First Rocket Heater Build - Newbie Questions

I'm about to embark on my first rocket heater build for my workshop at home, and have a couple of 'newbie' questions to ask, before i start! - Apologies in advance if they have been covered before

How much benefit is there to be gained from establishing a vortex in the riser pipe? ...and therefore is it worthwhile building the riser with a round pipe, rather than box section steel?

I'll be using an old gas bottle to construct it into - should the riser pipe be brought to within 2" of the top of the bottle dome? Would it be better to keep the dome, or replace with a flat lid?

I have limited options with regard to the chimney - the garage roof is asbestos, so it wont be possible to take the flue straight up from the back of the heater and through the roof. I could either take the pipe straight through the garage wall at floor level horizontally and make a 90 degree turn up the outside of the wall, or run the flue up the inside of the garage, and make an s bend through the wall just below roof height. - Are there pros and cons to either option, and which will exhaust the gasses better?

Thanks for any advice

"and therefore is it worthwhile building the riser with a round pipe, rather than box section steel?"

Here is your first big issue - you can't build a properly functioning and durable rocket mass heater using steel in the combustion core. If it works correctly, it will corrode and destroy the metal in a short time - one person reported that his 4" structural steel core was badly damaged within about 14 burns.

You need to use refractory ceramic material for the core - feed tube, burn tunnel and heat riser. This can be firebrick, castable refractory cement, or even old soft red brick. Modern hard brick or concrete brick will also fail in the conditions the core experiences.

There are many videos on youtube showing a rocket heater built of steel... but few of them posted again showing how their great idea failed miserably. One of the few is kntryhart; look up his videos for instructive visuals, especially his "Rocket Heater Autopsy".

Hi everyone,

I don't mean to hijack this post, but I'm new here, and the topic heading fits me exactly. I have a question about draft, and adapting a mass rocket heater to an existing chimney.

I have a wall, nine feet long, where we have an existing Ashley woodstove. Roughly half this wall is brick chimney, going up through the middle of the house. The other half is a concrete block wall which has been poured solid with cement, and then had red brick laid all across the front to make it match the chimney. The wall sits on a poured cement floor, covered with ceramic tile. The floor is poured on top of solid limestone, cut out of a large hill in middle Tennessee. So I've got a pretty good thermal mass going already.

What I'm thinking of doing is building a vertical mass storage, with a six inch pipe running serpentine throughout, and then bringing the pipe back to the existing chimney flue.

I'm attaching a drawing that will better explain what I'm trying to describe.

My question is, has anyone else tried something like this. My big question is, will it draw all the way through and out the chimney?

Suggestions and comments are invited and appreciated. Thanks.

This has been proposed and discussed before, though I don't know of a specific build in this configuration. There is no reason it wouldn't work, assuming the duct length, number of elbows, and chimney draft are in proper balance.
It would be just as effective if not more so, in your situation, to make a single masonry enclosure set against your existing mass, with entrance and exit at the bottom, to hold the hot gases while they cool before dropping to the bottom of the "bell", as this is called, and leaving for the chimney.

In my last post I meant to note that the pipe in my drawing measures out to roughly 27 feet long, even though it's only taking up a relatively small amount of space.

After the pipe finally winds its way to near the top of the ceiling, I'm asking the exhaust to travel straight down for three feet, before making it's way through the thimble and into the chimney. This seems to go against what I used to think of as common sense when it comes to woodstove exhausts.

Richard; You would do better by keeping your pipe inside the room and give it an "s" curve to exit. This will keep most of your exhaust stack warm and drawing well. Glenn was correct in telling you not to use any metal in the core unit, metal is only acceptable after the heat riser when the temperatures are cooling. Round , square and even octogonal risers has been built. Personally I prefer to use the round matt walker style perlite/fireclay hand made riser on top of a fire brick core. 2" is minimum top gap for an 8" system, 6" system is less. I have found that if you use a 55 gal barrel that the lid moves up and down with heat so I went with a 2.5" gap on my 8" to allow for movement. But they all work if insulated. Nothing wrong with a rounded top barrel, just make sure it is large enough (dia.) to give the falling gasses room to move. The reason people would prefer to use a 55 gal barrel is the removable lid, it is very nice to quickly pop off to check things like ash build up or the condition of your heat riser. If you don't have one yet you should get a copy of ianto evans rocket mass heaters version #3 readily available online as a pdf file instant download or as a hard copy book. This is the bible of rmh building and most of your questions are answered in it. Also available on line is the new book by the "wisners" , I have not seen a copy of this yet (only currently available as pdf ) but it will be as good if not better (updated not better) than the ianto evans book! Good Luck and happy rocketing !

Another Noob Question or two:

I'm currently building a RMH but I'm using an 11 gauge steel core.  That core is surrounded by refractory cement both on the "J" tube portion as well as the riser.  The inlet is 3" square steel welded to a 5" round inner core chimney.  I'm using two 25-gallon steel drums welded fast to the J tube at the bottom and to each other across the lower one's top and the upper one's bottom, with a removable top for inspection and cleaning.  I have built many rocket stoves out of steel and to date have seen no "corrosion" as it were due to the high heats.  If I can figure out how to post a picture here, I'll do exactly that.  I'd rather find out now than later that the steel core is not going to work, although my guess is, if it did waste away to nothing, the refractory, hollow surrounding 'stone' as it were, would still operate.  By the way, I'm using a stainless-steel wire basket, which will slide into a 2" feed tube and terminate in a cone, which will be lowered into the burn tube until its apex bottoms out and I will be filling said tube with hardwood pellets (oak).  I have already tried this design with a traditional "rocket stove" and it burns like a champ, allowing lots of combustion air around all sides of the conical termination.  I'm also installing a "secondary air" outlet within the edge of the round internal chimney as the vortex begins to form, to assure complete combustion of the wood pellets and their associated gasses.  Hope it all works!

Can you give some clarification on a few details of your build?

You say the burn tunnel and heat riser will be surrounded by refractory cement. How thick will this be, what kind of refractory, and what will be around it to contain it?

What are the overall lengths of feed tube, burn tunnel and heat riser? Where will you exit the barrel space, and what will follow? What sort of chimney are you planning?

You speak of rocket stoves, but these are distinctly different in operation from rocket mass heaters. A rocket stove will generally exit straight up, likely after heating a cooktop or pot, while a RMH must push the exhaust through a horizontal duct or similar. Rovket stoves have been shown to work as small as two or three inch diameter, while 4" RMH systems are notoriously tricky to get right, and smaller than that is relatively useless and unlikely to work in most conditions.

You have a 3" diameter feed connecting to a 4" square burn tunnel, connecting to a 5" diameter riser. The feed filled with pellets is obviously not going to allow enough airflow for good combustion, so I presume you have air entering the end of the burn tunnel... is this correct? Other experiments I have seen with pellet feeds have described the grate burning out quickly even when made with heavy stainless steel bars, so I think you are not getting the high temperatures a RMH is known for. 1500F is on the low side of typical, and at that temperature, steel will be glowing orange and softening. The fact that it has not for you indicates that you are radiating so much heat that the steel, and the gases next to it, are much cooler and likely not reaching full combustion. When the steel core is insulated well enough that the gases are burning completely, you will see the deterioration described elsewhere.

Well, golly.  This is my first build, so my supposition is, a blanket "Yes" to all your concerns.  I'm flyin' by the seat of my pants, but I've used oak pellets in rocket stoves and they work well, feed well, do not burn up anything and get things really, really hot.

I'll try to extrapolate the build, since I did nothing on it today (too cold in my barn, where my welder is).  I'm using two, approximately 15-gallon steel barrels.  The bottom one has the 3" square, eleven-gauge steel "J" tube in it and the open area where it is placed inside the lower barrel (shown in my pictures) will be filled up 8" with refractory cement (if I can ever find any -- Home Depot and Lowes has never heard of anything like that).  I am then going to find a cylinder approximately 7 or 8 inches in diameter to surround the "stack" and pour in more refractory cement there, which will be dowelled into the first pour unless I can do a monolithic pour (all at once).  

A second barrel will be welded to the top of the first at its bottom, allowing me to use the clamp-on ring device as a top, so I can get inside to inspect and clean things down the line.  Another steel plate will be welded as an interface between the two barrels, because their walls are TOO THIN to accept a good, penetrating weld, so I'm welding them at their bottom rolled seams, but as it is, one is smaller than the other (to allow stacking of drums I suppose) thus the plate interface.  Another plate will be welded on the outside top of the top cover to act as a heat sink and place to put a pot to boil to add humidity to the room when things are cooking.  The exhaust tube will be down the outside a ways (not yet determined how far down) and will be 6" diameter, run 9 feet through a to-be-constructed concrete and steel bench or couch, then out the wall and up a stack to terminate above my roof.

As for combustion air, yes, the feed tube is open to the room.  Also, there will be a secondary-air outlet bring in room air to within the vortex chamber, burn chamber, whatever it's called.  I've used a 3" "J" tube successfully with pellets and am sure it will both feed and burn to my liking.  I may need to shorten my exhaust run through the stone bench, if it doesn't work well in testing.  I plan on testing it with a horizontal outlet BEFORE I build the stone bench.

IF it doesn't work, I'll have one hell of a rocket-stove cooker.  I plan on testing each step of the way  as I build.