Donald Saito

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.

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?

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.

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.

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...

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?

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.

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.

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.

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!