Kurt: Ok, ofcourse, dobble barrel
A very smart thing. From a thermal point of wiev, the bigger the better. Like in ancient thermal baths, which were heated beneath the flors and through the walls. But still no experience showed me any power produces through the barrels. As Hans also confirms, the barrel is an obstackle. But with dobble barrel the gasses have more time to get rid of the heat and therefore can flow out beneath with the least resistance.
Hans: "...draft has to be coming from somewhere" Its difficult to get a good picture; but after 5-10 minutes burn, I see the draft working correctly beneath and flames building higher up the sticks starting to point upwards at the same time. So some air is getting sucked down and some just enters and then is hauled upwards without getting into the riser. This just worsens until the draft stops. It becomes an open fire on top of the feed. Air is sucked in from outside, not downwards through the hot riser.
Now I just tryed with a short chimney. It works better, so its obvious, that the J-feed needs more draft, than my system delivers. Offcourse it would be sad to let go of the self feeding. But ericting a 5 meter high dobble insulated steel chimney, as would be required here, doesnt feel good. Maybe a fan would be a way out?
Im still wondering: Where are the people, who advocate J-feed and horizontal outlet? I really would like to see their systems running!