I have noticed several pictures, all drawings no actual builds I have found that show a J tube with an air intake near the base, and what looks like a lid placed over the top of the feed tube after filling. I have "the book" which I haven't read for a while but I believe references keeping the feed tube packed tightly for proper air intake amounts. It seems the separate intake may give you greater control with some sort of slide damper, although I haven't seen this shown in any of the drawings. Perhaps its also done to prevent smoke back and give a place to easily light the fire from the bottom? IF there are other advantages/disadvantages Id like to hear them.
Anyway I am about to embark on version 3.0 of my combination maple syrup evaporator shop heater set up this time with a much more permanent and hopefully better set up as I have proved my concept enough last year for me to be willing to spend the time and money on improvements this year. So is this something worth adding has anyone actually tried it? I searched but didn't find any real builds. I will incorporate it if it makes sense. I think it may help me really tune my boil when sugaring so that in itself may make it worth it.
If anyone has tried it have you tried filling the feed tube with any smaller fuel? It seems with this type of setup after a really good bed of coals was going you might be able to fill the feed tube right up with wood shavings or pellets or something similar as the air intake would be coming from the bottom. Some mods would be needed such as some sort of grate that let ash fall through and kept the fuel from smothering the air intake but it seems feasible...
The separate air intake at the base of the feed tube is generally an idea of people who haven't built RMHs for themselves. It defeats one of the first principles of the J-tube, the downward-rushing air that pulls flames down and keeps the entire fuel load from burning at once and making a strong competing chimney that would burn up out of the feed opening.
There is a common secondary air supply practice, called a P-channel (for Peter van den Berg who developed it), which consists of a way for about 5% of the cross section to be separated and protected from fuel blockage. It is generally implemented as a sheet of metal at the edge of the feed tube, extending down about 1/4-3/8" below the burn tunnel roof. This creates added beneficial turbulence and fresh air at a critical spot.
Operators generally find that at full combustion rate, the top of the feed tube can be blocked about 2/3 to achieve the best combustion.