William Bronson wrote: Just a question,how did you fit the duct so perfectly to the inside of the cinder blocks?
Satamax Antone wrote:John, a nice way to check if you can add more mass, is to add a barrel at the end of the horizontal run, before the chimney, set up as a bell. 1.86m² of extra ISA. If it works, leave it there, add cob around, or bricks or stones, whatever!
I was checking this pic. https://permies.com/t/59459/a/45239/image.jpeg
It's a bit tight.
Get a barrel with a lid. Drill a hole on the side, to fit where the elbow is, if your horizontal tube sticks out enough. If not lengthen it 4 inches. take off the pipe above the elbow; place the barrel, the elbow back on the end of the horizontal pipe, but 90° from what it is now, so it faces you. Seal with alum tape. Place the lid on top, project the top tube onto it. Drill the lid, fit loosely, make the tube plunge into the barrel. Attach the top, seal. Light the stove and see what it does. If good your gain! Then add mass around this bell.
The down tube is called a plunger tube.