My steel RMH with the air-cooled core, integral sand mass and external mass of stone and sand which I wrote about a few months ago is now installed and up and running. Will keep you posted. Here's a few pics.
Where ever the core would glow red when fired up, I had cooling fins welded on. The hottest spot was the sides of the burn tunnel at the base of the feed tube. So I welded a piece of 2.5" exhaust pipe welded to each side and then connected an "L" shaped pipe of the same size connected to each of these. The "L's" rise up through the integral mass and exit at the top. (See pic above.) So with convection, air is constantly rising through these tubes and automatically cooling the sides of the burn tunnel to keep it from glowing and corroding with excess heat. But this does not affect the flame path which consistently runs over 1200F at the base of the heat riser (melts aluminum).
nobody knows the need for good heat like a Canadian does! lol.
i also noticed the same "hot spot" at the bottom of the feed tube/burn chamber on my first rocket i built. it has metal and internal mass also, it still works but it sits outside, while i experiment with cast cores and firebricks. however it was the original one that SPARKED my interest in these units, and its the only one ive ever "completed" so to speak lol. and it was one of my favorites because it didnt take up much floor space. i neglected to see the photo you mentioned. i hope everything works out and lasts long for yeah.
The picture I was referring to is the one at the top in the first post on this thread. You can barely see the cooling fins at the far side (front) of the stove, but the pipes that go through the mass are clearly visible at the top and back of the rocket. Here's a couple of pics of the actual cooling tubes welded to the sides of the burn tube.
I promised to keep you updated on how my steel RMH with the air-cooled core "survived" the winter heating season. As I plan to build a 7" system with a vertical mass with a much smaller footprint for the coming winter, I took this RMH out of the house last week. Here are pics of the core to show how it fared the first heating season.
The first pic shows the exterior of the core. The blackened flaky areas are where there was slight surface scalling. (The top of the burn tube and bottom of the heat riser.) But notice there is none of this where cooling fins had been attached.
The second pic shows the interior of the burn tube. Again no scalling where cooling fins were attached but some slight flaking did occur during the heating season from the top and sides of the burn tube and bottom of the heat riser.
The third pic is looking down into the bottom half of the heat riser.
The whole unit is still very sound and I have pounded with a heavy hammer all over it and found no weakness at all in the metal.
I did experience a reduction in my draft near the end of the winter heating season and was puzzled by it. But when I dismantled the mass I discovered my problem....I had not installed a cleanout port in a short length of my ductwork at the far end of the mass. There was a buildup of ash there which had begun to restrict the air flow. The 4th pic shows the ductwork in that mass and the short piece on the right was the "culprit"!
Lessons I learned from this experimental steel RMH:
(1) A steel core can work very well as long as the core is sufficiently air-cooled.
(2) Instead of just a small area of the burn tube being air-cooled and vented through 2 small vertical tubes, My next steel core will have cooling fins on the entire top and sides of the burn tube enclosed in an "air jacket" which will vent through a vertical 7" diameter stack and allow cool room air to constantly circulate to the exterior of the burn tube and vent heated air to the room.
(3)I didn't provide any air cooling fins to the heat riser last winter, but this showed me that I'll need to attach air cooling fins to at least the lower half of my next heat riser to preserve it from any scalling whatsoever. Heat will be shed from the hottest portion of the heat riser to the cooled exhaust gases in the lower portion of my "bell".
(4) Although I don't plan to use ductwork through a horizontal mass on my next RMH, this project clearly demonstrated to me the necessity of having cleanout ports to access EVERY portion of ductwork in a horizontal mass!
I hope this demonstrates to others that by cooling one's core (rather than insulating it) you can actually make a STEEL CORE work quite well in an RMH. I welcome any questions relative to steel cored air-cooled RMH's or any advice re building an RMH within a large bell. (My plan is to put the steel core inside a large furnace oil tank surrounded by a mass of sand within a sheet metal casing. I'm seriously considering putting 1 1/2" ABS piping within the sand mass which will heat water which will circulate by convection through a water tank beside the RMH and thus use water for heat storage and thus reduce the need for a large mass for heat storage.) This will definitely be vented to the atmosphere but from experience with the integral sand mass in my RMH last winter, I don't think the mass will exceed the boiling point of water! Would anyone suggest a better mass material than sand in such a project? Sand worked well in my integral mass last winter and is easily drained should the system need to be dismantled.