I have constructed and deconstructed my RMH system twice because of poor performance. I thought I had a good draft, and the fire burned realyl well, but the barrel never really got hot. I mean of course it got warm, but never to temperatures that can radiate heat and heat up a space. ( a small space, I have, maybe 100 sq. ft) After burning the system for several hours I could still touch the sides of the barrel for ten seconds or so. So I decided to take out some of the variables and test out my J-tube core. Here are my dimensions
Feed: 15" high
Burn Tunnel: 22.5"
following the 1:1.5:3 ratio. All components with a 7.5" x 7.5" CSA
Mine is made with bricks and all is insulated with clay.
So after testing just the J-tube core I noticed that my fire does not really get very high. The highest it has gotten is just about halfway up the riser. I am wondering if something is wrong because I have seen videos online with the fire blazing out of the riser and also have seen videos of people with fire vortexes in their risers. Mine just doesn't seem to be doing that and I'm not sure if it should be. In both the videos that I've seen they used cylindrical risers, and I know the designs have evolved from square to cylindrical.
Also as a bit of side note - the hot coals build up so much during my burning that they begin to clog the burn tunnel. Is this a common thing or a sign of something going wrong with the burning.
Any tips or help is greatly appreciated!
Flames coming out of the riser isn't a good sign, combustion need to be complete before inside the core, where the highest temperatures are maintained. It's unclear where you made your measurements, I am an advocate of the 1:2:4 proportions measured in the heart of the core. That is, feed from top to halfway down the tunnel, tunnel from heart feed to heart riser, riser from halfway down tunnel to top. Tunnel as short as is practical.
I believe I said this in your other thread, but your clogging issues might be overloading. Do you have any photos of your core during a test burn? Or a photo of the core?
I wonder if the cross section of the core compared to lengths is a bit large, letting the draft be lazy, or bypassing the burning wood. Feeding method could be important here; exactly how do you arrange the sticks in the feed tube? Do you lean them forward toward the burn tunnel roof, or lean them back toward you? If air can pass over the sticks without going between them, you will not get quite as good combustion, and coals can potentially fall out of the airstream. I routinely have a glowing coal bed an inch or two thick as I close up the feed at the end of a burn, but all of that burns itself out. I have now been burning three or four hours a day for about two weeks total (not counting long weekends away), and have not removed any ash yet. I have about an inch of ash in the near half of the floor, less at the back.
what kind of bricks are use?
is the core/clay still wet?
is the wood too wet?
The corrugated tubing used from the "manifold" / barrel cutout and through the mass, if you're still using same, has a good deal of unwanted drag and really hinders the draft. Any restriction at the "manifold" or transition from the barrel to the mass ducting will add unwanted drag resulting in a stove that just doesn't get very hot. Drag times 2. That transition is the hardest one area to make successfully large enough to funnel gases from the barrel into the exhaust ducting, and requires extra care in construction to get it big enough. But you could first try extending the chimney height, to something like 10 to 15 feet, or taller than that as needed to put the chimney's exit 2 to 3 feet above the roof or highest point of the roof within 10 feet or so, since it is probably the easier thing to do first. A tall chimney will even help overcome some of the drag induced by the corrugated mass ducting if such is not excessively long, though corrugated tubing is generally warned against.
If you go out the wall near floor level and only have two feet of chimney, that would explain all of your issues. Without the draft from a decent chimney, a J-tube core alone does not have a lot of push. Horizontal ducting can use up all of the draft you have, and you would get lazy circulation and quite possibly coals building up. Do you get any sort of roar or growl from the fire when it is loaded up and burning well? If not, that would indicate slow circulation, which leads to the other problems.
Quoted as follows:
S Jones wrote: I am using flexible aluminum ducking for the exhaust and flue. I can put it directly in the bottom of the barrel and seal it in with more clay. I have it running along the floor and exiting through the bottom of the wall where where it extends upwards vertically about two feet. Mind this is not in a home in the traditional sense but in a temporary winter shelter in the woods.
So high-friction corrugated ducting and next to no chimney. Extending the chimney at least 5', better 10', straight up with smooth duct should help.
rocket mass heaters have to be site-built, and do not lend themselves to even kit construction, as each location has unique characteristics. There is one company, Dragon Heaters, selling precast combustion cores (the most critical engineered part).