Soooo, I kept coming up with this thread when searching for why my mass rocket heater became sluggish, with lots of back burn and smoke...inside the house! Yuck! We'd been heating with the system for 3 winters, with each consecutive winter, the system's performance declined. I first thought my 22'run with a 180° turn about mid run through cob bench was too long. I spent many hours tunneling an additional exit flue through the 2' thick cob walls. That helped, but didn't solve the problem completely.
I figured I must have a CLOG! I can run a chimney brush through all the exhaust tubes, so I know there's no rat nests choking the system. The only remaining area that could be clogged was between the heat rise and the barrel. The only way to get to that was to cut the top of the barrel off (I didn't foresee needing to access that area when building...these constructing new system's...be forewarned)
So, using a reciprocating saw, I cut out the top of the barrel, leaving a 1/2" lip on the rim. It was sooooo packed with black residue, lofty soot! I duct taped a paint mixing stick to the end of a length of pvc pipe, and pushed all that junk into the first clean out chamber. All cleaned out, I ordered a disc of 1/4" thick steel to be cut from the local metal shop. A nice thick braid of stove rope on the the ledge remaining on top of the barrel, with the steel disc on top, closed the system back up nicely. Now the true test....fired up, the system works better than ever!
So, had I to do it again, I'd plan for a removable barrel top from the beginning (the steel is much more beautiful than the top of a barrel, by the way) and a fresh air intake...that's next on the list. The cob house is so tight, we have to open the wood cupboard door a crack for the fire to get it's full of oxygen.
