Questions on a basic design using ducting

I've been reading a bunch and watching videos (a couple of great ones here), I made a few brick rocket stoves with sheet metal flus and I'm finally ready to try out a design for a rocket stove mass heater for my greenhouse. The basic idea is to use really simple sheet metal pieces that I can get from a used building store near me for almost nothing. I'll build as much of a frame as I need to to support and contain everything, but here's the first version: (I guess see attachment

I use a 6" T for the feed and air supply. The 6" T connects to an 8" riser (no insulation). The 8" riser is surrounded by a 10" piece (it had a 8" reducer on the top for the cap) that works like the barrel most folks seem to use. The barrel piece is 32" tall. Then I've got 6" ducting coming out of the end of the 10" barrel. I have an elbow and a 6' riser that goes on the end that I used to test the system, but the longer term plan is to have about 24' of duct run through a box of sand to distribute the heat. The air in to the system will come from outside the greenhouse and the final vent off the last riser will go out the greenhouse. I'm planning on putting a 6" cannister fan near the top of the exit riser to help the system create draw when it's getting started.

So, 6" T into 8" riser surrounded by 10" barrel, all sheet metal duct pieces. Sound problematic? Reasonable? Good?

A couple of questions.

1. First, I heard I didn't need to insulate the riser in the barrel if I have a final riser that creates draw there. Is that true? Any problems with not having the internal riser insulated?
2. Any magic numbers for the ideal height of the internal riser and the height of the barrel?
3. I've got about an inch or so around my riser for the gases to swirl and go down to the exhaust ducting. I have the barrel a bit off centered, so there's more room on the side where the duct starts. Does that seem like enough room or should I use a piece of 12" duct?
4. I want this to be somewhat modifiable/movable for at least the first few versions, so I'm planning on using sand, bricks and cement pavers in the framing box to help hold up the pieces and catch and hold the heat from the fire. I was hearing something about sand being too good of an insulator and that it doesn't work well for storing heat? Does that mean all the heat will stay in the ducts and I'll end up with hot exhaust coming out the final flu instead of having collected most of the heat along the way?
5. If that's the case, any material better than sand?
6. For water heating, I'm thinking of using some high temperature rubber hose wrapped around the exhaust duct as it leaves the barrel. This is for an aquaponics system, so I can't use copper. I don't want/need the water to get too hot, so I could wrap the duct a few times with something to distribute the heat a bit first, or I could cover/surround the ducting with sand, maybe only an inch or two on top of the ducting and then coil the hose on top of the sand and cover it with more sand? The hose can handle temperatures up to 160 degrees, so I'm thinking I could try a few runs with the ducts packed in sand and check temperatures and see what I'm looking at. It might work fine to lay the hose on top of a few inches of sand or I might need to put six inches of sand under the hose otherwise it'll get too hot. But the idea makes sense to me, any problems anyone sees?
7. A buddy told me that the exhaust ducts need to be accessible for cleaning. Is that true? How do the folks who have these built into benches and covered with cob clean them? Is it just have a clean out and turn on a leaf blower and let the ash/dust go out the exhaust? Is there some other trick for cleaning?

Thanks so much for all the good information here, I'd greatly appreciate any thoughts or comments on this design. It seems like it's a pretty nice, easy way for folks to get started with easy to access pieces, I'd love to get some feedback on whether I need to go in a different direction or how to improve this design.

Thanks -- Rick

Metal is doomed, and it has no insulation, it's not a rocket!

Sorry to be harsh, but that's the case. No high temp in the burn tunel and heat riser, due to the lack of insulation. So the volatiles don't get burned, and therefor it's not a rocket stove.

Rick Frey :Welcome To Permies.com / Richsoil.com, and a Big Welcome to the Rocket And Wood Stoves Forum/Threads. With 28,000+ Fellow Members You can
come here 24 / 7 and find someone who wants to talk about what you want to talk about !

Your attachment did not come thru, hopefully as you make contact with individual members part of our group conscious comes thru, Each of us speaks for themselves

and as our craft grows everyday, no-one person is all knowing or speaks for all of us !

However, Metal is doomed ! If you need to have the technical details here is a link :

High Temp Hydrogen Attack

Please note High Temp Hydrogen Attack starts ~500ºF~ and a well made Rocket Mass Heater RMH, Has working

Temperatures above 2000ºF, These higher temperatures Give us the High Efficiencies and clean burn that are the

Core of the Reason for a RMH

A quick word about all the crap that can be found on U-Tube, Every Minute 100 Hours of Video Content is added to

the big Video Vault that is U-Tube, There IS NO Supervision or control over its own content, anyone can put up a

"Look at what I Did Video" !

Rick Frey wrote:I've been reading a bunch and watching videos (a couple of great ones here), I made a few brick rocket stoves with sheet metal flus and I'm finally ready to try out a design for a rocket stove mass heater for my greenhouse. The basic idea is to use really simple sheet metal pieces that I can get from a used building store near me for almost nothing. I'll build as much of a frame as I need to to support and contain everything, but here's the first version: (I guess see attachment

I use a 6" T for the feed and air supply. The 6" T connects to an 8" riser (no insulation). The 8" riser is surrounded by a 10" piece (it had a 8" reducer on the top for the cap) that works like the barrel most folks seem to use. The barrel piece is 32" tall. Then I've got 6" ducting coming out of the end of the 10" barrel. I have an elbow and a 6' riser that goes on the end that I used to test the system, but the longer term plan is to have about 24' of duct run through a box of sand to distribute the heat. The air in to the system will come from outside the greenhouse and the final vent off the last riser will go out the greenhouse. I'm planning on putting a 6" cannister fan near the top of the exit riser to help the system create draw when it's getting started.

So, 6" T into 8" riser surrounded by 10" barrel, all sheet metal duct pieces. Sound problematic? Reasonable? Good?

A couple of questions.

1. First, I heard I didn't need to insulate the riser in the barrel if I have a final riser that creates draw there. Is that true? Any problems with not having the internal riser insulated?

1a) If the Heat Riser is made of Firebrick, the firebrick Can be insulating and you can make a Type of lower performing RMH, Even with a Heat Riser Made With
Firebrick and 4''Thick, An additional wrap of insulation helps to create those freaky high temps that RMHs are Famous for

2. Any magic numbers for the ideal height of the internal riser and the height of the barrel?

2a)Unfortunately, there are competing Formulas and they all have to be understood independently, and in their entireties. I will direct you to help -directly

3. I've got about an inch or so around my riser for the gases to swirl and go down to the exhaust ducting. I have the barrel a bit off centered, so there's more room on the side where the duct starts. Does that seem like enough room or should I use a piece of 12" duct?

3a) Your System is too small for adequate gas flow, 12'' would better, but leaves you little room to make up the Transitional Area where the vertically falling
hot exhaust gases turn to flow horizontally through your Thermal mass ! Here your flow thru diameter should increase 300%

4. I want this to be somewhat modifiable/movable for at least the first few versions, so I'm planning on using sand, bricks and cement pavers in the framing box to help hold up the pieces and catch and hold the heat from the fire. I was hearing something about sand being too good of an insulator and that it doesn't work well for storing heat? Does that mean all the heat will stay in the ducts and I'll end up with hot exhaust coming out the final flu instead of having collected most of the heat along the way?

4a) The sand can be somewhat or a lot insulating, and may need to flow through 10% to 30% more sand and piping to flow into your Thermal Mass with
minor exceptions we normally say That with a 6'' system a bench made with cob needs a minimum length of 20' for sufficient heat energy flow to the
Thermal Mass !

5. If that's the case, any material better than sand?

5a) Cob Made out of Clay And Sand, with lots of heavy Dense rock in it !

6. For water heating, I'm thinking of using some high temperature rubber hose wrapped around the exhaust duct as it leaves the barrel. This is for an aquaponics system, so I can't use copper. I don't want/need the water to get too hot, so I could wrap the duct a few times with something to distribute the heat a bit first, or I could cover/surround the ducting with sand, maybe only an inch or two on top of the ducting and then coil the hose on top of the sand and cover it with more sand? The hose can handle temperatures up to 160 degrees, so I'm thinking I could try a few runs with the ducts packed in sand and check temperatures and see what I'm looking at. It might work fine to lay the hose on top of a few inches of sand or I might need to put six inches of sand under the hose otherwise it'll get too hot. But the idea makes sense to me, any problems anyone sees?

6a) Rubber is two insulating, Your 160ºƒ temperature is for air temperatures, and long periods of sporadic use, not buried in a higher temperature medium, and will fail
in the high temperatures it will be exposed too! You need Large Diameter 1.5 inch Stainless steel*, and your system has to be made so that it is thermo-
syphoning and unpressurized! The presence of valves that can be closed or pumps that can fail brings you perilously close to the" Boom - Squish'' Scenario, where
you are the Squish ! Think Boston Marathon Bombing with more Deaths, and more Full thickness, full body burns !

7. A buddy told me that the exhaust ducts need to be accessible for cleaning. Is that true? How do the folks who have these built into benches and covered with cob clean them? Is it just have a clean out and turn on a leaf blower and let the ash/dust go out the exhaust? Is there some other trick for cleaning?

7a) Hot off of the presses stuff, Common practice has been made to place stove pipe 'T's in the system with the unused leg closed off with a cap and positioned
horizontally towards the outside edge of the Thermal Mass Bench ! Attempting to place the Clean out leg facing vertically so that the clean out cap is on the top of the
Thermal Mass Bench creates a hot spot and appears to disturb the gas flow of the hottest gases at this point. Taking a clue from this more people are pointing their 'T's
downhill 45% off of the Horizontal where practical, which usually means well hidden !

Thanks so much for all the good information here, I'd greatly appreciate any thoughts or comments on this design. It seems like it's a pretty nice, easy way for folks to get started with easy to access pieces, I'd love to get some feedback on whether I need to go in a different direction or how to improve this design.
Thanks -- Rick

But wait ! Theres More !

This is where I strongly recommend you Go To Rocketstoves.com to download a PDF Copy of the Brand New 3rd Edition of Rocket Mass Heaters, This is-
"The Book'', With over 100,000 RMHs built to date they are all children of ''The Book'' and 95% of all the first time Builds (That worked) were made following ''The Book''

Buy this book, read the book and come back here to your fellow members for help as needed. knowing that you will be using the same words to describe the Sizes,
Shapes and Orientation of the parts to themselves and the Whole

"The Book'' will also save you time, money, materials, and Frustration as You Proceed further through Your Build.

For the Good of the Craft ! Big AL

*1.5' stainless is a good guesstimate without seeing your system

I tried a piece of stainless sheet metal used as a 'burn tunnel liner' ~~~mine got really hot, quick. By the way, I also tried clay slip coated bathroom tile instead of fire brick in the burn tunnel, in the same experiment (because I thought the floor & walls wouldn't get as hot as the ceiling of the burn tunnel)...total crack up, in less than an hour!

I hope Rick will share some of his 'after burn photos'...curious minds want to see how ya did! I had total fun with my experiment even if it was a failure.

james beam

Hi all,

Just ordered "the book". While I'm waiting can somebody answer a question? I've read several posts saying "metal is dead". What do you use for the barrel and piping if not metal? I'm researching a rmh for a new greenhouse build.

Metal is doomed to failure when used contact with the high temperature flame path of the J-tube (feed, burn tunnel, and heat riser interior). The exterior of the heat riser may be metal, and obviously everything further downstream too, including, obviously, the common 55 gallon barrel placed over the heat riser.

robert Hardin : If we use a little license and say that the very top of the Heat Riser just below the Barrel is the exit from the Combustion chamber, and compare it
more or less directly to the Fossil Fuel Fired Forced-air Furnaces Combustion chamber Then the 55 gallon Barrel compares even more closely to the furnaces Heat
Exchanger, They are both made out of ductile steel, with the barrel gage or wall thickness running a little stouter !

A F.F.F.F-a Furnaces Heat Exchanger is generally rated at 20 -25 years, Even then the problem that causes the Furnaces failure can always be traced back to un-
repaired damage in the combustion chamber .

So, The barrel while made of similar ductile steel is heavier and the temperatures exposed to are within the original safety spec of barrels made for interstate
commerce!

With over 100,000 Rocket Mass Heaters RMHs made world wide, I have not (yet) heard of a barrel failure! For the Good of the Craft ! Big AL

So, you're saying the feed, burn tunnel, and heat riser interior are the only things that CANNOT be metal? The barrel and connected piping is OK? What would be a good material for the feed, burn tunnel, and heat riser interior?

bob

Hi Robert, there are lots of internet pages to read up on RMH, while your waiting on your book. Try these out for starters, many, many pages of RMH related questions & responses.

https://permies.com/forums/f-125/rocket-stoves

http://www.richsoil.com/rocket-stove-mass-heater.jsp

http://www.zaugstoves.com/wordpress/

http://www.ernieanderica.info/rocketstoves

http://donkey32.proboards.com/

james beam

Robert H. : Here is a Forum Thread I started , as much to get things straightened out in my own head, as it started a little kerfluffle, And My answer was not as
Cogent as the one penned by Erica Wisner, Dec 20th 2013 I recommend you find that thread extension and read on from there

Link Below :

https://permies.com/t/30551/rocket-stoves/Fake-fire-brick

THis should be close at hand as you go through the book !

For the Good of the Craft ! Big AL

Sorry I missed a quicker response to the first few replies, thanks a ton Allen and everyone for some great thoughts/questions. Ok, a few followup thoughts/questions.

For the metal is doomed folks, you might be talking in terms of low-price, spare materials projects, but I got started with this idea from having been looking at wood gasification and the folks at All Power Labs (All Power Labs ). They make some beautiful wood gasifiers that operate at temps up to 2000 degrees F and they're all built out of stainless steel. Wood gasifiers have been used for running cars for years, all made out of metal, so, again, metal isn't dead, maybe just more expensive than folks want to pay for or than is easy to find from spare materials.

So looking back and doing a bit more reading, I think I did get suckered by some of the easy youtube videos and wanting to use easy to track down pieces. So a couple of key questions.

1. What is the typical/good temperature of the exhaust gas as it leaves the barrel and enters the exhaust ducting? Is typical 30 gauge stainless steel ducting you can buy at Home Depot or wherever ok for dealing with the hot exhaust or do you need something heavier gauge?

2. In term of the questions about perfect ratios of burn tunnel lengths, and chimney heights, it seems that many of these can be solved by using a low watt, low cfm, variable speed fan at the end of the ducting to pull air through the system. I get that might sound like sacrilege, but it solves so many problems, I don't get why it's not part of a standard design. Is there some problem with using a fan to create the draw and direct the final exhaust, assuming you've got enough duct so the air coming out doesn't melt your fan If you burn for 4 hours with a 25 watt fan, you've added the cost of 1/10 Kwh to your system but it seems like you've made life a whole lot easier.

3. So if one did build a riser out of old brick, a basic 8" square, and let's say you wrap it with a layer of 1" rockwool or superwool or the equivalent, and then cover it with a barrel, assuming a well built fire pit and burn tunnel, is that it?

Again, it seems like the fan solves a bunch of problems about perfect sizing of the fire pit and/or burn tunnel. As long as you have enough space to burn your wood and a large enough diameter burn tunnel to not choke your fire, if the draw is happening mechanically somewhere else, the real issue is a high enough riser to allow for sufficient time/heat to fully burn the wood gases and a material that isn't going to explode/melt at the temps that creates? Right?

Here's a pic of my system, getting closer to ready, but maybe ready for a redo of the riser material Don't worry, I won't be laying the pieces directly on wood. I was planning on using sand, but I might switch to gravel. From what I'm reading, it's much better at letting heat out and soaking up the heat. Maybe sand on the bottom and then gravel on top of that?

Rick, gasifiers arent insulated. And, they can't output 2200F° for long period of time, which the rocket can.

But if you know better, please do it!

Satamax Antone wrote:Rick, gasifiers arent insulated. And, they can't output 2200F° for long period of time, which the rocket can.

But if you know better, please do it!

Current gasifier designs are insulated, that's part of what makes a good gasifier, that every bit of heat from combustion gets used intentionally and nothing is wasted. It's actually more that every bit of heat is concentrated and focused, to achieve maximum temperatures for the cleanest burn possible. For a rmh, a tiny bit of dirty burn just means some smoke, but for a gasifier, dirty burns can screw up the generator, so the cleanliness of the burn (which mostly means burn temperature) is much more critical for wood gasification.

And gasifiers can run 24/7 for extended periods of time, so not sure what you're referring to. Maybe old time gasifiers had more problems, but current designs run way more frequently (i.e. for longer periods of time) than any rmh.

I was just at All Power Labs today, looking through their scrap heap, to see if I could find a nice piece of tubing they had screwed up on. I got a piece of 14 gauge 6" steel tube that I'm thinking to use as my riser, but I'm gonna get rid of my cheap duct pieces and either get some bricks for the fire box and tunnel or see if I can cut and weld the steel tubing into a design that could work.

Hey Rick, your kina talking about 2 different animals, a gasifier, or a RMH, and I can't see so far where 'the gasifier' part comes into use on your unit.

Usually a gasifier begins with a rather large barrel-like 'hopper' has a removable lid on top to load the thing with dry wood chunks. Inside the bottom half of a gasifier hopper is where the 'char zone' is located & there are various types of manifolds built within this zone that include 'syngas collector nozzles' and an ash collection area, I haven't seen anything like this on your RMH so far. And there are many variations out there.

It is true the gasifier creates a large amount of heat that needs to be radiated away from the syngas itself (which would yield you a good amount of radiant heat for your greenhouse). And could be operational for extended run times, based somewhat on your hopper capacity. And often the syngas is somewhat dirty directly exiting a gasifier hopper (thru the gas collector nozzles) and often the gas is cleaned (filtered) thru filter media such as wet hay or some other wet fiberous substance. Dry cyclone filters radiate a lot of heat and are often used. And a gasifier sometimes plumbs the gas thru a 'bubbler, water bath' type filter...all of this filtering, & plumbing radiates heat from the syngas, while cleaning it. Delivering a relatively cool/clean syngas to an engine or some type of burner.

An RMH doesn't do any of that, the major difference being that the RMH feed tunnel causes a good amount of air to be drawn in & support the efficient combustion of the wood fuel, gasifiers operate on a different principal of collecting syngas from a rather 'choked down' burning of charcoal in an atmosphere that uses much less intake air in the char-zone. Or at least that is the way I understand it. You might try checking out this site for more info. on gasifiers http://driveonwood.com/sites/default/files/styles/verylarge/public/wood%20heat3.png?itok=JbmTjkvh

james beam

It may be true that a small fan at the exhaust could compensate for poor RMH design, but that would make you totally dependent on continuous electric power. If the power goes out in a storm while you are burning, you have a lot of burning material that is going to go somewhere. Just damping it off is not safe either, as you then have combustible gases building up without oxygen, and if they get opened up before cooling off, you have a fireball in the making.

This is aside from the permie esthetic of avoiding conventional grid technology...

Rick Frey : Glen is right, this has actually happened with inexperienced owner operators !

As soon as the combustion core is up to temperature with a well loaded feed tube panicking family members dealing with badly smoking Rocket Mass Heater RMH,

have attempted to cover the top of their Feed Tube to isolate the RMH from the house !

They now have created the an-aerobic conditions found within a standard Gassifier without any outlet for the highly flammable hot combustion products!

Due to the amount of heat now stored within the firebricks surrounding your combustion chamber you are now committed to NOT Raising the improvised lid off of

The Feed Tube for literally hours until your Highly insulatedt burner has cooled below the ignition temperature of say paper (451F) !

Even though your initial plan for your system was for economy and a backup heat source for your Greenhouse, you can not operate the RMH, or get ready to relight

it until after it is again stone cold.

In Short, raising that cover prematurely will definitely cause someone to lose hair, and receive possibly severe burns ! You may remember a Famous Movie with the

Title ''Backdraft'', that dealt with these potentially deadly effects !

Think like fire! flow like a gas! Don't be the Marshmallow! For the Good of the Crafts ! Big AL

I'm not trying to build a gasifier, sorry for any confusion. There were just a few too many responses saying one couldn't use metal in a rmh design because it can't stand the conditions. I referred to a gasifier as a machine that operates under similar conditions that is made out of metal. Having now finished the Ianto book, they mention metal risers frequently, so I'm not sure what the concern was about a metal riser or about metal for any part of the system. I totally get that the cheap duct pieces I was thinking about using are insufficient for the burn tunnel and riser, but good quality, heavy gauge steel tubing gets used on all sorts of projects at extreme heats and has no problems--other than cost. If you can get bricks cheaply, that's great. I have better access to old tubing than I do to good bricks

Alan and Glen, I get the concern about the power going off and that shutting down the fan and messing up a system that depends on the fan for draw. My best sense is that I would still include a final exit flu, but just put the fan at the top of that. That way I can use the fan to control the beginning and end of burns, increase the flow rate if I want the fire to burn hotter, but turn off the fan once the system is hot and burning nicely. Otherwise, I get that power going out could be a big problem. When it's time for the fire to go out, that seemed to me to be a dirty time the few times I ran mine. The temps cool down but there are still some embers in the chamber that aren't easy to remove and that seemed to get smoky at the end. When I added my fan into the system, it let those last pieces burn up, exhaust outside the space then I turn off the fan.

And I'm not at all suggesting make a crappy rmh and just try to fix it all with a fan. The fan does seem to offer a few advantages, however, a smokeless, easy light, a smokeless, clean finish, and the ability to increase the heat of the burn by increasing the flow rate of oxygen into the fire. Those all seem like nice advantages and other than the work/cost of adding the fan somewhere near the final flu, I don't see the downside to it, other than the chance power goes off. I'm in the city, (Oakland, CA) and we haven't had the power go off once in 12 years, so I'm not too worried about it, but that makes a lot of sense if you live in an area where power outages happen with some regularity.

Rick, build the thing, and burn it for a while. Then report back when you have 200 or 300 hours burning time on the thing. I could be totaly wrong. But if your feed tube, burn tunnel and heat riser made out of insulated metal whistand the abuse. I'm keen to learn.

Rick Frey wrote:I'm not trying to build a gasifier, sorry for any confusion. There were just a few too many responses saying one couldn't use metal in a rmh design because it can't stand the conditions. I referred to a gasifier as a machine that operates under similar conditions that is made out of metal. Having now finished the Ianto book, they mention metal risers frequently, so I'm not sure what the concern was about a metal riser or about metal for any part of the system. I totally get that the cheap duct pieces I was thinking about using are insufficient for the burn tunnel and riser, but good quality, heavy gauge steel tubing gets used on all sorts of projects at extreme heats and has no problems--other than cost. If you can get bricks cheaply, that's great. I have better access to old tubing than I do to good bricks

Alan and Glen, I get the concern about the power going off and that shutting down the fan and messing up a system that depends on the fan for draw. My best sense is that I would still include a final exit flu, but just put the fan at the top of that. That way I can use the fan to control the beginning and end of burns, increase the flow rate if I want the fire to burn hotter, but turn off the fan once the system is hot and burning nicely. Otherwise, I get that power going out could be a big problem. When it's time for the fire to go out, that seemed to me to be a dirty time the few times I ran mine. The temps cool down but there are still some embers in the chamber that aren't easy to remove and that seemed to get smoky at the end.

Rick, we get that you are set on experimenting with iron and steel in your future builds. I for one keep coming back to counter your comments because others
following this thread extension in the future might be tempted to think you had proved your point ! Enjoy your research with materials that have failed again and
again and again, we will expect your future reporting to be just as straight forward and factual !

I am a little confused with your report of your system cooling down and running dirty, Any properly built J-bend Rocket mass heater should maintain high enough
temperatures to assure the complete burning of all fuels-especially charcoal ! Paul W.s brother put his hand inside the combustion chamber the following morning
after one of his RMHs first runs and came close to burning himself hour latter !

Perhaps your system has leaks and needs tightening up ! Regardless, this is exactly the kind of indicator of a problem that just should not be !

When I added my fan into the system, it let those last pieces burn up, exhaust outside the space then I turn off the fan.

And I'm not at all suggesting make a crappy rmh and just try to fix it all with a fan. The fan does seem to offer a few advantages, however, a smokeless, easy light, a smokeless, clean finish, and the ability to increase the heat of the burn by increasing the flow rate of oxygen into the fire. Those all seem like nice advantages and other than the work/cost of adding the fan somewhere near the final flu, I don't see the downside to it, other than the chance power goes off. I'm in the city, (Oakland, CA) and we haven't had the power go off once in 12 years, so I'm not too worried about it, but that makes a lot of sense if you live in an area where power outages happen with some regularity.

For the good of the Craft ! Big AL

james beam wrote:You might try checking out this site for more info. on gasifiers http://driveonwood.com/sites/default/files/styles/verylarge/public/wood%20heat3.png?itok=JbmTjkvh

james beam

OOps, I previously linked to a picture instead of the site http://driveonwood.com/forum/756

james beam