In all, not too bad. Due to numerous design flaws, there is some back-draft when the door is open, but when running at the right "speed", the door can be open and no back-smoking occurs. With bits of twigs and trimmings scattered about from chainsawing and woodsplitting for the stove inside the house, I've gotten the barrel up to 500 degrees a few times and the exterior piping is cool. The black stovepipe gets a bit warm, but I'm thinking of surrounding that whole run, once improved and completed, with bricks. Right now it exits a "flood vent" on the north side of the building (...if you have to ask, expect the whole story).....so I'm hoping when it's finished and ready to re-assemble in it's final resting spot, that I can create a proper exit hole and insulate it accordingly. Dare I say the weather has not cooperated? With a few days before Christmas, I've been waiting for one of those bone-crushing days of -20 to -30 F to see how it does in this un-insulated shop-wannabee. First few runs had enough smoke inside to chase the chickens out.....last couple runs, no problemo. The heat would allow to get the next phase of some wiring done, with fingers crossed of course.
Comments and questions would be welcomed. Without this site, I don't think I would have ever even gotten interested in the concept. Thanks!
edit: The exhaust port in the third picture need to be at least as wide as the riser and the chimney parts.
As to your draft issues, two things come immediately to mind:
1. Buy a roll or two of aluminum tape and get that pipe sealed; and
2. Get more pipe to raise the exhaust higher. Typical code says 2-feet taller than any portion of the building within 10-feet of the vertical exhaust stack.
As you say, this is a prototype. And as far as I can tell, an all metal one at that. Expect it to burn out. Which is OK for a prototype. But you get me wondering "why bother" when you then start talking about situating this is a permanent position. I do not think there is going to be a permanent time-frame for an all-metal design.
Thus, I say it is time to start collecting fire brick. You have some options here, but at least 30 of them, and upwards of 70 is better (roughly half of which can be half-size splits, 1.25" thick instead of 2.25" thick). OR start your plan for a poured core. But do one or the other. Temperatures inside a properly burning rocket stove will go well over 1,000 F, and perhaps twice that temperature. This is very hot, and is the primary reason materials fail in these conditions.
As you work out your dimensions you can get a solid count on your brick and/or dimensions (and therefore volume) for a poured core (remember to get the air bubbles out of the mixture).
Personally, I prefer simple for getting started. For my mind set, that means fire brick. I can pick up a brick and move it, and I like that. For me it is the best prototyping material. Plus, you can safely use it in a final installation, and know that it will survive the temperatures reached inside a properly working RMH.
Big box stores like Lowe's, Home Depot, etc. seem to have very expensive fire brick, way over-priced. Better prices are usually found by calling the brick sellers in your area. If for some odd reason they do not carry fire brick ask if they can order it. And whilst talking to them, you might as well as what temperature it is rated for, and if you have any options in that regard. The fire brick I got is rated for 2500 F and cost about $2.25 for a full brick (about $1.80 for a half-size split), in US dollars, after tax; the local box stores wanted $5 a brick!
While at the brick store, find out if they carry perlite (4 cu. ft. bags) and at what price, and fire clay too (usually in 50-pound bags).
Anyone else have trouble moving bags of perlite? The stuff I've gotten has such thin plastic wrap it splits along the seam as soon as I touch it! Next time I'm going to either take a tarp and wrap it immediately, or a big plastic bag and immediately put it in that. I just can't move it without causing a spill, and that is very fine particles, that I don't really like breathing, or trying to keep from going everywhere! In any event, I'd suggest at least wrapping and tying bags of perlite in a tarp for transportation and storage. Unless I am just getting exceptionally thin plastic bags. YMMV.
As you all indicate, lots of weak points in the system.....main burner will likely burnout, no mass being employed, main riser will likely burnout....all based on typical RMH temperatures. I'm considering the fact that, for an uninsulated shop, I'm not really looking to achieve RMH specs, but liked the concept for conserving fuel use, for portability of design, and for the flexibility of concept modification. At 500 degrees F, a brick oven placed on top of the barrel might work, and the chickens in the building get pretty active with the extra heat rising into the rafters [....unless they're just getting nervous over the talk of an added rotisserie.. ]. Although that's not RMH-grade heat, I still need to play around a bit with just what the needs will be to operate comfortably in the shop once a concrete floor is poured and some of the gaps in the building are better sealed. Yet with your comments and this prototyping, I'm keeping open the possibility in retirement of doing something more elaborate and more RMH-"true" on the main floor of our home to replace the present woodstove. [I was always really impressed being in Germany in winter and being blown away by the heat thrown off of a kacheloffen in many public buildings.] Biggest concern there would be reinforcing the floor to withstand the added weight, but I see there's plenty of info on that here as well. I'm not concerned about re-sale of the house.....with changes in FEMA flood codes, we are already in a zone that would prohibit ANY new buildings on this site and the house will likely be demolished when we leave.
Thanks again for comments, tips, and recommendations. Great site and great discussions here!
Have not considered this....was under the impression that the riser pipe *must* be insulated/insulating and so did not want to be cutting insulated pipe to various lengths. How well does regular stove pipe work for such tests? Will have to consider this during fine tuning....Thanks!
I think there is more to the formula than that, but the idea to grasp is that for a given temperature difference (between the outdoor air temperature and the temperature of the exhaust air inside the chimney) the higher the chimney the better the draw; also, the taller the chimney the better it will draw for a given temperature, thus, when dealing with smoke getting into the room, adding height to one's chimney is one option. This is why I mentioned it.
As Peter mentioned, increasing the exit in the bottom of the barrel is important. Peter knows his stuff, so I'd take whatever he says very seriously.
If that exit is the smallest cross sectional area (CSA) in your system -and it sounds like it is- it is going to cause things to slow down and back up. You want the smallest CSA to be in the fire chamber. Anywhere else and operationally others have experienced problems.
The "rockety sound" is from the fire riser, and it's ratio to the horizontal burn chamber (and feed tube). That relationship is what causes the rocket sound, and what creates the strong draft of the basic rocket stove design. Placing the barrel on top of this, and getting the rapid cooling effect of the air falling from the top to the bottom of the barrel creates a temperature siphon; that also helps move air through the system.
Ratios are very important, as are CSA relationships (which are also ratios).
1:2:4 is an often cited ratio of feedtube:burnchamber:fireriser. So if the feed tube is one foot deep, the horizontal burn chamber is twice as long (2-feet), and the fire riser is four times as tall as the feed tube is deep (4-feet). Smart folks say you can measure this from the center line of the passages. I usually base my calculations on the longest measurements, just for added insurance the system will draw well.
So if my feed tube is three sideway bricks tall (13.5 inches with the brick I am using) then I want my horizontal burn chamber, as measured from the most distant internal wall of the burn chamber, to be at least twice as long as 13.5" is tall (13.5 * 2 = 27 inches measured from the farthest wall of the feed tube to the farthest wall of where the fire riser and burn chamber make their shared 90-degree turn, from horizontal to vertical). Then I want my fire riser to be at twice as tall as the burn chamber is long: 27 * 2 = 54 inches tall.
1:2:4, starting with a 13.5 inch deep feed tube = 13.5 : 27 : 54
When building and thinking about changes, I keep two other rule of thumb ratios in mind:
1. I want the fire riser to be twice as tall as the longest horizontal measurement; and
2. I want the fire riser to be three times as tall as the vertical drop in the feed tube (13.5*3=40.5).
To get the greatest effect, you want the fire riser to be the undisputed champion of draft! This means keeping the feed tube as little like a chimney as possible. Therefore, keep the feed tube as short as is safe and practical, and as cool as practical. And by extension, keep the fire riser, as tall as possible and as hot as possible (I should say "practical" not possible - other experiments have shown excessively tall fire risers face a point of diminishing returns; *IF* I recall correctly, there is little to be gained by making the fire riser greater than 4-times the height of ? what?, either the feed tube or burn chamber).
Also, make the burn chamber as short as practical in the horizontal. Most builds seem to have about four fire bricks as their "bridge" - the bridge is the "roof" of the fire chamber, between the feed tube and the fire riser. You want that as short as fits with your design. You need enough room to fit the barrel in there, and to be able to build the feed tube wall. If the fire riser is off-center, as your design is, you should be able to measure some temperature differences on the walls of the barrel, with one side hotter and one cooler. This centering also effects how long the bridge/roof over the burn chamber has to be.
All of which is a really long way of saying, that maximizing the height of the fire riser, relative to the feed tube and burn chamber, increases the draft of the system. Add to this, the goal of making both the feed tube and burn chamber as short as is practical. And diminish the tendency of the feed tube to act as if it were a chimney, while increasing the tendency of the fire riser to act as if it were a chimney. And make the burn chamber the smallest CSA in the system.
And build with materials and standards able to withstand 2200 F and higher temperatures.
As I have come to understand it, these are the most critical design parameters.
Although, returning to your chimney and exhaust, this can be an important variable. Micro climate and the details of each building effect how well the building acts as a chimney and how well a chimney acts as a chimney (in competition with the building, and in competition with the feed tube). So in some cases, this is not a big deal, and venting out the wall at floor level works great; but at other times, it is required to get the top of the chimney above the roof. There are a lot of variables, so I'm not sure how well it can be predicted in advance. That's why folks say things like run your chimney up to 2-feet higher than the roof in a 10-foot diameter
Now having said all of the above, you may not even have built a basic J-tube rocket stove! In which case you'll need to apply some different standards to your measurements and ratios. If you have actually built a batch feed, I cannot help. But Peter is an expert at that.
When I look at your picture, am I seeing what is basically a wood stove with a fire riser/barrel on top? If so, that's not really either a basic J or batch feed. I suppose you could think of it as a type of L-style rocket stove. But the CSA of the wood stove should be smallest dimension if building as a rocket stove. And that might lead to some design/build difficulties. (I again vote for buying upwards of 70 fire brick, roughly half full size and half splits, and build your prototype that way - it is flexible and eliminates a great number of variables, which I personally believe to be of value.)
If you are not really building what others have found works well, my opinion is you are heading in the wrong direction, *IF* what you want is an operational heating system in a small amount of time. If you just want to experiment, that is a different situation.
I think I started my first prototype by buying 30 full size fire brick. I later added 20 or 30 half brick, and at some point I may have bought another 10 or 12 full brick. You might also check Craigslist for supplies. Sometimes there are good deals there.
John Weiland wrote:Thanks much, Erik, Wyatt, and others who weighed in. Really great suggestions that I hope to use in modifying my design. In addition to tightening up the exhaust ducts and adding some height to the external exhaust chimney, I'm going to increase the exhaust port size and possibly play with the riser height....one of the advantages of the removable top. Clearly there are better riser materials and designs for longevity and I hope to incorporate that in a final design. In addition, I'm just going to play temporarily with the idea of adding fire-brick inside the stove to take up some of the CSA and thereby perhaps create a narrow L-shaped burn chamber. [Biggest problem with that idea is that there is no insulation as is usually seen in cross-sectional drawings of the burn tunnel.] Easy enough to do as a test at any rate. I'll try to remember to take photos of a burn-in-action next time to see how things are progressing. Thanks again for great comments!
Depends how big that stove is inside. If there is room, you can fill in space, and get insulation, by mixing up some clay and perlite and shoving that in there, then line it with fire brick to make the burn chamber.