Hello and Thanks to all who have given me some thoughts on the problem. Your kindness is much appreciated! First to answer a few of your questions...
Yes it's in the basement, this is the only space that I have for a fireplace in my home. The exhaust exits through a 6" hole cut through a foundation block. It then goes into a short insulated piece of pipe (30") and out a short chimney ( 36") with a rotating cowl on it to reduce the down draft.
The brick column at left of my RMH was a chimney installed in 1900 when my venerable old house was built. Most of it was removed prior to me. It only goes up to the top of the basement and dead ends. I placed the RMH next to it so that it could act as additional thermal mass. Also, this location is in the center of the room.
I've tried burning with a window open. Doesn't help much.
The chimney inside the barrel has a diameter of 8 inches. The ducts are all 6 inch. (Possible problem?)
Also, the size of the manifold opening, made from a duct boot, may be too small to accomodate the 8" internal chimney. I tried to fix this by chipping it back to increase it's size convinced that this was the culprit. Nada! Nyet! Nothing!
It's an L tube right now because the J tube I constructed didn't work. It only seems to draft if I push the fire directly under the chimney and constantly babysit it. Then only a little bit. If I can get it to draft properly or, dare I dream, actually Rocket, I'll reinstall the J tube so that I can auto feed the fire.
There is no mass portion to the bench because the system works so poorly. Seems ridiculous to fill it up until the system is working properly since I may need to modify the ducts. I've covered the pipes with fiberglass insulation to keep them warm while testing and it does help, but not a lot. Also, I don't have access to clay with which to make standard cob. I live in a city and there isn't any clay in the yard, or for sale at the local dirt suppliers, Home Depot, Lowes, etc. I've checked in a 60 mile radius from home with local excavators, masonry supply stores, etc. but it's just not available. So I'm going to be using a product from the local dirt sifter place that clumps together well when mixed with a little portland cement and water. I have a few yards of it standing by for when the drafting problem fixed.
I removed the portion of the duct that extends into the second bench section shortly after the picture was taken last fall. That DID help the draft somewhat but definitely didn't solve the problem.
Another thing I've noticed is that the distance from the bottom of the barrel to the bottom of the manifold is 17 inches. In other words, the exhaust has to be forced down 17 inches before it can naturally rise through the ducts. Am I making the system try too hard?
I'll take some pictures now and get all the dimensions and be back soon. Thanks Again for talking me off the cliff....