I just built a new RMH, first build, in order to live in a tipi in Montana over the winter.
Specs: It's a small brick and cob mass, probably 1 to 1.5 tons. Firebrick J-tube, perlite-insulated firebrick heat riser. The feed tube is 7 wide X 6 long X7 deep, and the burn chamber and heat riser is 7x7. There is a big manifold (about 90 square inches total) that leads into 3 brick flues running thru the bed in parallel, about 7 ft long each, with a cumulative cross-sectional area significantly bigger than 7X7. All flues come together in another big-ish space before the flue exits into the chimney. There is a 12ft rise on the chimney, venting outside the tipi. The mass is currently wet, and temperatures outside are below freezing. I've been burning a lot of fires to try to dry out the mass.
The problem is that if I stack wood vertically, the rocket effect fails and the flow reverses. It starts with a quieting of the roar, then a wisp of smoke, then suddenly the flames are crawling up the wood and out the feed tube. It looks like there isn't enough draft to fully burn all the coals and the burn tunnel gets clogged with coal, interrupting the flow of air. I think it may also have something to do with heating up the feed tube and it acting as a chimney, but with such a short feed tube I thought I designed against that happening. If I feed it super short sticks (about 6") directly into the burn tunnel it burns great, but it's a ton of work to chop all my wood that short and hand-pack the burn tunnel every time I need to feed the fire.
What's up with the stove? Some of my friends who helped design this thing said they think the problem will work itself out when the mass dries and warms, adding enough draw to fully burn the coals. I'm not sure, since I burned the thing for 6 hours straight yesterday and still had the problem at hour six, when things should've been quite hot. Everything is still wet, though, and the chimney never got much more than warm to the touch. Please help me understand my rocket!
Hi Jennifer; I think your friends are correct. My mass took over a month or more to dry out , and yes it did smoke back at times , although not much after the first week. It is in a greenhouse/studio so we burn from morning till 8-10 pm every day and still use less than 5 cords. If I read your specs right, you have a feed tube 7"x 6" and that is 7" deep ? If that is so, 16" is more common as depth of feed tube so is 7"x 7"but 7x6 wont hurt. How long is the burn tunnel ? 7" x 7" is good size for the tunnel and same for the heat riser. How tall is the riser ? What internal size are the 3 brick tunnels? Length of the burn tunnel and height of heat riser are critical. Keep burning and it may dry itself out and work fine .
I would like some clarification on dimensions. You said your feed tube is 7" deep, and the burn tunnel is 7" high. This would mean that the top of the feed is exactly level with the top of the burn tunnel, unless you meant that the feed is 7" deep to the burn tunnel roof making 14" overall. This is a reasonable height, as long as the heat riser is tall enough in proportion (preferably at least 3 x 14 = 42"). It is common to give these length dimensions as overall lengths, from floor of burn tunnel to top of feed and riser, and for burn tunnel length from front of feed to back of riser. You could also state the length of the burn tunnel roof for the same result, though the 1:2:3 or 1:2:4 ratios recommended are for the overall dimensions (or centerline dimensions if you want to measure that way).
I forgot actually to write that the problem occured when -NO- wind. So that way I could imagin yours working well with it too. The thing is once this thing starts turning it will suck air out of the chimney and it is fairly impossible for it to start to than start to turn the other way around (meaning sucking air into the chimney). All I can say is that for me it was THE solution. And no I don't live in a very windy place.