I've read the books and looked on this forum. When using a steel pipe and using perlite as insulation in the heat riser, perlite is always mixed with clay/slip to keep it's shape, so when/and/if the steel heat riser wear's out, the perlite insulation will stay put. But it (perlite/clay/slip) takes a long time to dry if it's too wet a mix. Why mix the perlite with clay slip in the first place? Why not leave it dry (best insulate) and if the steel pipe wear's out, you'll know it when you see perlite in your clean outs? Wouldn't it be nice to know that your heat riser has worn out? and needs replacing? I'd like to use a clay/sand layer at the bottom and top of my insulation and dry perlite in between. Good idea? ______ Bad idea? ______
All this because I've know one person who built a RMH, had a problem weeks later with it, and found that their clay/slip/sawdust insulation was STILL WET after all this time!
when that inner lining of the heat riser burns out you not just going to see some pearlite in your clean out... the whole internal mass of pearlite is going to collapse inward if you dont have a binder
also note. the inner pipe is not the heat riser its merely there to hold the refractory in place so the it can cure into its structure its meant to burn out...