A linear RMH design idea for my tiny house.

So I'm designing a 10' by 12' tiny house. My idea is to install a long, narrow J tube rocket mass heater with cooktop along the south wall of the house, with the back surface of the stratification chamber also functioning as a Trombe half-wall.

To minimize space the design needs to have the smallest possible footprint and still function effectively as a mass heater and cookstove. I believe a linear design, long narrow and tall could be the most space efficient.

I'm envisioning a linear design something like this:

1. Beginning on the right side with a 6" feed tube,
2. A burn chamber no longer than absolutely necessary,
3. Then rising a total of 36" from floor height,
4. Into an inverted 6"x 12"x 21" 22 guage stainless steel steam tray pan in place of a barrel to serve as a heating and cooking surface,
5. Then falling to near ground level to empty into a  tall, narrow stratification chamber about 48" high and as long as the house will allow.
6. The exhaust would vent just above the chamber entry port and pass as closely as possible to the riser before exiting the house.
7. The whole structure should be as narrow as possible, 15" ideally to 18" if necessary.
8. Standard J tube masonry construction with a rock wool insulated fire brick core and red brick otherwise, with clay sand mortar where appropriate and standard mortar otherwise.

Ok do your duty and pick this idea apart. Cry havoc and let slip the dogs of war.

Hi John;
 10x12' sure seems small to me but its your tiny house.
Most important) Do you have a copy of the RMH builders guide yet?  Redily avalable on Amazon.
To start a drawing might help clarify things some.    

Ok) 6" feed tube?  Did you mean 5.5 x 5.5 by how deep ? burn tunnels are generally only 10-12" long.  A 36" riser made with what material ?  Ceramic blanket ? Insulated fire brick ? Heavy firebrick?
What is your plan for the transition area ? Below your cook top ?
The exit to your stratification chamber should be at the lowest point.
Your cook surface is going to be glowing hot at times, my 8" J tube can hit 1100F at the barrel top....

Finally, have you seem Matt Walkers tiny house cook stove ? Here is a link to it https://permies.com/t/71700/Tiny-House-Cook-Stove-Heater

I know you were expecting challenges to this design, but aside from the long-term viability of the stainless steel steam tray pan, it looks like you're going "by the book".  

Long and (relatively) narrow masonry heaters have been built for many years, and a properly constructed rocket mass heater is simply a masonry heater made from less expensive, locally sourced products.

I think your plan will work well, provided you insulate the wall very well from the stove, and the stove from the wall, lest you wind up attempting to heat the entire outdoors.  

thomas rubino wrote:Hi John;
 10x12' sure seems small to me but its your tiny house.

Thanks for your response! 120 sqft (actually a small fraction less) is apparently exempt from universal code requirements. My county is not zoned but I would rather build small now, avoiding any Imperial entanglements, and add on later as needed.

Most important) Do you have a copy of the RMH builders guide yet?  Readily available on Amazon.

Yes I do. An older version.

To start a drawing might help clarify things some.  

I have ... It does help. Fun fact: The front elevation I sketched up bears a small resemblance to the letters in the name "Jill". :) (J tube on the left.)

Ok) 6" feed tube?  Did you mean 5.5 x 5.5 by how deep ?

I'll use the standard 1:2:4 ratio, or as close as I can get it, and follow the generic plans I have as closely as I can.

burn tunnels are generally only 10-12" long.

That's good to know.

A 36" riser made with what material ?  Ceramic blanket ? Insulated fire brick ? Heavy firebrick?

36" overall from floor to the top of the cook top. This is to locate the cooking surface at standard kitchen height. I'm thinking a riser of rock wool lined firebrick. I may start out with just the firebrick and add the rock wool liner as soon as I can. Ceramic blanket is new to me. I'll have to compare costs.

What is your plan for the transition area ? Below your cook top ?

Steam table pans have a lip so they can hang inside a standard sized opening. This firm rectangular rim should snug up nicely to the firebrick below once inverted and clay-sand mortar should make a good seal between the sides of the pan extending below the red brick structural top. I wonder if nesting two pans together to increase the overall thickness would be advantageous. I'm not wedded to the steam table pans, but it seems like they would serve well as a smallish rectangular channel between the riser and the diversion tube -- like one independent section of a barrel from riser to barrel edge. The 6" depth and 12" width should allow for good flow across the inner surface of the pan, and the outer surface should get plenty hot enough to set cooking pots and skillets on to be heated for cooking. In my mind it should function very similarly to the top of a barrel, just smaller and more rectangular.

The exit to your stratification chamber should be at the lowest point.

I know that's the standard design. I just don't know how I could have the overall width of the stove be basically the width of the tube plus outer structure only, and still be able to have the exit below the entry tube. Since the hot gasses mix and stratify in the chamber rather chaotically, I don't think it would cause any problems to set the exit tube right on top of the entry tube at the bottom of the structure. Maybe extending the entry tube along the bottom of the chamber a foot or two would be enough to avoid any potential problems with the gasses entering and exiting too quickly without filling the entire chamber for effective mass heating.

Your cook surface is going to be glowing hot at times, my 8" J tube can hit 1100F at the barrel top....

You've hit on one of my most serious concerns with this design. My hope was that using a 6" system, making the riser as tall as possible within the overall height dimension, and giving plenty of clearance between the end of the riser and the inverted bottom of the pan would be sufficient to keep from overheating the cook top while still achieving rocket stove temps for complete combustion. Perhaps a removable cover made of thicker, more massive material would keep the pan from burning up when not having heat pulled out for cooking. I've seen barrel stoves with cob covered plates sitting on top of the barrel to avoid overheating the top. I would also need to experiment to see if there would be an effective range of temperatures across the cook top for cooking. You can't be a very good cook if the only setting on your stove is 'incinerate'. I've considered using trivets of various thicknesses to control the amount of heat being transferred to the cooking vessel.

I am also very concerned that a stove like this will quickly overheat such a small space making the tiny house uncomfortably hot. It seems that the three best solutions to overheating would be mass, mass and more mass, so that the heat is absorbed and released slowly over time. Problem is, this runs contrary to the stated goal of taking up as little floor space as possible in a house with such a tiny floor plan. Since the stratification chamber is tall and thin, it could be acceptable to use seasonally temporary additional thermal mass, such as a drum or two of water right up next to the wall. I'll need a water reservoir inside the house anyway as it will be completely off-grid. Might as well let the water serve two purposes.

Finally, have you seem Matt Walkers tiny house cook stove ? Here is a link to it https://permies.com/t/71700/Tiny-House-Cook-Stove-Heater

Yes I have. It's a great design. Much to emulate there. Unfortunately, I don't really have room for a glass range top, and also it appears to take up a minimum of (I'm guessing) 16-20 sqft of floor space for just the stove and wall clearance. I don't know if I can afford to use that much space all in a chunk in the middle of the house. I've also considered a design like the tiny stove with the gothic arch masonary bell on the top, but I don't know how I would cook on it. I'd rather have a design I can cook on during the cold part of the year and extend the thermal mass along the periphery of the floor plan to take up as little of the most valuable estate as possible.

Jason Broom wrote:I know you were expecting challenges to this design, but aside from the long-term viability of the stainless steel steam tray pan, it looks like you're going "by the book".  

That's great. Would you expand on your specific concerns with the SS pan? According to the info I found, 18/8 stainless has quite a high temperature tolerance.

Long and (relatively) narrow masonry heaters have been built for many years, and a properly constructed rocket mass heater is simply a masonry heater made from less expensive, locally sourced products.

I think your plan will work well, provided you insulate the wall very well from the stove, and the stove from the wall, lest you wind up attempting to heat the entire outdoors.  

Since there will be a 6-8 inch space between the back of the strat chamber and the glazed and insulated south wall I'm not anticipating any issues. I want the back of the strat chamber to pull double duty as a Trombe half-wall.

Hi John;
I'm starting to wonder after reading your concern about overheating a small area.  You are correct in that it could get over warm at times. Maybe a rethink about options.   For your space and needs I wonder if it would be better to build a batchbox similar to Kirks casorole door build. Much smaller and being all brick it would heat slower but steadier. Using a cast iron propane burner for your cooking would limit how hot it will get in the shoulder seasons. When lighting the rmh just to cook dinner might be a little to much.
Batch box building is more technical than a J tube, but Peter Berg has made all the building information "free share" any information about how to build a batchbox is easily found here or at Donkeys pro boards.http://donkey32.proboards.com/
Your stainless pan will work fine, but not if you double it. I believe that would cause the lower pan to melt down.
As far as risers.    Think about Morgan super wool. ( https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket)  Cost is higher but performance is over the top.

EDIT)  A steel plate can be built into a brickbell, near but not over the riser. It would work fine for cooking on a setting of less than incinerate...

I'd like to hear a little more about your idea to add a cooking plate to the masonry rocket heater. Where do you envision it being located?

I've also been investigating masonry stoves (non-rocket type) but I'm a bit hesitant because rocket stoves with their high combustion temps seem safer to live with. Some masonry stove designs without a riser worry me when I think that they could be giving off CO2 and CO in the living space. In videos when they take them apart the inside bricks are always black from soot which tells me the temps aren't very high and there are a lot of products of incomplete combustion being produced.

I guess another option would be to create a largish stratification chamber beneath the floor to absorb and slowly release heat and to mitigate a possible overheating situation. In a 10 x 12 foot house a 30" x 72" chamber beneath the floor doesn't seem that hard to incorporate into the design. Especially with a tamped earth floor like I plan to have. I may lose some heat to the ground, but a layer of insulating  dry sand around the chamber should cut down on that.

john Harper wrote:I'd like to hear a little more about your idea to add a cooking plate to the masonry rocket heater. Where do you envision it being located?

I've also been investigating masonry stoves (non-rocket type) but I'm a bit hesitant because rocket stoves with their high combustion temps seem safer to live with. Some masonry stove designs without a riser worry me when I think that they could be giving off CO2 and CO in the living space. In videos when they take them apart the inside bricks are always black from soot which tells me the temps aren't very high and there are a lot of products of incomplete combustion being produced.

I guess another option would be to create a largish stratification chamber beneath the floor to absorb and slowly release heat and to mitigate a possible overheating situation. In a 10 x 12 foot house a 30" x 72" chamber beneath the floor doesn't seem that hard to incorporate into the design. Especially with a tamped earth floor like I plan to have. I may lose some heat to the ground, but a layer of insulating  dry sand around the chamber should cut down on that.

Any stove design presents possible dangers from CO and CO2 emissions.  Wood-burning masonry stoves, including RMH's, must be properly sealed.  While the temperatures in a RMH get high enough to burn a lot more of the volatile compounds, that does not make them "safer", per se.  It makes them more ecologically conscionable, but they will kill you just as dead, if the emissions are leaking into your home instead of going out the chimney, as they should.  The inside of any masonry heater is going to get coated with carbon by-products, including the generic term, "soot", along with fly ash and other particulate matter.  That's just the nature of burning with wood.  If anything, experience with RMH's has taught us that the gap between the top of the riser and the top of the heat exchange (typically a metal drum) can become clogged with that carbon by-product, leading to less efficient operation of the stove and possibly dangerous operating conditions.

The Korean ondol heating solution is well-documented.  Any heating solution that is in direct contact with the ground...will heat the ground, which is a losing proposition.

Ok, Jason. Good food for thought. Thanks.

Hi John;
I envision a cooking plate, next to the riser not over it. Brick bell covers your riser ,  directly next to it. You create a flat spot for your 1/2" plate. Hot but not incinerate.

As far as trying to heat the ground …  it steals heat … you won't succeed in heating it.   Sand would help, but cob loaded with cut straw would work better.

A correctly built masonry stove is double skinned , to eliminate any chance of c02 venting indoors.  A rmh burns up most particulates but c02 is still present.  If any wood burner leaks there is a chance of suffocation. A C02 detector in your home along with a smoke detector protects you from this happening.  

Hey John;   When you said you had an older version of the builders guide...  did you mean Ianto's original book or the new book from Ernie & Erica Wisner?  If you have the older one I highly recommend getting a copy of the Wisners book.  It is considered the new builders bible.