WHat do you mean by double wall. Is there an air space inside? I was under the impression the masonry was solid between the interior flue area and the exterior . Also ,what would say the average weight of a small-medium one is. Yes there is a small air space or better said an expansion space between a core and an outer layer. Instead of air this space could be cardboard, mineral wool, or fiberglass material. A masonry heater has an inner part (made of firebricks, soapstone, and/or that thermal cement stuff). Once the builder has made this inner part he raps it with cardboard or fiberglass material and than builds the outer part (sometimes they build the 2 layers at the same time). There has to be an air gap/expansion joint between the 2 layers or else you'll have crack city - that is the inner firebrick part will expand and start cracking the outer layer (the part you see) - because the heat is trapped. Too big a gap an you have poor heat transfer from the inner part to the outer part. Too small or no gap and you have great heat transfer but cracking. I'm not an expert on masonry heater weights. But I would say a very small one could be 2000 pounds and a large one 12,000 pounds. In general a small MH is probably 3-5,000 #'s, and a medium 6000 to 8,000 pounds. Here are a couple of very small masonry heaters. The first one has firebrick on the inside (the inner core) and the builder is using 24" x 24" flue tile. He just straps the 2 halves together with metal strapping. And the second one is an experimental MH where there is only one layer - a core.