Ideally the exhaust / flue pipe diameter should match RMH system size, i.e. 6 inch inside diameter for 6 inch RMH, the stove's system size being defined by the size of the wood feed-tube opening. What size system did you all build?
BTW, only the portions of the flue that are outdoors and in un-heated attic spaces need be insulated.
Hi Linda, the ideal situation for a wood burning stove is that there be a good strong natural draft even when the stove is stone cold. A strong natural draft helps keep the stove under negative pressure at all times, pulling any residual combustion gasses up the chimney and out of the dwelling. With the proper chimney, even a freshly built, wet, stone cold RMH will exhibit a good natural draft. This is of even greater importance for stoves of "single skin" construction.
What's your chimney like? Is the exterior (outdoors portion) of the chimney insulated? Does the chimney exit at a height of 3 to 4 feet above the tallest point of the roof? For us mortals that don't reside in a magical valley with consistent unidirectional winds (re Ianto Evans) there's just no getting around having a proper chimney attached to our RMH.
For heat riser insulation, you could try fiberglass. I.e. to protect the fiberglass, perhaps make a two layer insulation by using fiberglass for the outer most 25mm thickness layer, and plain sand for the innermost layer, to let the sand take the full brunt of the heat from the riser's brick.
What's your heat riser to barrel top gap like? I.e. the distance from the top of the heat riser to the barrel's inside top horizontal portion? From your photos it looks like the top gap may be a little tight. I typically shoot for a minimum riser top gap of 75mm. Making it a little larger won't hurt unless you intend to use the top of the barrel for cooking etc. But an advantage of a slightly larger gap is that it will allow the stove to draft stronger on marginally short chimney systems.
Still making custom cuts to come up along the side of the barrel. Using Super wool as an expansion joint to hopefully keep it sealed...
Interesting build Thomas. The steel barrel to masonry vertical gas seal, that's going to be the trick maintaining a tight leak free seal in that critical area. Lots of thermal cycling forces at play there. Perhaps some kind of spring loaded strapping could be implemented to hold the barrel tight against the compression seal? Myself, I'd probably just go the second route, i.e. add/build the bell's fourth wall straight up against but not touching the barrel. Best of luck, and looking forward to the test burn results.
I would like to have, but no, my build notes come from E&E's early permies posts, that have long since been buried in all the thread traffic over the years. If you dig deep enough you'll find them. Good that you have the Rocket Mass Heater Builder's Guide -- that's a must have and simply the best J-tube RMH publication ever!o) IMHO. My own stove is patterned after their compact Cabin-8 model, via their plans.
Jim, sounds like you've got a good plan of action there. BTW, when these stoves are running full tilt it will be uncomfortably hot within 3 feet of the barrel. Needless to say, the intense radiant will heat up the stovepipe that's angled above the barrel, so there is really no need to insulate the first (lower) three feet or so of that 45° run, if any at all. And it goes without saying, it's a good idea to closely watch the temperature of the tent's ceiling directly above the blazing hot barrel.
As far as priming time, typically only a few minutes is required with the torch, as Max mentioned. Once the stove is run consistently for a time, the thermal mass bench will dry and warm up. Then daily start up will become much easier and exhaust priming should no longer be needed. To get a bit more heat from the combustion unit into the mass, you may want to consider insulating (Rockwool, CFB, etc.) the half barrel manifold, at least from the base up to the barrel clamp.
If you have any sections of 6" dia. HVAC duct left over, with insulation added, that would serve as a quick and light weight way to test the draft improvement of adding chimney height. Because it's very lightweight, none guyed support could be done indoors using something like sections of inexpensive TV mast pipe, running from the floor up through the same ceiling opening as the stovepipe, and extending above the tent's roof. You will be amazed at the draft strength improvement that a few feet of additional chimney height makes.
Another suggested area to improve upon is J-tube's wood feed. Build up the area around the wood feed entrance with several courses of brick (fireclay/sand mortared in place) to form a vertical wood feed "tube". I'd make it exactly the same measurements square as the current opening, and tall enough so the depth of this vertical wood feed channel (measured from it's opening down to the burn tunnel floor) is about 16 inches (common firewood length).
Hi Roger, from my notes (via E&E Wisner) the maximum horizontal ducting length on a really good quality built 6" RMH is about 40 feet with no turns. Subtract 10 feet for each 180 degree turn, 5 feet per 90 degree turn, 2-3 feet per 45 degree turn. Don't count the last 90 degree turn exiting your mass that goes vertical to connect to the chimney pipe, nor is the vertical chimney pipe length counted.
One way to help avoid a sluggish system is to test for natural draft before making your build permanent. I.e. lay out all of the horizontal ducting where it's going to be, all connected up with elbows, Tees and etc. and connect it to the chimney. Test for a natural draft at the other end that will be connected to the RMH barrel manifold. With your 14 ft. chimney you should have a fairly good natural draft, and with that you're good to go. The draft can always be improved by adding additional chimney height too.
I'll mirror what Max said about the chimney pipe insulation. Exterior stove pipe insulation is mandatory between the ceiling up to the chimney cap. Inside (living space) stove pipe insulation is optional. Insulating the stove pipe between the thermal mass exit to top of barrel is counterproductive, that's where the bare stove pipe is strapped to the barrel specifically to facilitate the draft.
RMH rule number one: avoid like the plague having firewood sticking up out of feed-tube opening, it's a recipe for smoke back, to say the least.
Starting a stone cold stove, especially in really cold weather, in a cold dwelling, is often problematic and frustrating. Break the cold starting procedure rules, attempt to rush it, and the RMH will bite you in the butt, rewarding the living space with smoke-back. Experienced RMH folks have their individual ways of dealing with cold start scenarios. Here's my tried and true method:
1) Remove the cold air plug in the chimney, using a propane torch to preheat the chimney's interior air. This is done at the exhaust Tee that's located right above the thermal mass bench exit's 90-degree upward turned elbow. The torch flame is directed upward into the open Tee until the stove pipe is warmed up and a good natural draft established.
2) Cap the exhaust Tee and light the J-tube kindling fire at the base of the heat riser. As the fire grows continue feeding it and work it back towards the base of the J-tube vertical wood feed. Use only small kindling wood. Continue feeding the fire until the barrel and J-tube warms up and a hot bed of coals form.
3) If number-2 fails, repeat starting again at number-1.
4) Once a well burning hot kindling fire is raging, then and only then, begin stepping up the size of the wood to larger pieces. 2-inch diameter and up firewood is added after about 30 minutes into the burning of fine splits and or small sticks, only when the stove is warmed up to operating temperature.
Hey Jim, I like that tent setup, nice. The condensate issue will improve with improved (hotter, better drafting) stove performance, by pushing the condensate (as steam) up and out the chimney. Guess that's pretty obvious. Nice photos, and the one of the truck head-on with tent behind clearly shows the draft problem -- all those close in surrounding trees. Want to know the optimum height of the chimney?o) Make it a little taller than the close in trees. I'm thinking erect one of those telescoping radio/TV mast, guyed off of course, insulated chimney pipe strapped to the mast, if you want to experience the best an RMH has to offer. Using insulated double-wall stainless steel Class-A chimney pipe sections, this top notch setup shouldn't set you back much more than the cost of the tent itself!o)
Okay, maybe that's extreme, on to the second plan: add insulated chimney sections one at a time, inching the height up in stages (about 4 feet at a time) and testing for draft with each new addition, until you get the desired RMH performance. You'll know when it's right, firing the stove will result in all that blue barrel paint flashing off in a cloud of noxious stinking smoke, if it's not 1200° F. rated paint of course. After burning off my barrel, plus a bit of sanding, it was repainted with high temp 1200° F. woodstove paint. The high temperature paint is holding up very well, with exception for looking gray'ish at the very top of the barrel, only the horizontal portion. That hottest area easily hits 800° F. to 900° F.
Anyway, with the right chimney setup you'll get that mass warmed up in its current configuration.
Hey Jim, nice looking build. H'm, how is your draft when the stove is cold without a fire going? A really good natural draft will pull the flame off a lit match held at the entrance of feed tube.
Max mentioned the chimney and that reminded me of my system. Living in a hilly area with shifting wind patterns, I've found that my RMH performs best with a tall chimney. Currently that consists of the following: single wall stove pipe exits the RMH rising to the ceiling (Selkirk ceiling box) where it transitions to double-wall insulated Class-A and passes through the attic and up through the roof to a height of 4 feet above the roof's peak. Total length from RMH to chimney hat is between 15 and 16 feet. My RMH initially had problematic draft issues on a shorter chimney, and extending the chimney made a world of difference.
For my PvdB 6" batchbox I went with an IFB (Insulated Fire Brick) heat riser (2600° F. rating) sourced from a local Harbison Walker refractories outlet (many locations scattered around the US). Anyway, three seasons down and the heat riser is going strong, and is expected to last for a good many years yet to come. It took three cases of IFB for the heat riser construction, and the cost at that time was in the range of $120.
From what I've observed in my own stove, pulsing occurs from a momentary pressure increase inside the firebox, when the burning wood is producing more combustible gas than the draw (draft) can pull through the port into the riser to feed the secondary heat riser's combustion zone. The greatest tendency of pulsing is when all the wood is catching at once and out-gassing like crazy. A portion of the excess gasses cause a flareup inside the firebox, firebox pressure increases briefly and that pressure simultaneously pushes gasses towards both the port and the primary intake. The velocity stack mitigates this effect by encouraging improved firebox throughput, especially handy for burning hardwoods that tend to "coal" heavily.
Jay Hughart wrote:Ok I scored 25lb of Hawthorn bond fireclay. Should I just mix it to a mortar like consistency with water or do I need to add anything to it? Sand pea gravel portland cement ect? I think the clay alone will be fine but I have never actually worked with it and chipping the motar from the top of the barrel was dusty and messy wanna get it right this time! Thanks for everyones help and input!
Just like making clay-sand cob, one part fireclay to between 3.5 ~ 4.0 parts masonry sand. Use as little water as possible, to cut down on cracking as it cures. One method I prefer to use, first mix fireclay with water to form "clay slip", about the consistency of pancake batter. Let it sit overnight to insure the fireclay has completely absorbed the water. Then mix in masonry sand to form a stiff cob that can be formed to the desired shape etc. Such a surface made from this will conduct heat well and boil water, but it will be subject to abrasion and wearing away with heavy use like dragging pots across the surface.
Matthew, I agree with Max that your stove more resembles an ever decreasing-CSA flue run system. BTW, that is very similar to my own stove. If your first bell was much larger, then it would be more like Peter's stove.
One thing I noticed about my batch-box is that there is much less tendency of pulsing when the burn is occurring just in front of the port to no further forward than about 6 inches in front of the primary air intake. Through experimentation I also discovered that a velocity stack added to the primary air intake, to promote straight-line primary air flow into the base of the burning wood, to encourage the burn at the back of the firebox to just in front of the port, significantly improves performance under all conditions from a full firebox load to the coaling stage of the burn.
The velocity stack protrudes through the firebox door's primary air intake into the firebox by 2 inches, thus directing primary air straight into the firebox's V-floor. As a test I made a temporary experimental velocity stack from coffee can tin, formed into a square tube tightly fitting the primary air cutout in the firebox door. The velocity stack is flush with the outside face of the firebox door, with a half inch wide 90 degree edge-lip to keep it from being pushed through the primary inlet while using a poker to tend the fire (shove embers and coals back away from the primary air intake). Anyway, maybe give this a try and see what effect it has for your stove.
Sounds good Matthew. Leaks indeed cause a performance hit.
I don't notice any smoke when burning seasoned wood, not after startup, only steam that vanishes within 4 to 6 feet of the chimney cap. The door has a viewing glass and I can see clear back through the port into the inside of the base of the heat riser. When secondary combustion is occurring, the inside the heat riser is illuminated in golden yellow light. Pretty cool, er uh hot.
My usual start-up routine is to first burn a handful of kindling to verify the draft and warm up the fire box a bit, using wood of about 1" diameter and thinner. All my wood is cut to a length that provides the recommended clearances between the fuel and the primary and fuel to rear port (about 2"). Once the kindling catches the secondary combustion inside the heat riser begins in less than a minute or so, noticeable by a change in sound as the rocket roars to life. After burning down to gently flaming coals and the secondary combustion subsides, I'll load in a full batch of 1" to 2" diameter sticks first followed by 3" to 4" diameter ones, filling the firebox to 3/4 full. Secondary combustion lights off within seconds of the firebox door being shut. HTH.
Hi Matthew, my batch-box (firebrick combustion unit, IFB heat riser, 6" system size) had the identical pulsation to your stove, at exactly the same frequency, with small flame bouncing out of the primary air. Likewise, slightly opening the firebox door stopped the pulsation and the stove behaved itself. The stove is going into its third heating season and doing very well, after a bit of fine tuning. Each heater build will naturally be a little different; construction materials, flu path, chimney draft, wind effects on the chimney, fuel-wood burning characteristics, cold start-up time, and etc.
I'm burning mostly ash and beech, and the primary air opening ended up needing to be 3.125 x 3.125 inches to correct the problem. I got an idea of how much larger to make the primary air intake by observing how much, on average, that the door needed to be cracked open over the course of the first heating season. In windy conditions that significantly increase chimney draft, I'll close the primary air by one third to one half.
For very close to the same heat output of an 8-inch J-tube RMH, a 6-inch batch-box is the equivalent. An 8-inch batch-box will run super hot, dump a lot of BTU, enough to heat a small warehouse.
To keep a PvdB batch-box going continuously for hours at a time will require reloading it during the coaling stage, about every 45 minutes. Matt Walker has been working on extending the burn time of the batch-box out to several hours while retaining clean burning characteristics, details this thread:
Aaron, I'll pass along a few thoughts that will hopefully help save time and effort on the reworking of the exhaust "manifold" area. The 1/2 barrel manifold works best with smaller diameter heat risers, so as not to inadvertently build in an unwanted exhaust restriction. With that in mind:
1) Let's say the 1/2 barrel is shifted right up against the heat riser directly opposite the exhaust cutout in order to get 4" of clearance. As Peter mentioned, it would be best to increase the riser to barrel top gap, since the exhaust gas path is now being narrowed out of the top of the riser (no longer a full 360 degree exit path). 4" or more top gap would be the minimum, I would think. The 2" top gap value commonly suggested makes for a hot cooking surface on the barrel's flat top, but that's not important for stoves built primarily for heating. The riser top gap on my stove is currently 13" (I experiment a lot). You could try it first without the 12" to 8" exhaust port reducer, but that's an excellent suggestion via Peter, and will improve the performance of the stove.
2) Let's say that shifting the 1/2 barrel produces only about 3" of exhaust port to riser side clearance. I'd definitely do as Peter suggests, make the 1/2 barrel exhaust cutout larger and fit a 12" to 8" reducer between the 1/2 barrel and the Tee, and also increase the riser to barrel top gap to at least 6" or more.
Aaron McKinley wrote:
Question, if I shift the barrel to get four inches on the exhaust side that will put the barrel and the heater riser (with a two-inch perlite ring) very close on the opposite side. Is that OK?
It is okay, and as they say "a necessary evil". Yeah, there's an exhaust restriction working against that stove for sure. With the Tee protruding 1" into the 1/2 barrel manifold, and only a 2" gap between the exhaust cutout and riser, the effect is that there's only about 1" for the exhaust gases to squeeze through. No wonder the barrel is not heating up. Once that restriction is corrected, along with the vertical chimney extension, be prepare for some serious heat! With the stove up and running I'd monitor the temperature of anything flammable within about 3 feet of the barrel; wood framing, window frames, greenhouse plastic, etc.
So right now I have an 8 inch T coming out of the drum with foil tape to seal it, then cob to seal it. This will not work?
Hi Aaron, you're using Ernie's 1/2 barrel "manifold" technique, in which the recommendation is to have 4" of space between the exhaust hole cutout in the 1/2 barrel to the heat riser. The minimum, according to their book "Rocket Mass Heater Builders Guide", is around 3" but the closer to 4" the better when going straight in (perpendicular connection) in order to keep exhaust gas restriction to a minimum. An angled or offset connection works even better, as it requires a somewhat oval hole cutout in the 1/2 barrel, which has a larger cross section thus less drag on exhaust gases. One spot to take care with is where your Tee connects to the 1/2 barrel "manifold", to insure a flush non-inside-protruding connection etc. Perhaps you've done that, since it's in the Wisner's book and plans.
BTW, it is okay to offset the barrel to get the required 1/2 barrel manifold exhaust hole to heat riser clearance.
Hi Aaron, I'd extend the chimney vertically by another 5 to 7 feet, whatever it takes to make it at least a few feet taller than the roof's peak. The stove will draft better, run much hotter, and smoke back much less if any at all. In wind the stove's draft will actually increase rather than smoke back, with a proper / tall chimney. I use just the standard dome style chimney cap sold for Class-A type metal chimney, works just fine in our 25 ~ 35 MPH wind gusts.
Edit: As far as how hot the barrel gets etc., it will be uncomfortable to stand within 3 feet or so of the barrel at full/peak burn. Without even pushing my stove I've IR'd the barrel's top at in excess of 750 degrees F. when burning moderate amounts of dry high fuel density wood; ash, maple, oak. And it only takes about 15 to 20 minutes for the barrel temperature to reach that maximum value.
When the mass of my stove has cooled to ambient, aka cold start, I'll fire the stove continuously for between 4 to 5 hours to bring the mass up to temperature. From then on I'll fire the stove for 2 hours each morning and each evening to keep it all warm. HTH.
Aaron McKinley wrote:Thank you for the answer. So would I place the 1/2 barrel on top of the ceramic fiber at the bottom, and then stuff the blanket in the seams between the firebrick and metal lip?
Yes, that's the idea. It helps if the areas of the 1/2 barrel's steel edges making contact with the ceramic insulation, if there is formed a broader contact surface by bending the steel edges to form an "L", making a lip of about 3/8" to 1/2" wide, so the ceramic insulation is not cut by the steel.
"Clay/sand mortar is used as fill between the blanket insulation and the face brick."
Aaron McKinley wrote: I can't picture this, it's too early in the morning. To stick the blanket to the brick?
Just to fill the gap between the insulation and facing (brick, block, etc.) to hold the insulation firmly in place. Other fill options include stuffing in additional ceramic fiber in the gaps, or use clay stabilized perlite for filler.
Pure clay without any sand? Even when clay is used in the recommended mortar mix with quality (sharp granules) masons sand (typically 1 part clay to 3-1/2 ~ 4 parts sand), it will still crack when in direct contact with steel (1/2 barrel "manifold" area etc.) unless an expansion joint is employed. For a "straw cob free" expansion joint on my stove, the 1/2 barrel "manifold", including the burn chamber, is wrapped in 1-inch thick ceramic fiber blanket insulation. Clay/sand mortar is used as fill between the blanket insulation and the face brick.
I've noticed that in my completed and fully dried out RMH, condensate will be developed in the ducting when the mass is cool. Once the mass is brought up to temperature with the first burn of the heating season, condensate is no longer developed (when burning seasoned wood, about 15% moisture content). Condensate will naturally be a little worse in an incomplete stove with really cool exhaust temperatures.
Hi Martin, your barrel's top should be to hot to touch after just a few minutes to ten minutes of fire burning, from a cold start. The top gap (heat riser to barrel's top surface) can be in the range of 2 to 3 inches (for heaters) and no less than 1.5 inches (for cooking on top of the barrel).
Some additional info. about your heat riser would be helpful:
What is the construction material of the heat riser?
What is the insulation material around the exterior diameter of the heat riser?
What is the height of the heat riser, measuring from the burn tunnel floor to the top of the heat riser?
About the burn chamber, is the burn tunnel insulated on all sides + top + bottom? What is the insulation material and the insulation thickness?
What is the construction and measurements of the exhaust transition chamber leading from the barrel to horizontal heat exchange ducting? Photos?
The barrel does get mighty hot with intense heat at the peak of the burn. In a properly built rocket that poses no barrel longevity issues. Some folks have replaced their barrels at the 20 year mark as part of general maintenance, only to find the old barrel in perfect condition.
If you're in a rather cold climate where you can have a fire every day, and if your new place is to be highly insulated (i.e. R38 ceiling, R22 or greater in the walls; US R-values) then a full masonry bell style stove may be an attractive option to consider. For some ideas check out the 6" batch-box all masonry designs on Peter van den Berg's site:
When I installed my duct work in the mass, I aimed for 6" spacing . . . so front 6" mass . . . 6 inch exhaust . . 6 inch mass . . 6 inch exhaust . . with the back of the mass at 6 inch . . .
My bench run ducting is also in the same "hairpin" shape, but 8" ducting. To aid in a little more uniform bench temperature to the far end, and to keep the exhaust temperature up, I tapered the mass thickness via positioning the ducting as follows: Beginning of duct is 6" x 6" (front and top mass thickness), the end is 4" x 4", and the return loop is 4" x 4" (back and top mass thickness). Spacing between the two ducting pipes is 4". The ducting has a slight rise, as can be imagined with the mass thickness measurements. No condensate buildup problems, so I'm pleased with the result. After four to five hours of continuous firing the exhaust pipe is almost to hot to touch when you first grab it for the first few seconds, then one can hold it tightly for a full minute or more.
Dave, have you tried a larger 55 gallon barrel? The extra space around the heat riser, and greater riser top gap, together with less drag on the gases, will make for a little less barrel radiant heat but may raise your bench and exhaust temperature just over the point of generating condensate.
Aaron McKinley wrote:
Right now the bottom drum is quite short and thin around the fire chamber. (Picture..) Is this too thin? I can recut another drum higher if needed.
Yeah, that is way to thin, it needs to be broad enough for proper sealing, and I'd make that a minimum of 4" wide (where the lower barrel cutout crosses over the burn tunnel). Wider is even better, to better accommodate a flexible seal that will need to be built-in for the full circumference of the lower barrel portion, to combat the effect of cracking masonry from thermal cycling / expansion of the steel exhaust transition.
Secondly, for the "half barrel manifold", turn the "lip" end up. You can check your barrels first to see how they'll go together. For mine I found that the barrel "bottoms" would easily "band clamp" together, or that one barrel bottom end could be "band clamped" to one barrel top end.
Dave, is you chosen system size due to the recommended 8" RMH in "the book by that couple"? As per Peter van den Berg, a 6" batch-box has about the same power as an 8" J-tube rocket. The larger 8" batch-box is much more powerful, and with just a single barrel will be way to hot to stand anywhere near it during the burn cycle.
As with Glenn and Satamax, how many sq. ft. of floor space are you planning on heating? Shop/building insulation values?
For an example, a 6" batch-box topped with a single 55 gallon steel barrel easily handles my 1350 sq. ft. cabin with power to spare. (R11 walls, R38 attic)
Paul is heating his double-wide with a pebble-style RMH and measuring wood usage, details in this thread (link below). There are some nice photos on his stove and it'll give ya an idea of daily RMH life:
John Harrison wrote: I was planning to build a 5" batch rocket using 25mm (1 inch) thick hard firebrick splits backed up by 20mm ceramic fibre insulation. I'm hoping that this combination of hard but thin and well insulated firebox components will provide the best of both worlds as regards durability and performance.
What do you think?
Since your question is a little off topic for the other thread, I'll add my input in this thread:)
20mm ceramic fiber blanket (CFB) insulation is a little thin for that application, since it will be compressed a bit when used between the firebrick splits combustion unit and the encasing brick (or cob, stone, or etc.) Personally, I'd double the insulation thickness. I.e. my 6" batch is constructed of full size (2.5" thickness) firebrick, with 1" CFB between that and a solid brick surround of 7" thick (doubled common brick). The surround heats up nicely to being almost to hot to touch after firing the stove 4 to 5 hours straight.
Insulating the barrel will do the trick, as Satamax indicated. Wrapped mine in some scrap R11 fiberglass (foil layer to the outside), then a section of "chicken wire" (could use bare wire etc.) to hold the R11 in place. Set it up as a Pocket Rocket and used a scrap 5' section of HVAC duct for a makeshift chimney pipe. Lit it off one evening just after dark, burning small pine limbs, twigs, cones, etc. Just a small fuel load adding more material as it burned down. After about 20 to 30 minutes the whole barrel was glowing a dull cherry red.
From what I've read by Peter van den Berg, the output of a 6" batch-box is very close to that of an 8" J-tube.
My own 6" batch-box is driving 40 feet *equivalent* length of 8" horizontal flue, encased in "claycrete" style cob and faced with solid brick, but otherwise classic bench style RMH layout. *Flue run consists of 20 feet of steel pipe sections plus 4x 90-degree horizontally oriented elbows for an equivalent drag of 40 feet. Vertical chimney is 15 ft. of class-A insulated pipe.
I used 8" flue because the 6" batch doesn't play well with back pressure (exhaust restriction). The half barrel "bell" bench Satamax linked to is another way to effectively and inexpensively construct low back pressure thermal mass benches.
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]
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.
@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.