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.
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.
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.
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.
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.