Rocket mass stove engineering

Hello everyone,

I'm new here.  I tried to answer my question reading through the forums so forgive me if I missed it.  I have also ordered the book "Rocket Mass Heater"...

All that I have read and watched on the videos (cool videos by the way) suggests that the heat is being captured in the heat-exchanger where it is conducted to the inside of the room.  What remains appears to be absorbed into the mass through the bench as the residual heat and smoke exits through the chimney.  Ok, that all makes sense... but the little things are escaping me.

Ok... here is are the questions:

1.  Why does the burn chamber have to have a vertical feed?  Has anyone tried a horizontal feed with a typical stove door?

2.  Has anyone tried a fresh air supply running parallel with the chimney but terminating just outside the house?

3.  How do you know that the gasses are being burned in the riser?  With this happening how long does the heat exchanger last before the barrel needs to be replaced?

4. How tall does the inner stack need to be?  How much space does the heat exchanger need between it and the pipe? 

5. How much rise should there be above the burn chamber and the heat exchanger? 

6. How much slope should there be on the stove pipe through the bench?

7. How big should the heat exchanger barrel be? 

8. Am I asking too many questions?

In all appearances this is the same idea and principal as a kacheloffen but on a much more affordable scale.  I am definitely going to build one and put it to use.  We already heat with a wood stove and the consumption is just embarrassing.  I’ll build this in the basement and let all that heat rise up into the rest of the house.  Has anyone tried incorporating a recycled gas water heater into the chimney path?

Crazy ideas!


Essay0ns from Missouri

I'm going to answer your last question, since others far more experienced can answer the others.

You've had a number of people here trying to incorporate water heating into the RMH system.  None have been very successful.  The problem is that there is a limit to the amount of heat energy you can extract from a RMH before that energy extraction impacts the efficiency and effectiveness of the system: take out too much heat, and the exhaust gas can't make its way to the chimney and out of the house.

Unless someone corrects me here, I'm going to say that if you want to use this technology you should build a dedicated system that acts as a pre-heater for your existing water heater.  Instead of making a bench, embed the salvaged water tank into a mass of cob.  By the time you've brought 40 gallons (80 gallons?) up to temp, the fire should have died down, and the cob mass will keep it at temp for hours.

Question for the rest of us: In building this system, would it make it more efficient to cover the cob with insulation so all the heat goes to the water tank and very little goes to the house?

And in regards to #8:

YES!  Go watch some TV or something and let me read my paper...

or...

Hey!  Look at that bird outside!  Is that an eagle or a hawk? 

essay0ns wrote:
1.  Why does the burn chamber have to have a vertical feed?  Has anyone tried a horizontal feed with a typical stove door?

It doesn't, but you want to anyway. Rocket cooking stoves are often loaded horizontally, but they are generally run for a shorter time and attended to all the time. A vertical feed allows semi auto feeding and keeps the ash in one place. Those are the ones I know about... there may be more...

3.  How do you know that the gasses are being burned in the riser?  With this happening how long does the heat exchanger last before the barrel needs to be replaced?

it does without the barrel... yes it can be run that way in the backyard... there are a lot of videos pointed to from forums here to watch this. Follow the video at the top of this page to utube and on that page you should find some of them. While I have no personal experience, others on these forums have had the barrel last 20 or 30 years at least (meaning it still hasn't failed).

4. How tall does the inner stack need to be?  How much space does the heat exchanger need between it and the pipe? 

5. How much rise should there be above the burn chamber and the heat exchanger? 

6. How much slope should there be on the stove pipe through the bench?

7. How big should the heat exchanger barrel be? 

4A) There is a formula to determine min riser height... should be in the book and other places on these forums.

4B, 5 and 7) In general the gas path should be constant CSA 9Cross Section Area) from one end to the other. Get out your calculator as each unit tends to be a little different. The minimum that seems to work is 28sqin. or 6 in diam. The riser inside should be the smallest CSA in the whole unit.

6) Some. The book will be more specific

It is generally best to do a mockup outside in the back yard (minus the mass) and fire it to full temp to make sure what you are about to do works. Start with the feed, burn tunnel and riser... make sure that burns right. Add the barrel and a short flue. try again. then with a long pipe in the configuration you intend to use inside. Make sure it draws right to working temp as this tests the riser insulation. Move it inside... try to test each step there too. (you have to have a flue setup first even if just out the window, but it should be close to what you wish to end up with). Make sure you have a CO alarm and smoke detector and they work BEFORE you ever light it off inside. RMHs work different that an iron box (air tight or otherwise) They are fired different too. If you have old fire burning habits you will have to relearn.

Its all good. I'm on the learning end myself... I've put some bricks together (dry) to the top of the riser... gotta cut the barrel next... when it warms up and stays light longer

I built boilers and heat exchangers for years.  you dont have to spell to weild.
Im thinking if i build the box out of steel and line the out side with fire brick ,i could case it in.  then put the feed ramp on a hinge ,horazonal or vertical feed ?
and a larg flat plate for the barrel , just havent thought of just how to run the exaust yet.
? if a barrel will last 20 or 30 years, the exaust pip is the weak spot.
  the army used a drip type gas stove , ideas

Well Bull you're right where i am right now. Don't forget to use an insulated heat riser to get the rocket effect, cause the 'gas pump action' to work, and burn the wood gases hotter.

You don't need a hinge - you can make it a horizontal feed cube and make  a steel plate that slides in vertically into the horizontal feed tray.   You remove the steel plate on start up and use as a horizontal feed. After the burn chamber and heat riser heat up, you can put the steel plate in and use as a vertical feed. The vertical steel plate could be moved closer to the insulated riser to shorten the area of the feed cube as needed.

I want a large flat plate to use for cooking and heating dish water and wash water on as well as for radiating heat into the room.

I'm still working on the rest of the exhaust bench as i don't know if i can turn it down after the flat steel plate or not. I'm going to try as i want the exhaust to move downward all the way out.

I just closed my cafe too, so i have extra flat grill and deep fryers ? just thinks to think about. if it can be built and cased then ,moble too, corn rosters theres just to many thing we can do with this. the feed shoot ? bigger longer , then melt lead, or use at the forge ?
boil water for working chickens. wow my brain may explode.
Im rebuilding a huge smoker right now so ill start with picking up the metal to do a build then kick it off. i have axcess to scrape stainless steel to case it and fire brick and refrack cement.  with a slanted exaust pip and a SS barrel then i could put a hinged lide on it so i could melt lead or pre heat steel for the forge ? ceramic wool insolation is what they use on boiler to hold the heat. lots of it around.    For a cabin ,i could put the box on wheels and cased in we could run single walled pipe over and out a window , with one of those little none electric wood stove fans i think you could heat 3000 sq feet ?

Sounds like you have a lot of material to work with.

3000 square ft is a lot of room... I don't need near that amount of space to heat. I guess i like the idea of a small central place that is easy to heat and cool and then surround that small life pod area with cool storage rooms to add space for things that don't need to be heated or cooled.

I advise everyone to not vary from the standard rocket mass heater that is in the book. Only those who don't mind a few failures and don't mind re building should experiment and even then - caution is a priority. Don't build an experimental stove in the home first - build in a shed or out house or somewhere that won't be missed if you accidentally cause a nuclear explosion when you recycle a Iranian brick or something.

its the failures that make it fun ,so long as you try .
i want to work out the bugs before i put anything in doors.and there may be a point where you cant build a RMH over a certion size? but these could be used to burn trash ,and heat boilers too.or burn  Pallits ,bails of trash ,or round bails of straw ?

3000 is just a number , and im a trader, and enjoy making some thing out of something else. it just looks like junk. reuse? im excited to try a biuld . It seem just limitless in posabilitys.

Sure hope you post some pics of those failures as well as successes. I would love to see a large Rocket Stove running a steam engine (SAFELY). Or even a large rocket heating a 3000 sq ft house.

ronie wrote:
Sure hope you post some pics of those failures as well as successes. I would love to see a large Rocket Stove running a steam engine (SAFELY). Or even a large rocket heating a 3000 sq ft house.

If I was of an age to be going to college, I'd get an engineering degree, specializing in steam.  Then I'd like to design a steam locomotive using a rocket stove instead of the typical polluting boiler we've all seen.

I know a kid who inheritted a 1/4 skail case steam engin , scared to run it .
so i fixed it up to run on air from a compresser , just once around the yard and time to refill.
But im with you on this a steam engin or trip hammer, generator? Limitless!!!
Or the new tig electrode , I was playing with hydrenge but 1 little explosion in the house and im not allowed to play inside any more.

Hi Everyone,

Ok... so I got the book, I have collected most of the materials and I have a set of plans.  I have even figured out most of the math.

So the only thing that I have not figured out is how to size and build the exhaust ducting from the drum to the bench.  I read that - going in - you should maintain the same size (no. square inches) throughout the system.  I am going with 8".  It appears to me that to get the same volume moving out of the barrel you will need to extend the collection ducting around the bottom sides of the drum... am I over thinking this?

essay0ns 

Hello everyone,  I hope you all had a good summer. 

Fall is here in Missouri and it's time to get back to work on the rocket stove.  I have all the math done (I hope) and have even made some 3d models on the computer.  I'm still having some trouble with the height of the riser but it seems that it's not too big an issue as long as it's a couple of inches below the drumhead.

I'll be restacking brick and testing again starting this week. 

Has anyone made any progress on similar projects over the summer?

Essay0ns

essay0ns wrote:
Has anyone made any progress on similar projects over the summer?

I have cut the barrel... Using a hot water tank... put my feed and exhaust holes in the bottom and started cutting the pipe to fit. I hope to get to fire ready soon... then add mass to barrel, then think about adding bench or other radiator... all this outside. I had been playing with a brick tunnel/riser, but have decided to go metal. The original RMH that I saw pictures of had a narrow gap between the burn tunnel and the barrel to force the hot gasses closer to it so it got hotter, but I notice the later ones have a wider gap. My thinking goes something like... Constant CSA would make the gap about a half inch... but the reasoning behind making 6inch pipe the smallest is surface friction... so 28sqin is the minimum only so long as the flue path is quite square... I think there is also a minimum width. In other words, if the flue path has a rectangular cross section, the smaller width must be bigger than a minimum length. I don't know how small it can be though. My barrel is 18india. on a 6in system... this leaves 5inches gap... but I need insulation, so I only have 3 or 4 inches... probably ok.

Having said all that... because the gap is wider than 1/2in. and the gasses can move freely, the ability of the barrel to transfer heat from the gas is diminished. This could be good or bad. If I am trying to make a small unit and keep my mass close to the barrel, it is not so good because much of the heat is coming out the bottom of the barrel... If I have a lot of bench to warm up then it is good, cause i still have heat left.

I want to try different head gaps too, The normal head gap for a 6inch system is about 1.5 inches for constant CSA. I want to try more just to see what happens. I think the top of the barrel would be hotter and I could therefore put more of my mass there to soak it up... actually I was thinking of an oven too.

I want to try the radiator/bench at different heights too.. Maybe 10 feet higher?

I am using brick for mass. I have lots of 3 hole clay brick which I will stand soldier around the barrel and put a metal strap to hold them against the barrel. That way it will be easy to take apart to change or clean.

Lots of ideas... now to try them out.

essay0ns wrote:
Hello everyone,  I hope you all had a good summer. 

Fall is here in Missouri and it's time to get back to work on the rocket stove.  I have all the math done (I hope) and have even made some 3d models on the computer.  I'm still having some trouble with the height of the riser but it seems that it's not too big an issue as long as it's a couple of inches below the drumhead.

I'll be restacking brick and testing again starting this week. 

Has anyone made any progress on similar projects over the summer?

Essay0ns

Erica's formula for height of riser is;

H > a+b     Where;

H = height of riser

a = depth of load chute

b = length of burn tunnel

I think it's best to not put a lot of cob around the base of the barrel until after you have run the rocket and are happy with it. Even Paul had to change the gap he had, between the riser and the barrel at the top, on his portable RMH. I think that would be found near the beginning of the thread by Paul called portable rocket mass heater.

Here's Erica's post:

Ernie Wisner
Posts: 258

Rocker Researcher and Grumpy Old Sailor


View Profile WWW Email Personal Message (Offline)
January 27, 2011, 03:43:05 PM Reply with quote
This is Erica on Ernie's login ....

I'm late for work on our actual rocket mass heater project, but I just wanted to drop the idea that for portable systems where you have plenty of money, brick might be a good thermal mass.  You can line certain sections of the pipe with brick and a small amount of mortar, or cast little corner wedges from refractory / concrete.

Welcome, Divonbriesen, and good luck with your projects.  i think you'll find most of the proportions you need in the book.  Height is mostly determined by H>L+h, where H is the heat riser, L is the length of the horizontal portion of the burn tunnel, and h is the height of your feed tube.  In practice, most heat risers are 3 feet or more. You can make a large part of this from brick or masonry, but for a portable system a whole barrel is more convenient.

Yours,
Erica
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Anyone ever consider a venturi directly where the burn chamber is.
Also thinking of introducing adjustable fresh air inlet
directly after the burn chamber in order to get as complete burn as possible.

I have not built one - just thinking about the design.

Jen0454 wrote:
Anyone ever consider a venturi directly where the burn chamber is.
Also thinking of introducing adjustable fresh air inlet
directly after the burn chamber in order to get as complete burn as possible.

I have not built one - just thinking about the design.

Can't remember who said, but the venturi effect doesn't work at the speed the gases travel in a rocket.