performance difference between square and cylindrical heat riser

I am wondering if there is a difference in performance between the RMH systems that use a cylindrical heat riser rather than a square heat riser

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"
Riser: 45"

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!

You don't say anything about the bricks being layed flat or on edge. On edge is definitely the best option, less mass in the core. I understand the insulation around the core consists of clay, but clay isn't an insulator, rather it's mass. Use something that has real insulation value like perlite, vermiculite, Leca expanded clay granules, Morgan Thermal Ceramics superwool or Roxul mineral wool. The fact that there are lots of coals is a sign of a burn that's too weak.
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.

Hey S Jones, sorry you're still having troubles. I would agree with everything Peter said, and you should too. He's at the forefront of RMH technology, and won't lead you astray.

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?

Measuring along the centerlines of the core, as Peter describes, gives the most accurate sense of relative element proportions, while measuring along the outside surfaces of the elements gives the most strict differences between elements. I think either way can work. For a 7" square core, 16" x 24" x 48" along the outsides would give 1:1.5:3, while the same system measured on centerlines would be 12.5" x 17" x 44.5", or 1:1.36:3.56. So the edge measurement gives a bit more safety margin for the same calculated ratios.

The OP's system measured on centerlines would be 11.25" x 15" x 41.25", or 1:1.33:3.67, which sounds quite respectable.

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.

i wonder why the barrel does not get hot. i mean, if it burns, the heat has to go somewhere...

what kind of bricks are use?

is the core/clay still wet?

is the wood too wet?


good luck
tobias

@S.Jones; still using only 2 feet of chimney height? Many a RMH requires the draw afforded by having a good (insulated) and tall chimney, to achieve a really "rockety" hot burn.

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.

I haven't found a place where you have described your chimney in any way; you didn't answer my question about it in your other thread.

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.

Glenn, that's what I've gathered from S. Jones 4th post in this thread, where there was mention of the ducting passing through a wall and extending up about 2 feet.

https://permies.com/t/60956/rocket-mass-heaters/liftoff-don

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.

Oops, yes, that was four days after my post, and I apparently missed that bit among the others.

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.

Hey all thanks for all the responses. It seems I just don't have the proper materials to get this thing done properly. I thought I could get it done with just some bricks mud and some clay (and some flexible ducting) but there is definitely more involved. I plan on trying it again soon when there isn't such a mad dash for warmth. For now it will just be a regular fire place on the side of my hut. It may not be the most efficient but at least I'll get to watch a fire. Till next time! And thank you again

You could do it with just bricks, mud and clay, if you were experienced and followed a good proven plan. But as an experiment in the middle of winter, you may be best off with a conservative build for now.

Does the tall chimney also apply to regular wood stoves? I have an initial draft problem. Has anyone marketed a decent rocket heater?

A well-insulated tall chimney can improve the draft for any wood-burning appliance. The key is having the chimney warm before the fire is lit, either by being inside the building or by priming it in some way.

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).

Walker Stoves (walkerstoves.com) produces CFB core kits and also ready made rocket combustion units in both J-tube and PvdB batch-box configurations. Either of the latter cores combined with the "primary radiator" accessory makes a fairly light and portable heater.   [edit: video link addition]

I've built 3 RMH hybrids, and I really like the rectangular riser in my latest builds.   When turned width wise and combined with the van-den-berg Venturi, it seems to be most beneficial with the double Rams horn vortex.   I think two vortexes simply fit better inside the rectangle but I can't explain why it works so darn good.  But I like it.   It seems to outperform my 6" round riser on my previous build, giving me the highest bell-top oven temps I've ever seen during my research and development