Basic dimensions for rocket mass heater system?

I'm getting ready to build a somewhat smaller RMH using a 30 gal steel drum, and was wondering what the relatively best dimensions are when considering the size and length of the horizontal burn chamber in relation to the heat riser. As I *think* I've read, the horizontal burn chamber that feeds into the core/riser should be roughly half the length of the core/riser itself, and the top of the core/riser should be about 1.5" - 2" shy of the inside top of the barrel. Does this sound about right to you?

Here's my questions in a numbered format:

1.  How long should the burn chamber be relative to the height of the core/riser?

2.  With an 18" wide steel drum, what should the inside and outside dimensions of the core/riser be?

Hope I'm using these terms right. I've searched around, but couldn't find anything readily on the subject. Thanks for any advice you can give me on this!

The first question when considering an RMH is, what are your heating needs? How big is the space you want to heat, how insulated, and what is the climate? If you are in the Sierra foothills in California, you presumably don't usually have a very harsh winter. Thus, unless you are trying to heat a large house, a 6" system may be right, which just happens to be the largest size that will conveniently work with a 30 gallon drum.

The standard recommended proportions for a J-tube core are 1:2:4, feed tube height to burn tunnel length to riser height, all measured along the outer edges of the cavity. So you might have, for a 6" system, 12" feed, 24" burn tunnel and 48" riser. This will want its fuel cut to 12" lengths, for best control of the fire. A 6" system will have round sections of flue 6" diameter and square (brick-built) sections 6" x 6" in cross section. The burn tunnel can be a bit less if necessary, but not larger.

The riser will be 6" diameter inside, and with common inexpensive construction, at least 2" thick so as not to lose too much heat; 10" outside diameter total. You can make it thicker, but I don't think there is significant benefit in going from 2" to 3" insulation thickness for a 6" system. You want to support the barrel so that there is a 1 1/2" to 2" gap from riser top to barrel top. More is okay; the closer the gap, the more heat will be concentrated in the middle of the top. This may matter if you want to do much cooking on it, but if that is not important, you can make the gap larger.

I posted a couple of pictures of a 6" system I built a few years ago in this thread: https://permies.com/t/40/53224/Portable-Compact-RMH-Hybrid-internal#442157

Hi Glenn, thanks for the quick comeback and valuable information - very much appreciated! I just purchased the fire brick to build my RMH, and now have a few more questions:

My 30 gallon drum has a width of 14". The bricks are 2.5" thick. If I try to make the inside dimension of the heat riser 6", that would leave only 3" left to center the heat riser core inside the drum. So centering the riser /core inside the drum would leave only a 1.5" gap between the outside diameter of the core and the inside wall of the drum.

1.  Would a 1.5" space around the core be sufficient, or

2.  Should I instead make the core narrower, and if so

3.  By how much (5"? 4"? 3"?). And if I made a narrower core...

4.  How would that affect the dimensions (specifically the inside diameter) of the burn chamber and exit flue?

Hey Don;  If your 30 gal barrel is 14" dia. it is a 16 gal barrel...   a 30 gal is 19" dia.  That 16 gal barrel is perfect size to make a cast riser. See the current post entitled "Using concrete sono tube for cast riser"

Hi Thomas, oh, *really*. Shows you how much I know about barrel sizes 🙄. But actually I would still like to make my RMH that small (16 gal) if possible. What do you think? Could it be done with 5" or 4" or 3" "throat?"

Thanks mucho for your help!

That small doesn't work as well according to those that have built them. I really recommend buying the Rocket Mass Heater Builder's Guide, which explains all the dimensions you want to use and shows various examples. 6"-8" is the most common sizes and work best with standard systems.

Mark is correct Don.  6" is the smallest size that is easy to build.  Some 4-5" units have been attempted ... most are no longer with us.  Buy the book ! Its available at amazon.  A whole lot of very smart people have and are building these.  Take their word and stay with a 6" ... could be built with a brick bell to have a smaller footprint.  

The full size 2 1/2" thick firebricks are not really suitable for a heat riser; what you want in that section especially is a very low-mass, highly insulating material, the opposite of standard hard firebrick. Perlite-clay cast in forms as suggested is a very good and inexpensive way to go, and there are several other tried and tested methods.

A 6" core with 2" perlite-clay walls will leave 4", or 2" all around, between riser and your 14" barrel, which should be adequate.

Mark Tudor wrote:That small doesn't work as well according to those that have built them. I really recommend buying the Rocket Mass Heater Builder's Guide, which explains all the dimensions you want to use and shows various examples. 6"-8" is the most common sizes and work best with standard systems.

Hi Mark,

Well, the space I have to heat isn't really very big - a 10' x 12' storage shed (though this "shed" is built like a house), so the RMH for it doesn't need to be super-duper. Just enough to take the edge off. I hope.

thomas rubino wrote:Mark is correct Don.  6" is the smallest size that is easy to build.  Some 4-5" units have been attempted ... most are no longer with us.  Buy the book ! Its available at amazon.  A whole lot of very smart people have and are building these.  Take their word and stay with a 6" ... could be built with a brick bell to have a smaller footprint.  

Hi Thomas,

Do you know why anything smaller than a 6" system hasn't worked out? Do they just not work unless they're at least 6"? The space I have to work with is small; as I mentioned to Mark, it's only a 10' x 12' floorspace, so I'd like to try keeping any RMH as small as possible.

Glenn Herbert wrote:The full size 2 1/2" thick firebricks are not really suitable for a heat riser; what you want in that section especially is a very low-mass, highly insulating material, the opposite of standard hard firebrick. Perlite-clay cast in forms as suggested is a very good and inexpensive way to go, and there are several other tried and tested methods.

A 6" core with 2" perlite-clay walls will leave 4", or 2" all around, between riser and your 14" barrel, which should be adequate.

Hi Glenn,

You encourage me! I'll give it a try and see if I can get it to work.

Don; When you drop into the 5" or less size rmh ,apparently they do not draw reliably, and are described as problematical .I have only built 8" myself and they rock -et .  You do not need a build a tunneled large mass. Consider making more of a masonry style with several brick chambers (bells) instead. That would easily fit into your limited space.

The exhaust gases don't flow well through the narrower widths, as the resistance increases significantly. A smaller space just means you will burn less wood for a shorter time to be sufficiently warm, but you should stick to a minimum of 6" to avoid performance issues.

thomas rubino wrote:Don; When you drop into the 5" or less size rmh ,apparently they do not draw reliably, and are described as problematical .I have only built 8" myself and they rock -et .  You do not need a build a tunneled large mass. Consider making more of a masonry style with several brick chambers (bells) instead. That would easily fit into your limited space.

Thomas: Just for yokes, I assembled a 4.5" J-tube out of bricks and lit a bit of fire gel in it - I got no draw. Of course, it was a little windy outside, and there were gaps in the bricks, so that might have had something to do with it. I tried again with an approx 6" assembly, and it did seem to draw, but again, I couldn't quite tell if it was the wind that was doing it.

I broke down and purchased the Wisner RMH book. It seems the more I get serious about actually building one, the more questions I have. For example:

1.  Do the chambers for the fire have to be square/round, or can they be rectangular/oval?
2.  If I make a 2" thick riser out of clay, sand and perlite, will it last very long, or is thicker better?
3.  I have local clay in the ground, but would it be better/more sturdy to make the riser out of actual fire clay?

Mark Tudor wrote:The exhaust gases don't flow well through the narrower widths, as the resistance increases significantly. A smaller space just means you will burn less wood for a shorter time to be sufficiently warm, but you should stick to a minimum of 6" to avoid performance issues.

Mark: Yes, I experimentally tried a 4.5" channel, and it didn't work at all (see my response to Thomas, above). Demitol! I wouldn't mind a system that didn't put out as much heat as a larger (6" or 8") system, but it at least has to create a draw, and my test failed totally. I'm right now looking into how to build a 6" core out of clay and Perlite. The project goes s-l-o-w-l-y...

The thickness of the heat riser isn't really a point of concern, instead the goal is thermal insulation between the heat riser gases and the gases dropping down the outside of the riser and into the manifold. If the heat riser isn't well insulated then the riser starts to bleed heat out into the barrel which will rise and can cause stalling. So as long as you have enough which the book will explain then you are good, and depends on what you use as to how thick it will be.

Keeping the internal runs close to square or round is better than rectangular or oval to minimize the surface area to CSA ratio. The more surface area the more drag, so round and square are best for minimizing that.

Using the clay in your soil for mortar will require screening as you don't want any tiny bits of rock or sand or silt which will throw your bricks out of level or not seal. A tiny tilt at the bottom will get really bad once you are 15 courses of brick up in the riser. Make sure you dry stack everything once to be sure it's totally level, then use clay slip on the brick edges to hold them together.

You're the gov'nor, Mark! Thanks for the info and advice. I just got a bag of perlite and will be mixing up some clay to cast my 26" high riser core with 2" thick walls and 6" inside diameter.

Well, I purchased the Wisner book, but now have another question: since my little 16 gal barrel is only 14" in diameter, if I make a 6" inside diameter heat riser core out of clay and perlite with 2" thick walls (making a total riser core width of 10"), will that be sufficient to insulate the rising heat (inside the riser) enough to prevent the space outside the riser from warming, thereby causing the system to stall with resulting smoke backup? It seems the book recommends insulating the riser core to prevent this from happening, but with my narrow 14" barrel and a 10" outside diameter riser core, I'll only have 2" of gap space between the riser core and all around the inside surface of the barrel.

So, do you think an otherwise un-insulated riser core with a 2" thickness of clay and perlite will work?

Thanks for any thoughts anyone can give me on this.

I think that will be close but should work if you can form it to be 10" outer diameter for the riser. 2" thick perlite slip should be decent insulation. Once the riser dries and hardens, you can light it up and lower the bell over the riser to see how the draw behaves. Measure to leave 1.5-2" between the top of the riser and the bell. Good luck!

Yes 2" gap will work fine,  As long as you have a good size transition area it will flow.
No need to wait for the riser to dry,  set the barrel over the riser ,set your top gap, mud the bottom of your barrel in and light it off. A riser dries in no time with a 1500 degree fire inside and several hundred around the outside.

Hi Mark and Thomas, thanks for your quick and encouraging responses! Two last questions and I will begin building my riser core: 1) Wouldn't it be better to wait for the freshly set clay and perlite slip to fully dry and cure slowly to prevent cracking, or is that not a concern with clay? And 2) Should a goodly amount of sand be mixed with the clay and perlite to also help prevent cracking?

Hi all: Ironically, after reading the Wisner "Rocket Mass Heater" book, I have decided to SCRAP the project, as I don't really have the space necessary to build the smaller RMH I had envisioned and still keep it far enough away from the walls (to prevent fire hazard). The shed is already small, and once I add shelves and store boxes and tools and such inside, there's really no place to put the RMH without exposing combustibles or getting in the way.

Phooey!

Glad I bought the book, though. And THANKS for all the effort to help and advise me. Perhaps others will benefit from the subject discussed.

This RMH heats a very tiny cabin, and would probably take care of your space fine. Being all brick and plaster on the outside, it does not need much clearance on any side.
tiny house rocket mass heater: the cyclone batch style

This video may have some insights into the problem of riser ratios. It's only a clip, but there is a more detailed version available for only $1!