I'm looking to install a RMH in my "office", which would be about 300 square feet, and attached via a pair of doors to a non-insulated 600 square foot "party room" in one of our sheds. So I bought Ernie and Erica's 6" system plans and the Rocket Mass Heater book by Ianto Evans and Leslie Jackson.
Before I build that, or even start collecting materials, I wanted to have a better understanding of what makes RMHs work (and not work). I took a quick look at the diagram of the core in Ianto's book, and then a friend and I spent ten minutes chipping cement off some old bricks, and another 10 minutes stacking them up, giving me this:
We fired it up, and it sucked (no, not the good rockety way). Basically we lit a fire in the chimney to get some draft happening, and also lit the fire in the bottom of the feed chute. We did get a horizontal burn when the fire was right at the bottom, however as soon as it started to burn up into the fuel there wasn't enough draft to pull it back down. We tried removing a course of bricks from the feed and adding it to the chimney, and that helped some. We noticed that when the fuel was pushed into the burn tunnel, it drew better. So we took the feed chamber down entirely, and watched that for a while
(here's my friend watching - he works at power stations. I think he's marvelling at the fact that fires can consume less than 2600 tonnes of coal per hour)
At this point, I remembered that the diagram had shown the burn tunnel being taller. Not wanting to start over, we pushed the coals into the chimney, and then dug down in between the walls using a mattock
(I'm posting this picture because the short side was what we used to remove the cement from the bricks - a single tap would take it off!)
We resurrected the feed chamber, and it was a little better. Then I scooped up the ash and dirt we'd dug out and covered the worst of the gaps in the burn tunnel. This had an immediate and dramatic improvement. We then added another three courses of bricks (they had holes in, so we had to lay them the other way) and that made further improvement. Anyway, this was the finished product.
We discovered that if you dropped a long strip of eucalyptus bark in the chimney, it would burst into flames and cause some serious rocketyness. We also discovered that if you threw a half dozen brown coal briquettes (calm down!) into the feed chamber, it created so much heat you could no longer look down the chimney. My friend commented something along the lines of "Yeah, there's a reason fossil fuels kick ass".
My plan next weekend is to make up some cob (never done that before) and try and cover up all the gaps in the lower part of the structure.
One question that springs to mind is would a diagonal burn tunnel (maybe rising one brick width over its length) help reduce the risk of smoke back? Seems like it would make it easier to light and easier to clean too.
There are lots of ways to reduce the risk of smokeback. Trouble is, many of them just end up increasing the risk of 'smoke forward' - inefficient, polluting burn.
The 'conventional' solution in RMH's is to increase the heat riser height, and/or shorten the burn tunnel, until things balance well. You want constant, good draft; but you want to also maintain good mixing and complete consumption of fuel.
The right angles in the burn tunnel help mix the gases while in the hot part of the flame path for complete combustion. There have been a few stoves that use a gentle slope, e.g. Lorena stove, Good stove.
In a RMH I would expect a gentler slope to have a similar effect to a rounder angle at the heat riser join: smoother flow, less mixing, more smoke out the top.
Please try it, though, since you've got the test bed and the idea. See what happens. It might be a great option. There may be an angle that still gives adequate mixing, or a way to do a 'smoke shelf' or 'smoke throat' that replaces the lost corner with a fin-like vortex shelf.
Adding undue complications to an experiment with a much simpler answer ...
I wonder if you are using gum fuel-wood, if it will burn rich or pitchy, like our pitch-pine or madrone. Ianto's ash-trap under the fuel feed can be useful for certain dense or pitchy fuel woods, as it provides a way to get longer fuels completely into the feed without waiting for their lower ends to burn down. Wet wood, and pitchy wood, both tend to burn up the outside before the ends collapse. The extra space only lasts for the first little while until coals/ash build up, but this can be enough to heat things up and get a good draft going ahead of the flame-creep.
(Good, dry, seasoned fuel wood is always preferable. But in a rainforest you're going to have more than 15% moisture because the air itself is above 80% most of the time. And you have what you have as far as local fuels. You can balance pitchy with punky or scrap to a certain extent, but sometimes you can find a work-around that suits the local norm.)
I wouldn't mind seeing a coal-fired RMH; they have coal-powder briquette tea stoves in Japan that are pretty cool. But a friend doing early calculations in the late 90's told me we have maybe 50-80 years of coal left. I hope we have wood much longer than that. I want us to learn to use it well, so that we still have it.
A lot of places Ianto went to help improve their 'primitive' kitchen fire technology, they had a special trick, and if he was very good they would show it to him. A squirt-bottle of kerosene, or really stout matches, or parrafin-and-sawdust firelighters, or a conduit tube to blow air through. Ernie used to carry a scrap of car tire in the Arctic for unquenchable tinder. At the edges where it matters, people appreciate the benefits of fossil fuels. Even where they are expensive, a little goes a long way. If you don't have bitumin, tar, coal, whale-oil, wax, etc, you get really good at spotting fat-wood and fire-mushroom and all the other useful natural boosters.