12" X 8" pipe reducer in the manifold

We are new to building mass rocket heater but have read most of the books and watched a lot of you tube. Now we are getting ready to build this spring. Our question is of course on the manifold. We are planning to use a cut down 55 gallon barrel for the manifold, with a whole 55 gallon on top. We have read where the manifold exhaust exit to the channel should be a least 2X that of the CSA of the burn tunnel. We will use 8” 26 gauge galvanized steel duct pipe for the channel and thinking of using a 12” X 8” 26 gauge galvanized steel duct pipe reducer to connect the manifold to the channel and coat the hole thing with  a couple of inches of  a mixture of high temp. clay and sand. The 12” end of the reducer gives you a CSA slightly more than double the 52” CSA of the 8” channel pipe. Any thoughts?

Your plan sounds fine, with one caveat. Using galvanized steel so close to the barrel risks having the galvanizing offgas zinc fumes, which are hazardous to health. It is recommended to use only black steel for the first 5' or so from the barrel to avoid this danger.

If you must use galvanized steel in this location, you could heat the parts in a fire outdoors with good ventilation until the galvanizing turns a dull gray, at which point it is stable and safe to use indoors, unless you expect to get it glowing hot or weld on it, when there is serious danger of zinc fumes. (Welders have gotten sick from welding on galvanized steel without protection and ventilation.)