Gary's Rocket Mass Heater

Kirk--you may be correct there. I did the math and made that "invisible wall" area a little bigger than the normal square inches of the system, but I may need to cut out some and weld in a oval shaped metal to seal it back up, in order to get 75-100 square inches and allow good flow there. I'm currently working on getting some cob on there to help absorb the heat that builds up so quickly and seems to cause backpressure as the exhaust tubing doesn't have a "mass" to absorb into yet. That's my thought anyway. I did have substantially more success once I filled in the bottom of the barrel gap with vermiculite, so that the side hole was at the very bottom rather than just close to the bottom.

Here are some project pics from my work today.

Erica Wisner wrote:

#### 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. ####

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

I think there have been many times when I have gotten a little lost in the technicalities of one of Ernie and Erica's explanations. Am I correct in assuming that you both talked about this one, and Erica typed it up ? But sometimes there's a simple gem of information such as the one highlighted. That particular bit of news seals the deal for me so far as base materials are concerned. I like everything about the workability and re workability of cob so that's a wheel that I won't try to re invent.
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You mentioned pictures and also referred to consulting fees. Check the picture below, to see a little token of my appreciation. I'm going to send Ernie and Erica $20 every time I complete a wood burner where I've referred to their work for guidance. This will probably happen 10 times and won't hurt a bit. I'm sure there are hundreds of other lurkers out there who could afford to pay a little for all the help they've received. A simpler method would be for everybody to buy the book so that they can ask informed questions.

I'm hoping to get our fiery friends to Vancouver Island for a build and workshop after their N.Z adventure. The plan is to put together workshops where I haul 24 people to the event and supply tents and cooking that are part of my bus business. If you're near here and want lots of help building a RMH, send me a message.

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I'm not sure if you stock things like this but I found a huge amount of firebrick on this govt liquidation site. They're on an army base in Utah --- http://www.govliquidation.com/auction/view?auctionId=5613322

Gary,
Something I've only just noticed and I don't know if you've discussed it before.. Your use of flexible joins may be or become problematic.
If gunk is GOING to build up, it's most likely to do it at the flex-pipe. Those corrugations love to catch whatever flows by. They will also create a good measure of friction to the system.
In particular, where your bench pipe turns 180 deg. doubling back on itself, that is likely to cause a GOOD deal of friction and back pressure on your stove..

Also, There should be a port (or two, or some such) at THAT point, where you can get into both pipe runs for cleaning.
Fine ash and what-not WILL blow down into there and find THE MOST inconvenient place to pile up.

I built the entire run of pipe with a decending slope with the intension of being able to run water through it to flush it out, maybe even just slam a 5-gallon pail of water through once or twice a year and see what comes out. As far as the corrugated pipe, I can see if the whole run was that(which I first was going to do and decided against for that reason), but with only 1 bend using it, and it being a double bend, I think the smooth curve will be an advantage and the ribs a disadvantage that will balance it to a net even, if not better in my opinion. One thing I think is not logical is putting a cleanout in a bend--it is always a "T" pipe, so the gases would back up in the cleanout part, and would have to basically make a 90-degree turn. The best would be to build a swollen curve with smooth sheetmetal like you see on race-car tubular manifolds at tight bends, but this is not rocket science, it's rocket-mass science. ha ha.

Gary Park wrote:I built the entire run of pipe with a decending slope with the intension of being able to run water through it to flush it out, maybe even just slam a 5-gallon pail of water through once or twice a year and see what comes out.

Now there's an interesting and different idea.. It's either genius or nuts, I really couldn't tell ya which..

As far as the corrugated pipe, I can see if the whole run was that(which I first was going to do and decided against for that reason), but with only 1 bend using it, and it being a double bend, I think the smooth curve will be an advantage and the ribs a disadvantage that will balance it to a net even, if not better in my opinion. One thing I think is not logical is putting a cleanout in a bend--it is always a "T" pipe, so the gases would back up in the cleanout part, and would have to basically make a 90-degree turn. The best would be to build a swollen curve with smooth sheetmetal like you see on race-car tubular manifolds at tight bends, but this is not rocket science, it's rocket-mass science. ha ha.

Umm actually, it works out fine. It's the best place for a clean-out, one port for two pipe runs, stacks functions and reduces the number of parts. Gasses dwelling in the clean-out causes no harm, actually this effect can be accentuated by making a slightly over-sized cob chamber that both pipes stick into. This wider chamber will cause gasses to slow for a moment, which encourages ash to settle in a more predictable location WITHOUT causing too much friction.

In fact, forgetting the pipes entirely and building the bench into a "bell chamber" is my current favorite. Better heat harvesting potential, more even distribution, more versatile in flow-path(s)... All 'round marvelous solution, me thinks..

Got a few more hours of work done today. Poured concrete around the last bit of pipe that I set a granite slab over to keep it from getting crushed. I did have to weigh it down into an oval shape, but not too far as to constrict flow. The cobbles are a preview of what the entire stove floor trim will look like. Planning on all the sides to have a cobblestone wall look, and the top to be smooth of course. Also chopped a large trash can of straw(from ornamental grass cuttings last year-super strong stuff).

Great looking work.

any updates on how this is working?

David Brock : Thank you for 'bumping' this thread up to the current time, it is worth a re-read for the many insights Clearly presented by both Erica Wisner and Kirk Mobert !

Often gems get lost amongst the Chaff that accumulates daily, you have done a very simple but much needed service, in the future I will trust your judgement as to what should
be 'Bumped" ! Big AL !

(must have spaced on taking pics of the mod...oops)...So here we are today after cobbing over the hole. Basically I cut a triangle out of the "armpit" of where the gases come down and enter into the horizontal flue piping. Then I cobbed in the gap and made sure the inside corner was nice and contoured. After this mod, I have much more "rocket" in my rocket heater, and I was able to start burning with hardly any smokeback at all.
Here's two pics-one of each side of the modified area.

Here's a little descriptive pic of the mod.

Took me a bit to figure out the video link--a short burn video, because we all love to see and hear a nice fire burn. (have to click on it)

Hi This is really looking promising. Firstly I know nothing about RMH that is worth adding however I do know that the corrugated pipe needs to go. I know this from building car intakes and exhausts, a typical crinkle bent pipe will have much more resistance then a smooth mandrel bend. Think of it like this every ridge in the pipe creates an eddy and swirl effect in the flow, in these areas the air starts to almost act as a solid reducing the diameter of the pipe. So your making the pipe more restrictive at the turns which are already more restrictive then straight runs. Its like reducing the pipe diameter bu 1" at the bends.

Another thing we did with exhausts to make them flow better is step up the diameter of the pipe over distance. So 2" pipe into the muffler and 2.5" out. This is to counter the resistance to flow as the gasses cool even with no muffle the principle still applies. I don't know how this would work in a RMH but something that is going through my mind.

You're not the first to suggest that the corrugated pipe could be an issue. I believe the smooth radius being a positive overcomes the corrugations as a negative, just as much as the sharp angles in normal stove-pipe elbows and tees being a negative then negate the smooth walls as a positive. If I could find a mandrel bent style 8" tubing, I'd be all over it! The best flowing option would probably be to have a flat-walled and flat roofed(kindof a square) with a swollen chamber at the elbow/turn, similar to dirt bike exhaust manifolds. Lastly, the speed of the flow has a lot to do with how much negative pressure a short section of corrugated tubing will put on the system. On a car, the exhaust flow is pretty huge. On this heater, the speed of the gases 1/2-way through the system is like a light breath exhale, so the affects of eddying and backpressure are close to zero. All that being said, at this point it is working very nicely, so I'll be leaving it that way for now.

Just had an idea; Maybe I will remove that 180' elbow and coat the entire inside of it with refractory cement, creating a smooth surface. The heat at that point is never that hot(not enough to cause massive expansion/contraction and subsequent cracking/breaking of the putty pasted into the grooves). The grooves would provide a lot of surface area for adhesion, and I could even coat the surface of the cement with a high-temp slick paint that would mimic the surface of the galvanize piping. This may be the next evolution in efficiency. :O)

I talked to one of the HVAC guys that I work with and asked him about this project (I have my own interest as well ) Anyhow he said that when they calculate their systems they account for the corrugated pipe to have an 100% increase in resistance in a bend over standard pipe 90s. Basically the corrugated pip has major efficiency losses and thats why they typically don't use it. He also said to check on the heat resistance of the pipe because that stuff is pretty weak compared to steel. I wouldn't be so worried about the heat rotting it away but having the aluminum sag once youve got the heater covered and working for a while could cause a major flow issue that requires complete gutting to repair. Bottom line its a nice bend but maybe not practical for other reasons.

It is stainless steel. HVAC systems have massive flows of air, so yes in that use it would have resistance.

Stainless is good. He didn't think the reduced flow rate would change the dynamics. Don't get me wrong you gotta do what you want in the end no matter the opinion.