Un-freakin'-believable!
I couldn't concentrate on my work, so I knocked off for a long lunch, put in the tubes, and lit a fire.
I got a small bed of coals with a quick, hot burn using kindling and wrist-size rounds, and loaded it up with good splits. When it was burning nice and hot after the reload (450 on the furnace front and about 500 on the flue), I knocked down the door air supply on the door, waited about 5 minutes, and reduced the spin draft on the ash pan door to about 15%.
The furnace doesn't have a window, so I can't post pics, but I can crouch on the floor and peer through a couple of tiny holes in the door's flapper. What did I see? FLAMES DANCING ON THE CEILING
Yeah
After it stabilized, my furnace front temps are around 500 and my exhaust temps are around 400. That's the first time I've ever seen my flue temps lower than my stove temps without smoldering the fire, and even then I've never seen it with either of them that high.
After 1/2 a burn and very little concrete data, I'm already thinking that I should increase the air supply to the secondary tubes. I think that 3/4 inch supply manifold pipe may be too small, or that the 1/16 inch holes I drilled in the secondary tubes should be greater in number or size, but I'm going to give it a few weeks before I make any more changes. If I make a change, the first thing I do will be to drill out the secondary tube holes to 3/32 and add a few more. Hopefully that will pull more air through the supply manifold.
The important thing is that we're burning clean and closed down, which is unreal. I've burned clean, and I've burned closed down, but never at the same time!
I'll post pictures of the installed tubes after this fire dies down and try to figure out some way to get a video, but that's doubtful.