New to forum and stoves, rocket and otherwise. I did build a very crude barrel stove in early 70's I probably should have been shot for, but that's about the limit of my experience. I am in planning stages, mostly set, to build a 300 sq ft one room house that will be insulated extremely well. I would like to build some sort of rocket mass or masonry heater but am concerned that the conventional barrel design will dramatically overheat the space. So far as I have been able to tell, there seem to be definite size limits on the low end. Obviously I need to get a book and read up so here is the question:
I know of two books for sale, Rocket Mass Heaters, by Ianto Evans and Leslie Jackson and RMH Builder's Guide, by Ernie and Erica Wisner. Would one or the other be best for a total newbie? Are they mostly two different presentations of same material or very significantly different in content?
From what I know so far, I am most interested in using a bell set up (cob or adobe brick) for heat transfer and my current idea would be to have the riser open into the first bell but with a thick metal plate above it to use as cook top plus some small source of quick heat for the room. I will need details details details including how to scale something properly down to what I need. Am I correct that 4" rocket heater is the smallest, consistently successful design? Also, at this point not sure between J tube and batch box. I am hoping all that sort of info will be covered in one or the other of the books?
So bottom line is which book to get (first?), for total newbie interested primarily in building to heat a very small space?
Hi Geoffrey; Bottom line you want both ! However the wisners book has not been released yet, so start with ianto evans book first . In answer to your other question, 4" is too small... 6" is the recommended smallest size for a first build.
Geoffrey; yes , you are correct it is available as a download , I have not seen it yet ,waiting for the kickstarter to get a hard copy. If you absolutely can not get both books then I would guess that the wisners book would have better numbers to build from. If you cover most of your barrel with cob , a 6" won't run you out ... you just won't need to burn it very often. These stoves only need to be burnt every so often the rest of the time you just relax and enjoy them . You could try a 5" system but that is the smallest a first time builder should try, the heat just can't travel down a small pipe very well.
How handy are you? How analytical are you? People have built successful 4" RMHs as first builds, but I think they have tended to be the type who are very experienced with making, analyzing and tweaking things.
Ianto's book does not cover batch boxes exept in mentions of some case studies like Peter van den Berg who has developed them to a high degree. Ernie and Erica have worked with J-tubes, and I think if their book has any real information on batch boxes it will be a chapter contributed by someone else. The truly effective batch box RMH is a recent development within the last few years. The best information on them will be in the forum at donkey32.proboards.com (look in the Reference Library for much of it).
If you have a straightforward flue path, you can put a masonry bell around your riser core with little metal so most of the heat is absorbed for later release, and a short fire will take care of you for the whole day.
A J-tube needs to be fed every 5-15 minutes or so depending on the size of the wood, but you can stop whenever you feel you have put enough heat into the system. A batch box is supposed to be loaded full or it will not run efficiently, and you can't really stop it until it is finished its hour or so burn. The batch box tends to put more heat into the system faster. So your living patterns will influence the best setup for you. If you want to tend and cook on it every day, you probably want a J-tube.
In fact last night it occurred to me could maybe have the riser open to the first bell, no barrel or preliminary tower. Could put a metal plate directly over it for some but much less direct heat to room and use as a cook top. Already have a preliminary design for insulate oven to set on top of cooking surface, easy to remove and put back, which would help further moderate heat release in addition to being ta-da! an oven. WOuld need a lot more info on bell heat exchange designs, so far have only found a little.
I am moderately handy, no where near to what I would call a craftsman, but I do like to tinker. Have made my own surfboards a number of times. Zero experience so far w/ masonry, welding, all the really relevant stuff to this project. But where I live that is easily source-able from friends and neighbors.
Welding is mostly irrelevant unless you want to make up a batch box door, and you need little in the way of traditional masonry skill. The firebrick layup technique is different than the ordinary mortar bed for bricks, as you just dip the firebrick in a fireclay soup and set it in place, checking and tapping to make sure things are true and level. A bell is more like masonry than the tiny J-tube layup. If you have access to clay or clayey soil, you can make a nice thick cob layer around the masonry bell core to mediate the heat transfer. Following the patterns of DragonHeaters (who have a reportedly very good 4" J-tube design) should do you well. Their systems are taken (licensed) from Peter van den Berg's designs. I would suggest that castable refractory for a small system will make a difference in effectiveness over firebrick, with the better insulation and less heat going into the bricks.
Thank you. Yes I found the thread on Ghetto casting and plan on making J tube that way or batch box if I decide to go that route. Looking like bell design most likely at least from what I know now; would use homemade adobe brick for that inner layer of the bell(s). Lot of reading to do first though
I don't see any reason that a batch and a J can't exist as two parts of the same RMH. Two entries into the riser, a batch door air entry that can be shut off and a J tube feed tube cap. You might not need two burn tunnels if the J tube can be built at the back of the batch area.
Life is too short or my list is too long, not sure which.
If you try to have a batch box and a J-tube feed the same riser, I would make refractory plugs that can close off either one exactly at the inner face of the riser. Having an unheated cavity open to the riser would change the flow dynamics and add a cooling element right where it can do the most harm.
RMH does not, and I doubt Ernie & Erica's book will either, as its focus is on building a J-tube system. Both will give a lot of important information on other aspects of building a RMH, with Ernie & Erica's having more technical data.
Thank you. I am traveling right now but will almost certainly get both books and build a J tube or two in my back yard first. Then either get lazy and go with that or start experimenting w/ batch boxes.
Geoffrey, I have a 500 sq ft house in Wyoming, but mostly close off half of it and only heat 250 sq ft. I built a 4" rocket heater (in a huge hurry, with little research) 3 winters ago and have set it up differently each winter. Just posted a photo of the latest version here. beautiful rocket mass heaters
It may be a little colder where I'm at, but I can tell you that my heater does not in any way overheat this tiny space. It burns most efficiently when it's cold out, but at -33 F it's difficult to get the inside temp up above 64 F. The heat output is slow, and a 4" system just doesn't produce that much heat. A system with a more efficient design might produce more heat, but 4" systems just have limits. Mine works, but with some issues. The biggest difficulty I face is that the chimney temperature tends to be too low, producing condensation and creosote.
My burn tunnel is 8" high, 12" long, and the heat riser is 48" high, currently with 9' of 4" duct though cob mass, exiting through a 6" chimney, just FYI.
I plan to put a 100 sq ft addition on my house this summer just to put in a bigger RMH - will probably go with an 8" system, though I haven't made a final decision on size yet. I think I will also keep the 4" system and work on tweaking it more, just for fun. Hadn't heard about the Dragon Heaters before, so I'm having fun looking at their systems.
If your 4" RMH almost heats your 500 sf house, a 6" will take care of it easily (even at 600 sf). That would allow you to keep your existing chimney, which is generally one of the more expensive parts of a system.
Having a system slightly oversize is not a problem, just burn less often.
Your description of an 8" high burn tunnel is puzzling - that is twice as big as it should be. Is it really a 4" x 8" cross section, or were you referring to something else? That detail would likely cause poor combustion; the burn tunnel is supposed to be smaller than any other part, if not the same size.
Glenn, my 4" RMH is barely adequate to heat 1/2 of my 500 sq ft house during our coldest temps (-30 to -40 F). I have to keep stuffing wood in every 10 to 15 minutes. The small mass is unable to keep the house warm for any length of time without the fire actually burning. When the power is out, I can keep pipes from freezing, but I'm up till 3 or 4 am keeping the fire going, sleep a few hours, then at it again. I am thinking that an 8" system would heat the mass a little more quickly than the 6" system, so I wouldn't have to be home feeding the fire all day or up all night (though a 6" system might be perfectly adequate.) I think I'm dealing with a little colder temps here than most people. I believe Paonia is slightly warmer than Pinedale, WY but it's still pretty cold country.
The new RMH will have a new chimney, as the whole thing will be located in the addition, which will be built specifically to accommodate the weight - so I'll be throwing down some money no matter what - but the external part of the chimney (the most expensive part) will be very very short because it will exit at the highest part of the roof. We may have some extra pieces of 6" chimney laying about, so that could sway my decision if money is tight when the time comes.
Maybe I didn't describe my system correctly...? It is constructed of 4" square tubing, changing to 4" round duct in the mass. Maybe more correct to say the feed tube is 8" vertical run (with extra 2" below to catch ash), 12" horizontal burn tube, 48" vertical heat riser - all constructed of 4" square tubing. Does that make more sense?
Geoffrey, sorry for the thread jack - just suggesting from experience that even in such a small space, I'd go with a 6" or bigger system for cold climate heating.
Given that activity profile, I would go for an 8" system too As long as there is enough mass, you can just burn less often and coast in warmer weather.
4" systems are notorious for not working well in a RMH situation, and if it does work well, when made of metal, it will burn out the core in a season or less. If it isn't burning out the interior metal, it is not getting up to efficient temps. I suspected you might be talking about feed tube height and not burn tunnel height.
Chimney condensation is just a factor of not leaving enough heat in the duct after it passes through the mass, but creosote only happens if you are getting incomplete combustion. How much insulation do you have around the burn tunnel and riser?
Kelly, thank you! Not a hijack at all, very relevant. I was thinking 6" bell mass heater but from what has been said, might even to w/ 8".
My idea was to mostly enclose the riser in thermal mass rather than a barrel and have only a metal "cook top" for immediate heat radiation. Also playing w/ ideas for removable oven to build that would sit over the cook top for baking but also as thermal mass or at least damping the immediate heat, and could be fairly easily lifted off for quick heat. If that would be too heavy, maybe an insulated, lighter removable and some small area masses like cob pavers to cover the metal. That way the mass would be in pieces and not all one super heavy chunk to lift.
Of course first I have to get back home, get me a book or two, and read up and then make my learning curve mistakes in the backyard.
My girlfriend heats her house with a woodstove, and cooks and bakes on it all season. She has several bricks that she arranges as needed to accommodate her pots, and puts a tray or big pot over them to enclose a space when appropriate. You won't need much of a mass or box to do some good baking.
I am priming her for a RMH, after I get mine finished and working
Thinking about this a bit more, if it is built so that the metal top is easily covered and uncovered, even an 8" will put out almost no immediate heat to overheat even a very small space. Equivalent to an approx 12" square fireplace with a very powerful inward/upward draft. 12" because of bigger size of firebox on 8" batch box as compared to firebox/feedtube on a J type set up.
Glenn, yes - my 4" rmh has flaws. I don't want to take up anymore space on this thread for a system that no one should replicate - I have promised Mr. Lumley that I will post story and photos (3 winters, 3 configurations) soonish, so anyone interested can see what went right and what went wrong.
Geoffrey, we are in a somewhat similar situation - deciding what kind of RMH to build for a small space and cold climate. My thoughts for the 8" system are that it's easier to cool the place off if it's too hot, than to warm it up if a smaller system proves inadequate. For the 6" system: it might provide more than enough heat, and the parts are certainly more easily obtainable and cheaper. I'll be following along with interest as you sort through the options!
For a really small space, I expect that the larger dimensions required for an 8" system might be a significant factor. Especially if you have an exposed barrel as in the standard design, it might be hard to avoid uncomfortable overheating while burning. The full bell design would reduce this issue.
The theoretical volume of an 8" system would be about double the volume of a 6" system, but the material costs would not have to rise accordingly. Some parts cost close to the same regardless of size, and the main bulk will usually be low- or no-cost in money.
Also, quick calculations from Peter's chart for dimensions put the top of the riser on 8" system over my head. 6" would still require significant "split level" kitchen floor (some kind of platform) to be able to use top for cooking unless someone knows another way to set it up other than metal plate above the riser.
At the RMH cob workshop last fall, Matt of Walker Stoves (who was there for the innovators event and was building cool stuff a few days early) built an outdoor RMH with the top of the feed tube level with the ground, and the burn tube below ground, specifically to make the top of the bell a nice height for cooking and smoking. Hoping this will be shown in the new set of DVD's. Not sure how that would work indoors, (would require a bit of bending over to feed the fire) but might be worth considering (in floor heat....?) especially since you still have the possiblity of designing your structure around the stove.
oh my! I am indeed arithmetically challenged. When it comes time to build I think I will hire a high school kid
to check my math.
Though looking at a converter online it says 1440mm = 56.69" which is just shy of 4 3/4 feet
Regardless, the underground box interesting idea as I am planning on most likely doing a tamped adobe/cob floor over scoria for inuslation. I currently do have strong preference for batch box though so not sure how that would work, maybe a ramp sort of thing down to front of box or mini "split level", just wider hole to put it in
About length of the batch box riser: It's 8 to 10 times the base figure, measured from the firebox floor. Base figure for an 8" would be 5.76", for a 9" system 6.48".
Nominal length for an 8" riser would be something between 46" and 57.6", a 9 incher would do with a range of 51.8" upto 64.8". I would suspect the chart is giving you the maximum lengths.
I have to add, it's safe to say the riser of the bigger systems could be somewhat shorter due to the better area / volume proportions. I've never experimented with the bigger systems, apart from that one at the innovator's gathering. So in reality I don't know for sure what the length limits are.
Yes, that's correct.
The horizontal floor of the firebox is in effect just as wide as the port, a little bit over 2 inches in a 6 inch system. At first it's a bit confusing for some people, the sloped sides are only there in order to collect all the glowing charcoal in the middle, thereby shortening the glowing phase. Mind you, when you leave those out, you have to shrink the height and/or width of the box a bit in order to compensate for it.
Thank you Peter. I was 1/2 thinking that might be the case about shrinking the depth if flat bottom. I love the idea though of sort of a self-tending fire; it stirs itself and keeps all the bits and hot parts well piled together saving us (me) the trouble.