Gary's Rocket Mass Heater

A picture is worth a thousand words, but what is a thousand words plus a picture worth? This Rocket Mass Heater(from now on referred to as RHM) with 6" i.d. riser and flue, 4" cold air intake, extra height riser(maybe 48"?), and 30 foot exhaust=single piece stainless steel chimney liner attached to a sealed box to ensure no exhaust seepage through cracks in the cob/concrete surrounding the firebox area. The thermal mass will cover ~46 sq. ft.(1 1/2 sheets of plywood sized) about 2' high average. The entire setup will be underlayed with 1 or 2 inches of foam board and 3/4" plywood. My cob mix will include some cement(which I think will keep cracking down when it dries), and I plan on smoothing it all over with that lightweight fiber concrete spackle stuff and painting or sealing it somehow. I have 2 small children and plan on keeping the house relatively air-tight, so a standard system with joints everywhere and sealed only with cob/clay which cracks as it dries and allows air gaps is not acceptable to me, so I'm going the extra mile/dollar with the expensive flue liner piping and welded/sealed box around the lower part of the riser where it feeds the horizontal run of flue piping. Hopefully this will also decrease drag with no right angles and a smooth wall surface without joints, and make cleaning easier-again since it is smooth and no right angles. I do want to have a bolt-on, sealed cleanout/inspection port on the box part. I plan on cleaning it with a super powerful leaf-vac from outside combined with a blower inside.

Picture is somewhat to scale as far as the people and dog.

So I'm diving in. ~$800 spent and climbing. Decided on 8" exhaust. Just bought an 8" x 30-foot light flex stainless steel flue pipe, fire bricks(getting more later), inlet feed piping(4" metal dryer vent type), morcoset-(furnace/flue joint sealing putty), also refractory cement(same as morcoset?). Tomorrow hopefully picking up a BIG CO2 tank for the outter "barrel". Thinking about buying a plasma cutter for making sealable joints on the barrel/tank as well as cutting the top or bottom off etc.
Pic is of the two types of furnace cement I bought.

The 4" inlet tubing, and the 8" x 30' s.s. "light flex" flue liner.

Pics of some more collected items for the project. A big 'ol bag of vermiculite(comparable to Perlite I'm told) and Ceramic wool/fiber--a whole box was 100 square feet about 3/4" thick on the roll(may expand when installed?).

Here is the outter tank and rear and front pictures of the design I'm most happy with currently for the firebox.

And lastly for today; a pic of the current progress. The LP(propane) silver color tank will be the outter wall of the flue tube. The thin goldish (stainless steel chimney pipe) color tube will be the inner flue pipe--possibly coated with refractory cement, wrapped in ceramic wool, and coated over that with refractory cement again and then the final gap filled with vermiculite. That way, if the inner stainless pipe rots out, there will still be multiple layers of protection against the vermiculite pouring into the burn chamber. After 2 trips gettting bricks, I still need another 10 or 20. I could use normal brick for a lot of it, but the sizes aren't exact so it would be much more hassle lining things up.

Some updates as it's gotten colder and I'm really pushing to have it burning in the next few days. Decided to not use the silver propane tank and rather just connect a few galvanized flue liners with rivets for the outter wall of the internal flue riser. Cleaning the paint off the CO2 tank was a major pain. Cutting it was a major pain. Finding out it had another tank inside it was also a pain. So things are going slowly but surely.

Tank birthing process...

One pic showing where I'm at now.
One pic showing how the top is being cut off, which will be the bottom on the RMH.
One pic showing inside the lower part of the outter drum where the inner flue riser pipe will rest and outter barrel will be welded to support plate.

Note in the center pic--to put the barrel on a rotissery I hammered a stick/log in the hole, drilled a small hole, hammered a drill bit in that hole, and rested the bit on the top lip edge of a 5-gallon bucket full of rocks. For the other side I cut a tiny hole in the center, then nailed a nail into my deck post, cut the head off the nail, and slipped the barrel onto it. So then I could spin the barrel slowly around making the cuts(with a plasma cutter, but similar could be done using an angle-grinder). It worked well.

I know it's not much progress, but here are some pics of todays work.
First is the setup to hold the pieces together as I packed in my vermiculite/morcoset(foundry/furnace cement) mixture under the burn tunnel and around the intake tubing--I removed the top row of shortened bricks to do that.

Second is a pic showing where the intake pipe ends.

Third is where I stopped since I may like to cut the edges off of some blocks for building the burn chamber.

Got a massive amount of work done today, proably only one more good day of work from being able to fire it up for a test run(which may be next week due to work...doh!). I put descriptions with the pics. One thing that bit me was that I welded a flat "ring" to the outside of the drum at the split--just to one end of the tank, that way the top end could set into the ring and it would seal(using morcoset), and be stable. Well, once it was fully welded and cooled, I found that the welded metal actually shrunk, so the ring was actually a smaller diameter than the upper drum seat, so I had to grind the outter sides of the very end of the upper tank as well as go all around the lip and bend it outward to make it fit. Otherwise it's going well today.

A few more pics, since pics are better at explaining than words.

impressive. how much space will there be between your outer tank and the LP tank containing riser insulation? the only potential concern I see is that the outlet into your exhaust ducting might not be big enough. it may be fine, but I've heard that's a common bottleneck.

are you building this in an existing house, or is will the house be built around the stove?

tel jetson wrote:impressive. how much space will there be between your outer tank and the LP tank containing riser insulation? the only potential concern I see is that the outlet into your exhaust ducting might not be big enough. it may be fine, but I've heard that's a common bottleneck.

are you building this in an existing house, or is will the house be built around the stove?

I decided not to use the LP tank b/c it's so heavy and I was a little worried about cutting it for fear of explosion. I will instead be using a stainless steel 8" inner riser, galvanized outter shell of inner riser, and pack vermiculite between with a morcoset/vermiculite mixed cap. The oval hole to the exaust duct is 10" x 6 1/2", so something like 52 square inches, which does match the system. I would have enlarged it a bit had I known that would be the value in the end, but without using a larger outlet pipe(like a 10" duct), it would be difficult. I do have a trick up my sleeves for allowing a smoother transition from air flowing down and out...TBA in upcoming posts... :O) There is also about a 4" tray inside the tank that will hold a lot of ash and keep it from blocking any flow--as well as a cleanout to access that gap.

This is being built in an existing house with a poured concrete basement. It took me a while to be ok with smearing the morcoset on the nice smooth concrete floor. It will also be difficult chiseling the flue hole in the wall(not the act, but the thought of it), as well as hauling in wheelbarrow loads of muck(cob), but I have high hopes of it working in the end. I'll have some videos up later once something is worth video taping.

More progress today building the firebox hole surround and mounting the plate/lower tank setup with lots of morcoset for it to squish into and seal. The front panel is thin bricks for later drilling a 6-inch hole for an "eye" using an old circular double glass thing I pulled from an old(50's) stove. Hopefully it won't cloud up with soot too quickly in between cleanings.

Gary,
You are doing beautiful work on this project. I like your descriptions of the rotisserie and the other solutions you've come up with, and your tidy welding and machining.

I'm concerned that you are repeating 3 known errors, and these may be compounded by your use of permanent cements.

1) The chimney liner is a spiral-corrugated material.
We've had problems with similar materials in the past, especially with corrugated aluminum ducting, which creates way too much drag for a rocket mass heater to draft properly. The spiral texture slows the flow and increasing the potential for smoke-back.
Fireplace inserts and stoves that use this spiral-flex chimney liner are designed to draft hot exhaust quickly past this surface, and tested for installation with specific sizes. I don't know if they size up for the extra drag, or if it just works better at high temperatures. In a rocket mass heater with a low-speed, low-temperature exhaust, the corrugated material doesn't perform the same as a smooth-section stovepipe or ducting in the same size. Too much drag.
If this does turn out to be a problem, the fixes I envision would be: to shorten the heat-exchange ducting or replace it with smooth pipe; to size down your burn tunnel and heat riser slightly; to make the heat riser taller; or to add a warm secondary chimney at your outlet to increase draft.

2) What's up with the extra air intake? It looks like you are fitting an air intake either into the manifold, or into the lower front of the fuel feed. Or is that how the exhaust gets to the heat exchanger?
Adding extra air anywhere in the system increases the potential for smoke to go up into the room, instead of being sucked down into the fire as air enters alongside the fuel. People going this direction often put a lid over the fuel feed to stop the smoke while letting 'their' air intake work, and then you get a massive smoke plume every time you open the fuel feed to add more wood. I encourage people not to add any extra air intakes for this reason. Have you tested this configuration before?
I know outside air intakes are a popular fad for various reasons, but these things have to be tweaked really carefully and may not work in a system that's already balanced on the fine edge of its draft limitations like the rocket mass heater. The RMH is designed to extract every last nip of heat from the exhaust, so its draft is balanced against a great deal more drag than other wood-burning devices. There have also been some problems in woodstoves with basement air intakes accidentally functioning as air out-takes under weird draft conditions; can your setup handle 800+ degree heat if the air intake starts out-flowing instead?
It looks like the outside air feeds to under the burn tunnel, which also creates a potential for thermal shock on the bricks of the burn-tunnel floor. I would make sure you can replace these bricks if they crack, or insulate them so they don't have to deal with the thermal shock.

3) Combining new and permanent materials in an un-tested installation, with limited cleanouts or options for removal and re-fitting of parts.
You will want at least one cleanout option to get ash buildup out of the manifold area annually. It looks like you are using the end of the welded pipe for this, a nice solution. We generally put a cleanout at every 180-degree bend in our heat exchange ducting, but perhaps your flexible pipe will not make any bends sharp enough to obstruct a wire brush or vacuum hose.

I haven't seen any pictures showing a mock-up or test-fire outdoors. It sounds like you've got a good handle on most of the details, but there are just these few places where it looks like you're winging it. I'm concerned that if you case the whole thing with refractory materials before testing it, any small problems you have will become permanent. I can't see the temperature ratings on your furnace cement, but I hope it's over 2300 degrees. 2800 would be better. Ernie's been able to make test-bed systems that melt the steel liners at the top of the heat riser.

You are using modern materials instead of cob or fireclay mortars, and some of your comments suggest that you haven't worked with cob enough to trust it.
I hope you do have some experience with the refractory materials you're choosing instead. If you haven't monkeyed around with these materials or with rocket mass heaters, how do you know what temperature ratings you will need, or what kind of thermal expansion or heat conductivity to allow for?

Cob is not clay; properly made cob is not as prone to cracking under thermal shock as some of the modern masonry materials are. And the great advantage of cob is that it has been tested in this application, and allows for re-wetting and adjusting if needed. If you have never seen proper thermal cob, it's understandable that you would be inclined to trust modern materials instead. But they have their own problems. I guess the ultimate is to work with whatever material you are most comfortable with, but test as you go. Here are some things to be aware of:

- Portland cement, or any material containing lime, tends to powder off when exposed to high temperatures. Its heat conductivity is also different from pure earthen masonry. You won't know what kind of thermal performance your system will give until it is cast and cured, at which point it is a little difficult to change things. Cementitious materials are also much harder and less comfortable than cob. So you will definitely want a surface temperature that you can safely put cushions on.
If you are going to use any Portland-cement-based or rigid refractory materials, you might look at the masonry heater ASTM standards for guidelines on where to place expansion joints to prevent cracking from thermal expansion.
Portland cements are also generally incompatible with earthen materials. Cement-stabilized earth can be weaker than either cob or cement; and cement stuccos can trap moisture against earthen materials, further weakening them. Please test whatever materials you choose to use, or get some help from an experienced local builder who has done installations with the blend you plan to use, as these effects can be delayed for a year to a decade before causing structural failures.

- Furnace cements are generally designed to seal joints in metal construction, which means they are often rated for woodstove surface temperatures (anywhere from 800-2000 F), just high enough for a nearly clean-burning fire. There are a few that are rated and designed for higher-temperature applications like industrial furnaces and glassblowing; we've been able to get ceramic refractory materials rated for 2300 up to almost 3000 F. The interior of our burn tunnel often gets to incandescent temperatures, I'd estimate over 2000 F.
Most industrial methods for handling high heat combine a non-sealed, replaceable lining such as firebrick, with an outer, sealed shell that is not exposed to the highest temperatures. Expansion joints are commonly made with ceramic-fiber felt (like you have), fiberglass gasketing, or even cardboard felt in the lower-temperature areas. In our earthen masonry designs, the clay-stabilized perlite serves as both insulation and expansion joint.

- Thermal expansion - We generally run the stove while the fireclay (clay/sand) mortars are still wet, to expand the metal against them and ensure they will not crack later when hot and dry. If they crack during the initial firing, we remove, modify, and re-mix the batch to include more aggregate or grog. This trick is usually not an option with the 'permanent' refractory materials; you have to get them right the first try and wait until they are cured before firing. If they crack later, the whole thing must be replaced, there is no re-mixing though sometimes you can get away with a partial removal and patch.


I would love for your system to work well, because it would be the first example I know of someone who has used cement successfully. Lots of people are interested. And I think you have the skills to make a beautiful job of it.

I guess at this point all I'd recommend is to test-fire it before putting any more cement-type materials in place. If the draft works perfectly before the cement is added, there is less chance of something puzzling that can't be fixed. And test-fire it for a good several hours, to make sure there are no problems with the burn tunnel materials at high temperatures.

I'd also give yourself an expansion joint and secondary seal around the firebox, so that you don't have to worry if the furnace cement doesn't live up to your expectations. Vermiculite is not as easy to clay-stabilize as perlite, but it should work fine for insulation. And we have recently started using the ceramic-fiber batting, and like it.

Thanks for posting pictures, and keep 'em coming.

Other readers - is this too much technical advice for a public forum? I feel like we've gone over some of these points before. We often end up sharing this kind of troubleshooting without asking for a consulting fee, and it seems like it should be valued. I could be saving someone a substantial cost of replacement materials if nothing else. This post should be worth at least several pictures.

I just think this project has the potential to be such a beautiful example, or a very expensive failure, hinging on a few details that come with experience.
In the past, we've had people post their brilliant 'improvements' on the rocket mass heater, and then disappear when their improvements didn't work. I hope Gary will stick around to tell the whole story, successes and failures, and let us help him fix any problems until he loves the finished project.

Good luck, and keep us posted.

-Erica W

one last thought -
we haven't done a basement heater yet, and the ones we've considered were tapping into a vertical chimney that goes up above the ridgeline.

Multi-story houses tend to draft warm air out their upper stories, creating negative pressure in the basement that can draw cold air in through a woodstove chimney or other outside air intake. (Same air flow pattern the Moorish and Craftsman architects used for passive cooling.)
Some basement woodstoves or fireplaces can have draft issues for this reason, and a rocket mass heater is at least equally susceptible.

In a 1-story building, a ground-level rocket mass heater can draft properly with a horizontal flue exhaust. But in a multi-story building, Ernie and I have generally used a vertical-chimney exhaust, indoors (therefore always pre-warmed) to counterbalance this house draft tendency.

I will be delighted to hear if your horizontal exhaust works out; if not, it's something to allow yourself options to change it later.
-Erica W

Wow, Erica. That was a lot of information! ) Thank you for all the advice and insider info, I really appreciate it. The intake "tunnel" under the burn chamber is a copy of a woodstove design my friends new woodstove had(around 2010). The intake air is preheated by running under the burn chamber(or tunnel in this case), and then directed through piping to super heat it, and then released in relatively small quantity(hoooopefullly not causeing reverse drafting) downward toward the fire/flames. With my friends woodstove, there were actually little flames on each of the 1/8" holes in multiple(6 or 8?) pipes that spread accross the top of the firebox. I'm looking to create something similar--maybe just 1 or 2 tubes in front and back and top part of the intake hole. Then above that would be a fresh air intake hole in a "lid" that is insulated toward the large drum to keep from preheating that air too much and to keep the draft going downward. I'm not one to easily give up, so if that doesn't work it may take 2 or 3 designs to get one right.

For the flue pipe--the corrugated piping (I thought) would provide the best safety against leaks as well as flexibility and ease of install(no joints to seal, no elbows etc). It was the single most expensive part of the system. If it does inhibit draft though, I could remove it since I won't have all the mass in yet when I do the test burning. Visions of a very low speed(geared down) electric fan outside have come into my head at times--thermally contolled--would turn on if the firebox temp. reached 100F or something. Cost would be pennies per month, and could be run on a battery in an emergency. That probably won't happen though. If it just doesn't work out, I can always do the standard piping with unions and elbows.

Thermal expansion is one thing that is hard for me to plan around. It seems like everything is expanding in every direction, so it's hard to figure out where to put any strips of ceramic wool, if anywhere. I did leave the firebricks in the center burn tunnel UN-cemented to the floor, so the whole assembly there could wiggle a little, but not sure if that was necessary.

Plan is to test fire it possibly even tomorrow--I have to wait for the morcoset to dry to allow safe install of the outter sheath of the flue riser and packing vermiculite in between. Then I may need to wait to have a hand to help load the top part of the tank down onto the assembly. For the first 1 or 2 burns, I'm going to run the flue liner outside through the open sliding door(about 10 feet out or so), and see how the draft goes and what kind of resistance there may be. Also having the doors and windows open will keep it safe if any smoke leaks out(supposed to be 60F temps all week). I do currently have a stone chimney that inside is about 7x10 or so, which would work as it is only about 10-feet from the RMH, but if the temps are only in the 100-degree range when the gas enters the vertical flue liner, I don't know that it would have much pull going up 25-feet to exit at the peak of the roof. Here's where I'm at today--my worms are holding the flue liner level and in place while the morcoset sets. Thanks worms!

Some pics of how I mounted the inner stainless steel flue liner(pulled from my wood-stove flue pipe).

After 2 days of the inner liner seal drying, I managed to make my custom width outter shell of the riser pipe by opening up 2 smaller tubes and self-tap screwing them together. First time putting it together I was going for a 2" gap, but that doesn't leave much room for error, so I ended up with about 3" for the side gap. The top gap is measured at 2.5"(both at the inner and outter riser pipes which are different lengths due to the slope of the outside tank). It's impossible to see in a picture, but I slightly tilted the outter shell to allow a larger gap at the rear-bottom(where the exit is), and a larger gap at the top front. This should direct a larger percentage of flue gases forward initially and then as they continue downward they will wrap around and exit at the back.

How's it coming?

Two hitches. 1. Ran out of vermiculite friday and won't get more till next week. 2. out of town 2 days for funeral. Otherwise I'm so close to a test burn I can smell it... :O)

Sorry about the funerals,

I know about running out of materials, I needed more vermiculite to finish my slab and had to run to the store mid pour...

Erica Wisner wrote:
Other readers - is this too much technical advice for a public forum? I feel like we've gone over some of these points before. We often end up sharing this kind of troubleshooting without asking for a consulting fee, and it seems like it should be valued. I could be saving someone a substantial cost of replacement materials if nothing else. This post should be worth at least several pictures.

This is really useful info Erica, and definitely not too much advice. I had better send you a donation . I understand the giving advice for no fee, but as long as overall I earn a living, I don't mind. I help others, others (like you) help me. It's less hassle than the Microsoft approach. In the days of steam in the UK Boulton and Watt who built steam engines (using customers' labour and customers' materials) used to charge for their expertise, but then used to charge for each revolution the engine made .

I just think this project has the potential to be such a beautiful example, or a very expensive failure, hinging on a few details that come with experience.
In the past, we've had people post their brilliant 'improvements' on the rocket mass heater, and then disappear when their improvements didn't work. I hope Gary will stick around to tell the whole story, successes and failures, and let us help him fix any problems until he loves the finished project.

Also good info. I am old enough now to understand it is better to play with some rustic lash-up first, and get some understanding. That way at the cost is low while I find out if I really want to do it. Worringly it does put one in the position of: "in the land of the blind, the one eyed man is king", while in reality there are "masters of the universe" out there (Erm, Ernie & Erica?)

Gary, I stopped using fire cements because they always cracked and fell out. I now use a ceramic paste for flue seals and combustion chamber repairs. It is water based, and when it dries it is soft and flexible. Never had it crack. Fraxfil is one name might be worth looking at. It will stand at least 1200ºC.

Our burn tunnel gets to incandescent, so I suspect it would be over 1200 c. But there are moldable refractory products that are rated above 2000.

In the world of the one-eyed man, a decent clay-sand mortar or fireclay mortar is hard to beat.

I would not pre-heat the air because I want cool air washing over the ends of the fuel to drive the fire down to the bottom of the sticks, not the top. The real firebox is in the bottom corner and horizontal feed tube, and I don't know a way to get outside air in there without increasing the backdraft problems in the fuel feed.
But I can see the temptation to go for the 'afterburner' effects even if it takes re-designing the entire fuel feed.
Do you have a plan for loading more fuel in when the top layers are on fire and you need to open the lid?
My biggest problems come when I've let the fuel feed get too hot at the top, and I have to decide whether to let the embers burn down for a while before adding more wood. Firebrick can hold a lot of heat, especially with the insulation that is needed for the thermal shock and clean burn on these systems. Sometimes I can poke a stick down and have it burst into flames just from the radiation between the walls.

It would not be that hard to insulate under the firebox if this air-feed idea doesn't work, and very hard to install it later, so I think it's worth trying the experiment now that you've got it lined up. Do keep a close eye on it during your test burn to make sure it doesn't draft backwards up the air-feed tube. And make sure to use short enough wood that you can hard-stop the system with a shutdown lid (spare bricks) if there's a problem.

Can't wait to hear the results of the test.
Also, I want to see your design ideas for a welded 'barrel' or bell for more elegance; your current design is certainly going to look slicker than an oil drum. See "Oil drum in my living room: Design contest" in this forum.

Condolences on the funeral's necessity, I hope the family and friends were good to be with.

-Erica

Sometimes I can poke a stick down and have it burst into flames just from the radiation between the walls.

Wow never heard that before... didn't mention that in the podcast...

Somehow I had missed the last few posts, which were encouraging, especially after a week or so ago listening to the podcast all about people making rockety type things and calling them Rocket Mass Heaters when they weren't and trying to improve upon something that's already at the peak of innovation and me thinking they could be talking about me(and probably were). Sooo...continueing on. I did finally get the 2'nd big bag of vermiculite, finished filling the gap, capped it with a morcoset and vermiculite mix, had 3 family/friend people help me lower the top part of the tank on and install the exhaust tubing(corrugated), and did a test burn for 3+ hours. 5 things to note. 1-It wasn't very rockety-I bet it is the corrugated exhaust tubing, but it could also have been hindered by damp green knotty knarled wood(exactly what the book says not to burn)--at night it is hard to tell what you are grabbing outside. 2-I had some concern of the tank getting too hot for as close as it is to the floor above. Found it not to be a problem at all, even when it was burning super hot for a long time. 3-Slight backflow of smoke, but only when I didn't have an extra brick or two covering part of the opening/feed tube. 4-The rocket heater effect of clean burning and exhaust being basically only water vapor DID happen. I took a few breaths throughout the burning process and it always had just a musty watery smell, like that of the clothes dryer vent steam. 5-The final outlet temperature was way low(without ANY mass yet). I think even more than backpressure due to the eddie effect of the ripples, the corrugated tubing causes excessive dumping of the heat and by the time the air exits, it is room temp and much denser and heavier, causing it to be harder to "push" out. Imagine stretching the tubing out until it was flat--it may be 60' long at that point.

Course of action at this point--buy a few straight pipes, cut the hole in the wall for the flue exit, and run the smooth pipe straight out to see if the backpressure or "rocket" effect changes. I may also add a draw inducer, which will be a very small fan that will be both thermostatically controlled(on when fire is hot) and switch controlled(for starting fires)-only if necessary though. If I use straight pipes for the main runs, I still plan on using the corrugated tubing for the turns/elbows. Pics soon...

Some pics before final assembly...

One of the flickering flames and one of the interior (almost) finished(cap on vermiculite insulation not on yet).

I wouldn't use corrugated for the 90's, each smooth 90 can have as much drag as about 5' of smooth 6" or 8" pipe, making it out of corrugated will only make that worse.

I'd sell that corrugated on craigslist and use the $ to buy some straight pipe.

Did you kill the idea of bottom heated feed air?

-Dan.

I don't know about the drag in each 90, since a standard 90 bend has only 3 flat turns, or if you use a "T" to make the 90 then you're making the air hit a wall and then turn sideways. I'm hopefully buying some normal pipe sections today to do some more R&D. I will be cutting the wall hole too, so I won't have to leave the door open the whole time.

I haven't totally given up on the hot air intake. I plan on having that in place maybe with a curtain-style air flow hole/slot, rather than through tiny circular holes like my friends wood stove was, but also having a feed tube lid that will not only slip fit into the feed tube to guide the wood smoothly downward, but also have an adjustable hole in top(like on some car trunk key holes where a flat circular cover spins out of the way for you to put the key in the hole). That way, when I wished to feed it, I would open that cold air hole all the way, letting a fresh air bath get rid of any risidual spoke.

if nothing else, it's a handsome stove. keep at it, and I bet you'll get the kinks ironed out.

Today I rigged a run of straight 8" smooth pipes straight out through the wall, with a downward gradient for the whole run and did a test burn(doors open of course). The draw was the same or if any, just slightly better. The fire does not have the rocket sound unless I burn small twigs(up to 1/2"), and I get some flame creep up the feed tube, even though it is not really even a tube at this point. I get the best draw if I block 1/2 of the square inches of the feed tube, so I think I have a restriction, and I know it is NOT the horizontal run(I even removed the cleanout at one point with absolutely no change in the burn). I did make sure to have the same CSA(cross sectional area) throughout the system, but I have a suspicion about 2 things. 1; The GAP at the top of the riser between the riser and barrel may be too small(2 inches per my design, but I'll be measuring that again once the bricks cool(tomorrow!?...). :O) 2; The fact that I used a steel TANK, and not a BARREL. I think the rounded top of the steel tank, even with the right clearance, may be creating a capping effect of the heat riser--like when sound hits a dish and bounces back. So I may be bouncing back the heat and/or flow of gases. My solution to this(If I dont' come to any other conclusion), will be to cut a disk out of the top of the barrel and either weld it back on upside down, or cut and weld a flat plate to it. So that's where I'm at. Here's my current test setup pic--I drew in where the back corner of the room is(pink line), and where the hole through the wall(cement) is(pink circle). I later plan to put a loop in the run to get about 30' of distance.

I may have had a "eureka!" moment tonight. I was thinking, "How is my RMH different from the standard design?" and going point by point through the system. Aside from the rounded top of the outter tank, there is a big(fixable) issue with heat transfer at the bottom of the drum. I have a thick steel plate that the drum is welded to that sits on the burn tunnel bricks--this absorbs heat quickly, which is not a problem in and of itself, but since it is attached to the outside drum it conducts the burn tunnel heat to the outsides of the lower part of the drum, which heats up the inside as well as the ouside, and so I think it's causeing a heat flow "fist bump" so to speak. The gases come up through the riser, are cooled as they hit the top part of the drum/tank, then as they decend they are hitting another wall of heat that is being generated at the bottom of the drum/tank. So my solution is to fill the bottom of the inside of the tank with vermiculite, as well as wrap the outside later with ceramic wool insulation, and hopefully this will keep the excessive heat from transfering in so quickly. I'll try this tomorrow and see how it goes. The top gap between the riser and tank was good(2 1/4"), and the outlet is something like 60 sq. in., so should be fine there. When I fill in the vermiculite, I'll taper it to the front to get a nice decending flow of gases going.

More updates; Today I filled in the bottom of the tank(outside the riser but inside the outter tank) with vermiculite, tapering down to the outlet. This helped the draft/draw a LOT--I guess the air pocket at the bottom caused some flow backup as the air churned down there, as well as the heat from the lower plate mount rising up the sides of the outside tank.

A second thought/thing I noticed that caught my attention; When I had the 30-foot corrugated pipe on, I had pretty much just water vapor exiting the pipe, whereas with the straight pipe, I had both smoke and creosote smells coming out. I did have a little bit of a draw problem with the corrugated pipe, but it wasn't much different than it was after changing to the shorter straight piping before adding the vermiculite to the bottom of the gap inside the tank.

A third thought/thing I noticed; The draw seems to be fairly strong with the current setup until burning for 20-30 minutes, at which point I think the heat in the feed tube as well as the heat in the outter tank is causeing some backdrafting once the fire is burning down to coals, no matter what type of covering I have over the feed hole or not. I believe this may be caused by a LACK of Mass at this point. Since I am now confident in my horizontal flue run plans, I'm going to work on bringing in the mass(cobblestones and cob) and assembling that part of the system and piping, and then do more testing with the same stove "guts"(feed/riser/tank/outlet hole) once I have that mass in place to absorb some of the heat and allow a longer hot burn hopefully.

Here's the pipe exit, a few inches off the ground, and a vertical flue pipe tester section--didn't seem to make any difference having it on or off, but it wasn't a sealed connection so that may have something to do with it.
Then a pic of the cobbles and the clay I dug out from the yard when I had a tractor(aged slightly now). :O)

I get the best draw if I block 1/2 of the square inches of the feed tube, so I think I have a restriction

When you use Charles's law of gases going from 70F to 1700F the volume can expand 4 times the amount. So blocking of 1/2 is not bad, I have noticed with mine that blocking 3/4 works well which matches the math.

When you choke down the cold air coming in the fire/feedtube has to heat less incoming relatively cold air.

Brilliant. I love what you're doing with this one...

Gary,
Interesting project, clean work.
Looking at your images and reading your reports, my immediate suspect is that there is a slight bottleneck at the junction between your "barrel" and the horizontal run.
I quite like how you went about it, nice neat connection, easy to clean.. Hopefully it won't need changing but hard experience has me looking there first.
I recommend that you measure the circumference of the cut out oval, multiply that by the distance across the gap to the heat riser assembly. If the area of the (invisible) junction "surface" is not HANDILY larger than system size, you have a problem.

It's any easy thing to miss, even though the pipe prepared to accept input and the gap around the barrel are both generous, the junction between the two may not allow enough flow. Many folks do, but I NEVER rely on the numbers here. I ALWAYS oversize this spot by as large a margin as possible. I'd say that so far, in 3 out of 4 stoves that I've seen misbehaving, the culprit is right there under the barrel. Number two problem is top gap or side gaps being too tight and the rest tend to lean towards a discovery like "oops, how did THAT end up down there?"..