of relocating to the Kenai Peninsula actually (currently in the lower 48). It will likely be spring or summer before we are there, depending on how things go. That is very helpful information about building materials and wood sources. I had guessed certain materials would be hard to come by considering I have had difficulty sourcing firebrick for a ceramic kiln project here in the lower 48. Have you been able to find/scrounge the materials you need, or are you still looking? Depending on our timing in the move, we may be interested in helping with parts of the build. A part of my concern has been having to depend on our first build for so much (heat, cooking, water etc.). I'm sure it can be done, but it would be nice to gain a little experience first.  I will also be very interested to hear about how the stoves work for you when you get them completed!

If you don't mind me asking, what size building will your stoves be going in? And do you have other heating sources you will also be using? Do you plan to use them to cook? exclusively or supplemental? If you don't have answers to all those questions, no problem.

Thanks again for your responses folks! Any others who have experience with a Rocket Cook Stove- I'd love to hear from you!

I have a rocket stove in my outdoor party house that I cook on regularly, I did have a pice of ceramic glass on the top as shown in the video but I have changed this for a solid 10mm steel top.
The 6” tube that runs through the barrel is very useful and cooks lots of great food like jacket potatoes and  garlic bread.
The hot plate is superb for cooking, steaks, fish, anything really!
I have had other rocket stove formats but the top Loading J tube style suits me best.
If I feed lots of small wood the top will glow red hot at over 1000f but you can control the temp to a certain extent and the hot plate has different temp zones.

 Absolutely! That's one of the many joys of living in paradise, it can be difficult to find certain things that folks in states have an overabundance of and getting things shipped here is an expensive endeavor.

 As far as materials go, I called Distribution International in Anchorage this morning they have all sorts of refractory materials in stock, so I'm good there. I might even make a trip over the pass to load up for the build this spring. It's the thermal mass portion that's been difficult thus far, but I just discovered a substantial amount of red clay on my property in Kenai and I have begun experimenting with it. I'm working on some Rammed Earth ideas utilizing my clay to replace the need for the red clay bricks and I'll start a thread on here so that I can share the results as I go along. There is also a solid surface countertop shop in Soldotna where one can source all sorts of amazing materials, but alas they don't come cheap.

 The first RMH will be a 6 inch J-Tube similar to James's posted above. It'll be built in a 16x24 ish foot passive solar greenhouse I'm breaking ground on this spring in Kenai. The structure on my property in Nikiski is slated to be around 800-1,000 depending on my wife's desires Ultimately I want smaller, but she wants square footage for the kiddos, so I'm sure I'll be the one compromising... I plan on using the RMH for absolutely everything that I can, cooking, heating, hot water, baking, etc etc. That's why I want to build a simple J-Tube style at home in Kenai first, so that I can experiment before planning the big one out north. I will have back-up cooking sources, propane fueled, for simple cooking purposes but my ultimate goal is to be free of the need for fossil fuels. One other idea I have is to have a standard secondary air wood stove in the home for a "just in case scenario." Something that can provide quick heat if there are issues with the RMH. We get a lot of good earthquakes on a regular basis so I'd rather not leave my heat source to just one unit, especially considering the fact that it'll be made out of the very stuff that isn't very earthquake friendly.

 I hope the information is helpful, and I do plan on posting my progress on various ideas and builds on this forum as I go along in the hopes that it can help other folks. If you need any information on the Kenai Peninsula I'd be happy to help, might be easier to do it via private message though so that we don't get too off topic from your question.

https://live.staticflickr.com/65535/51820992553_cf37db019c_h.jpg

https://live.staticflickr.com/65535/51820981888_012436676c_h.jpg

Thanks Fox James- That's an awesome looking stove. I have never seen a J-tube in glass before. I tend to gravitate toward the batch box style, in part because you could see the fire with a glass door. But having glass on the side & on top on yours is awesome. And the tube through the barrel is brilliant. How are you liking the steel top? I have contemplated doing a cast iron top so that you could actually cook directly on the cook top, similar to a Black Stone. If anyone has any input on that concept I'd love to hear it.

Jacob - Glad you found a source for refractories, and super exciting to have some clay on the property! Looks like you are doing some testing. I'd love to hear about your progress. I have never made brick or firebrick, but I have some background in ceramics and have done some reading in the chemistry/thermodynamic properties of various ceramic materials (in order to make clay & glazes for different applications). Not sure whether or not I'll know how to troubleshoot anything you come across, but I'd be happy to try.
You mentioned earlier that you had purchased the plans for the Tiny Cook Stove. From what you've learned, do you think this stove would heat your 800-1000sqft home? I was feeling like it was maybe too small, but maybe I am wrong.
Sounds like you and your wife are already doing much of what we hope to do! I'm glad you jumped in and responded to my questions. We look forward to hearing about your progress, learning what we can, and giving help when we are able.

Thanks again all! I am already learning a lot. Looking forward to continuing the process.

I have a few more videos, this one shows some of the construction details so you can see that the stove is not exactly the same as an average J tube.
I have since made a few more mods including the new steel cook top and a vortex adaptation.

The main disadvantage alongside batch designs is the regular feeding that is required but in my circumstance  I prefer that aspect as I can regulate the heat to suit summer, winter and cooking.

The 10mm steel top is much better that the old 6mm one, I would not recommend ceramic glass  as it holds no heat at all and only really works while cooking directly over the heat riser.
The thick steel is great as it hold heat and offers lots of cooking zones.

Hi Eloise
I currently have 3 RMH's.
A 6" batchbox with piped mass in our greenhouse/studio  permies.com/t/150380/Happily
A 7" batchbox in my shop with a brick bell. permies.com/t/94980/Brick-Bell-Shop-Heater    
permies.com/t/138779/Breaking-News-Shop-Dragon-admitted
And a walker BBQ oven with Matts  riserless core design as my outdoor kitchen.  permies.com/t/164923/rocket-ovens/Build-Black-White-Rocket-Oven

Matts core plans can be used in mutable applications.
From his riserless core on up, the upper portion can  be anything.
It can hold a large tank for water.
It can be a black and white oven like my outdoor build.
It can be a cooktop with attached bell/bench.
Once you buy Matts plan, the possibility's are endless.

Can one of Matts stoves keep your home warm in AK?
I think so, but you will be running it often or steadily to keep  your mass warmed up.

My other two RMH's are Peter Bergs batchbox design.
Both stoves started out as J tube RMH and I later converted them over to batchbox.

In your situation I would build myself a 7" batchbox as a main warming stove.
I would also have a Walker stove or two for cooking/ Baking/water heating/mass warming.

Here is why.
A 6" J tube in my opinion is just not big enough for northern climates. They require even smaller wood every 30 minutes.  
An 8" J tube is much better.   A warmed up 8" will need wood added every 45-60 minutes to keep it from going out.
J tubes are easy to build, they are a wonderful magical design that will roar like a dragon at full burn.
However if you have children in mind think about the 7.5" open square hole with a superhot fire in it....

Peter Berg Batchbox design allows larger wood to be used.
Wood is laid horizontal and is behind a door.
Burn time on my batch boxes is an easy 2 hours or more.

My thought's on this are seasonal based.
Spring and fall your 6" walker stove in any configuration will have you opening windows for cooler fresh air.
In the middle of an Alaskan winter... I would want a 7" batchbox for my main heat source and a  Walker riserless for cooking and water heating.

As I mentioned,  both my batcboxes started out as J tubes And were easily converted later to being a batchbox.
So you can start out with a J tube  and "upgrade to a batch later.

Now some information about the need  for clay bricks / cob as  "mass".
Away from the core and riser you can use concrete block...
Large rock is a wonderful mass.
If you use a piped design, simple dirt to fill air gaps and as many large rock as possible make a mass, contained with easily available concrete block or just about anything you wanted. Sheet metal  or even wood can be used on a bench.
Near the core you will need clay  brick and for the core you will need true Firebrick.



 











 

Fox James - Thanks for that video as well, and the recommendations for a cook top. Since you cook regularly, I have a couple more specific questions if you have time to respond:

What main adjustments do you make personally when going from cooking in a conventional kitchen to on the RMH? What does the flow of preparing a meal look like for you? Are you generally firing the stove with the purpose of cooking? Or are you running it anyway for heat?

Thomas Rubino - Thanks so much for taking time to respond, your perspective and experience is very helpful! I took a look at your links, and your stoves look awesome. With your BBQ, from a cold start, how long to you burn before reaching roasting temps (200-300) and baking temps (400+)?

I can certainly see how having 2 stoves could be helpful, and as Jacob Klingel mentioned earlier, would also provide redundancy in the event there is a need for maintenance. However in that case I'm wondering of the Walker Cook stove would ever really be used as a Masonry Heater, or if it would primarily be functioning as a cook top. Especially before we get an addition put on, 2 stoves in 600sqft seems like overkill to me? I'm also concerned about having the space for them haha.

You mentioned that the Walker stove would heat the house, but would have to be fired consistently. In my mind, if it is my primary or only cooking appliance, I expect I'll be heating it to cook on/in at least 2x/day if not more. Keeping the fire going in-between doesn't seem like to large of a burden to me. However if that greatly decrease the time we can leave home and not have the water freeze, I can see how having increased mass to hold heat longer would be very desirable.

In your opinion, Thomas, would simply adding mass to the Walker Stove, and putting in an appropriate bypass (to heat only cook top/water in summer) accomplish the same outcome as the two stoves? Would the concern be that adding mass may require increasing the size of the system (maybe an 8"), and therefore continuing to cook yourself out in summer? I guess other than redundancy, what makes you say 2 stoves would be better? I'm not opposed to the 2 stove option - just trying to understand more specifically what I would gain from it.

Also thanks for your descriptions for materials. I have wondered where concrete block would be acceptable to use. Sounds like it could be used for a bench which could be helpful. From the reading I've done I think I'd really prefer a bell bench.

Thanks again for all your responses everyone! Keep 'em coming

Hi Eloise, if your 600 sq. ft. house is constructed and insulated to Net-Zero standards, using a passive solar design, I imagine the living space will remain above freezing with nothing more than the waste heat given off by the refrigerator, and etc., according to what I've read on them. Given that, a Matt Walker style full size masonry cook stove with thermal mass bench will not only heat such a highly insulated space, but would have one opening a door or window in the dead of winter to keep from overheating the space -- or running the energy recovery ventilator in cooling mode. Here's a short video on one such Net-Zero dwelling in Canada that goes into the construction details, next best thing to an Earthship IMHO:

Hi Eloise, my stove has a bypass so I can cook on it without heating the room to much or I can cook on it and heat the room, weather and time of year dependent.

The bypass just isolates the hot plate from the barrel and mass.

It only takes around 10- 15 minutes to heat up the plate to  500f and probably  30 minutes to get the whole plate saturated with heat.
With experience you can adjust the heat by the size and type pf wood and of course the amount.

I have a removable insert in the hotplate that once removed offers direct heat from the top of the heat riser so you can place a kettle or big pan directly above the main heat source.

At one time I had a much bigger cook plate over a slightly different design stove, I loved that for cooking for big partys……..

Byron- Another interesting video, but right now I'm mostly interested in learning more about the RMH systems.

Fox James - Thanks for the info, and the extended cook plate looks super nice to use! Your cook times make using a stove of that type on a daily basis definitely seem reasonable. Especially with cut outs for direct heat.

If anyone else has input, I am still interested in hearing what you have to say. In the mean time, this info has been a great starting point.

Thanks again, all!

Jacob,

Do you have any updates on your build? Were you able to source or make some clay bricks for your thermal mass? We are mostly settled in Alaska now and would love to see your progress, and/or help you out if you are still working on your build.

-Eloise.

Hi Eloise! Unfortunately I was not able to source any bricks, but I have found many other materials that are available locally. I built a miniature "Geo-Air Thermal Battery" high tunnel on my property this summer and now I am sourcing materials this week for the first RMH build. I will be basing it off of Matt Walkers riserless core, with a few tweaks of course... The prototype will be built in the high tunnel since I have space for it there, and it'll give me a chance to tinker with it over the winter. I do have plenty of clay though, I used the mini ex I rented for the high tunnel build to pile up a large amount of it in the backyard.

Hey Jacob, can you elaborate on what a "Geo-Air Thermal Battery"  is?

Sure thing William. It's basically the same thing as a GAHT system, Ground Air Heat Transfer, but with a different name haha.

https://www.greenquest-library.org/

This is where the term comes from. I bought the package, studied the ideas, and then miniaturized the system based on my budget and time.

In this image is my mini GAHT, or Geo-Air Thermal Battery, which is a rammed earth thermal battery behind my masonry bed stratification chamber connected to my six-inch Batch Box Rocket Mass Heater.

I pull hot air from the top of my high tunnel and blow it through the corrugated perforated four inch drain pipe installed inside the rammed earth thermal battery.

The results of my system were amazing. We planted seeds March 26th of this year and I was harvesting crops in late May, when most people around here were still waiting for the snow to melt.

Wow!
I've flapped my gums about an above ground air to earth thermal batteries but you just went ahead and did it!

If I am  understanding correctly, the GAHT battery is built along side the rockets stratification chamber(bell).
Is the grow bed ontop of the GAHT battery, the bell or both?
Are both or either of the masses insulated from the ground?

Hey William! Sorry for the late response, crazy busy around here. I will do what I can to start a new thread about my build, there's a lot of stuff going on with it and I already hijacked this post enough...