The need to insulate the riser is driven by what?

Asked another way, with the 'rocket' roaring the velocity and temperature' of the flame would appear to not be affected by the riser inside surface temperature? Given it was up to temp doing what it should be designed to do, heat a mass for providing warmth over an extended period of time. I'll admit I haven't built a permanent Rocket Stove nor have I stayed in a Holiday Inn. Rockets were built of various sizes to become familiar with critical dimensions and clearances are being used by friends to heat greenhouse planting beds, they all work.

keeps the hot side hot.  
It makes sure the heat stays in the riser to encourage a complete and most efficient burn.

perhaps not exact, but pretty darn close I bet...

Hi Joe;
Good question!  It is all about efficiency, and how quickly you achieve that efficiency.
By having a super insulated riser all the heat you have created in the feed tube and burn tunnel now can blast up the riser at full burn.
No energy used trying to heat and maintain that heat in your riser, its all headed to be stored in your mass.

You might think it doesn't matter if the riser gets hot, as it is all heat in your room.
To a certain extent that is true, however you need to start and keep a steady draw thru the entire system.
A rocket that decides to flow backwards is a smoky thing to experience!
You also want the highest temperatures to get a complete burn, otherwise you will end up filling your system with ash.  

As youtube  demonstrates,  you can build any old thing and make it burn.
Will it last?  Will it be highly efficient?  Probably not...

Many hours were spent by the innovators with test equipment, fine tuning how to get the most from a rocket.

We encourage experimentation or safely playing with fire.
You can learn a lot in your backyard.
In the long run following the already supplied directions will get you what your wanting.

Trevor and Thomas, thank you for your responses,

Still uncertain about the need to insulate the riser especially if constructed of heavy fire bricks laid flat where flame/heat efficiently is a result of fuel - air mixture. Once the surface of the riser fire brick are at temp heat transfer through the width of the brick is constant but minimal/slow and not affecting the reflection of heat into the flame/combustion path/mixture.

When I do my RMH build in a year or so it will be of all new material designed for high fluctuating temperatures. It will be configured to actually heat the floor the heat sink, with little ambient heating to accommodate waking up to a comfortable sleeping shack. Not interested in anything metal in the feed tube, burn tunnel or riser other than the barrel for longevity. We will also incorporate means that I feel safe having a 10 year old grandson firing the rocket up when dead cold absent the common by-pass.

Another assertion that of the ‘push–pull’ theory doesn’t make sense that Thomas indirectly dispels himself by stating “... however you need to start and keep a steady draw thru the entire system.”, thus the bypass blastgate for sale on your site. For me the secret to system efficiently is at the feed tube by fuel size and rate of feed to assure a complete burn up the riser.      

There's a point at which it becomes impossible to continue without experimentation. I encourage that you experiment, and record your data, so that you can corroborate what others here have already done and are telling you.

It's just good science. Please keep us posted on your progress, and good luck.

-CK

I'll throw my 2 cents in and side with the argument that insulating the riser concentrates the heat for the last few fractions of seconds it takes to get the most efficient combustion.  Though I'd like to see what you would do differently.  I'm presently working on a variation of a "Riser" which traps the combustion gases in the ceiling of a vertical cylinder and forces them to spin their way down, up, and out again.

An important function of riser insulation in a traditional RMH with steel barrel around the riser is to keep the exterior of the riser from getting very hot and making the barrel space hot. That would tend to reduce the temperature differential from riser interior to barrel, and reduce the "push" to encourage gases to flow down and into the mass. People have found that a system with an uninsulated riser can stall out after a while as the riser heats through.

PULL or PUSH... one or the other – can’t have both: ‘draw/draft’ is where I’m at. I imagine “stall”ing could be an issue: just where it occurs would be dependent on the Delta-T at any given point from fuel/air input to circuit exit for any number of reasons. Being impatient resulting in overfiring with a third to half covered barrel base would be a viable cause.  

This whole RMH thingy is so subjective absent standard testing protocols that which makes it so interesting, a play toy of sorts.  

Heat or lack of heat (refrigeration) has a time factor when efficiently archiving target temperatures. Rushing either process can be done with specific arraignments of associated equipment and process accessories/controls.

My latest build was a backyard wood fired Napoli / Pompeii hybrid Pizza Oven for the wife in exchange for drilling a water well on our 36 acres up north. It was super insulated with ceramic board and blanket products where a six hour firing to a 750F floor temperature maintains a 450F equalized temperature at 24 hours and 250F at 48 hours. Cooking all kinds of stuff during the (temp) ‘drop’ is as much fun as the 90 second pizzas, triple onions and jalapenos on mine

You should build one. Only then will you both understand, and have a basis for experimentation, as do many others here.

Understand, many people have put years in on the development of the RMH. The ratios given aren't guesses, nor are the measures taken in any way accidental.

Difficulty starting and smokeback will be some of the issues that plague under-insulated risers. Also, RMHs are heat batteries, not radiators. An uninsulated riser risks leaving the area around it uninhabitable. It's not like you can just turn the heat down.

Lastly, the riser and everything before it are the hottest parts of the system. These systems degrade quite rapidly if the warm/cool cycle isn't attenuated to some degree. That's where the insulation come in, keeping the brick bell, or the refractory housing your barrel, whatever your design, from not-so-slowly spalling off into bits.

There are other reasons. These are just a few. But please, by all means, if you want to replicate the salient testbeds and do the thermocouple-based temperature readings on multiple different units with controls, we will appreciate all the data you can send. Yes you'll be replicating all the work dozens of people have already done, whose proponents are trying to save you such work, but it's good for science to get independent verification.

Please let us know how it goes on your end, and good luck.

-CK

You CAN have both push and pull in a combustion system. Not both from the same place, but the core riser/barrel component can impart some push, and the chimney can certainly give pull. A good chimney is the most important factor; a poor chimney is dependent on an excellent core "push" for function, and smokeback is likely to be a regular occurrence especially while starting.

A wood-fired combustion system is not closed, so I don't see the relevance of refrigeration systems. The major push is the heat riser, then contraction in the barrel makes the gases slightly denser allowing them to fall more easily. Calling that a push or a pull is questionable; perhaps you could define the changeover as the point at which the internal pressure falls below ambient atmospheric... The vertical chimney effect would I think clearly be defined as a pull on the rest of the internal system. A strong enough riser push effect combined with extensive cooling and thus a weak chimney draft might yield positive internal pressure all the way through. Maybe the bottom line there is that where heat is being generated -in a vertical channel- there is a push, and where it is being absorbed by the mass/flue/chimney there is a pull.

You can get smokeback by overfiring, but you can also get it by a draft not having been established yet, or from a gust of wind in the wrong direction pushing down the chimney. As you state, starting with a well-drafting chimney (ideally going straight up in the center of the house at the roof peak) is key; still, in bad conditions (warm spell while the house and chimney are cold), you can get poor draft which can only be resolved by (pre)heating the chimney.

A correctly built J-tube system generally once burning is very difficult to overfire - you just can't fit enough fuel to overpower the draft capacity. I suppose if you stuffed it with tinder in the middle of a burn you could do it.

By the way, when you build a rocket mass heater to your design, how will you know if it works better or worse than a standard by-the-book system if you don't also build one of those in the same location?

Glenn

When I build you and others will decide for yourself if it works better or worse than the standard by-the-book system, which seem to have ongoing issues today that need tweaking.  

My mistake, obviously it’s not a closed system, I misspoke: the only process I can see having push/pull within a system is the refrigeration cycle.

There is no push, you said it yourself “The vertical chimney effect would I think clearly be defined as a pull on the rest of the internal system.” What is the reason the by-pass came to be and recommending the final vertical flue riser be close to the barrel: to heat the stack, to create draw/draft/pull? Pull on what “rest of the internal system”... feed tube, burn tunnel, riser, barrel, battery and out.

It may seem the riser is pushing but it’s the downstream cooling effect that is pulling (“contraction”) the flame to behave the way it does in the J-tube. Even with the addition of the by-pass that is used to draw the combustion first horizontal then vertical up then vertical down to “PRIME” the system... to heat it up to establish a pull.

https://permies.com/t/159700/Retrofitting-Piped-Mass-Bypass-Install

Joe Danielek wrote:
It may seem the riser is pushing but it’s the downstream cooling effect that is pulling (“contraction”) the flame to behave the way it does in the J-tube. Even with the addition of the by-pass that is used to draw the combustion first horizontal then vertical up then vertical down to “PRIME” the system... to heat it up to establish a pull.  

The way I understand physics, this statement would seem to violate the Laws of Conservation of Mass/Energy, as well as Newton's Third Law. Burning wood, overall, converts solids to gasses. These gasses have a much greater volume than the wood from which they come, so they must go somewhere, as well as the energy they contain. If, as you say, the contraction of the flames creates the pulling force, it could never make up for the fact that there are more gasses after burning than before, because the gasses (even at atmospheric temperature) will always take up more volume going out than coming in. Moreover, it would violate Newton's Third Law, as the pull cannot exceed the push generated by the expansion of the flames. According to Newton's Third Law, there can be no uncoupled forces; to every force there is opposed an equal and opposite one. If there is a pull, there must also be a push.

The way I see it, what makes the exhaust flow outside rather than into the house is buoyancy (in a naturally aspirated system). Since a RMH puts out more gas than it takes in, it cannot create this in and of itself. It is the differential of the much larger system (the atmosphere) with the smaller system of the RMH that creates the buoyancy needed for draft. A RMH releases heat, and this heat makes the gasses thinner and more buoyant in the exhaust in relation to the atmosphere. Whatever pushing and pulling is going on in the RMH should be inconsequential. Here it is the macrosystem that is important.

There's a quote from Shakespeare I try to keep in mind, "There are more things in heaven and earth, Horatio, than is dreamt of in your philosophy." I have found this to be true countless times in my experiences with making things. There are almost always factors that I fail to consider for which I must account later and modify. Sometimes, I never even figure out what the X factor was; I still have to modify my plan to make things work. I get the feeling that many things (like the bypass) regarding RMHs are a result of this same experience. They are work-arounds to help ensure RMHs work in the widest variety of circumstances possible. If a building has a healthy natural draft, it's easy. It's when it doesn't that things like the bypass can help.

The riser in a J-tube contains fiercely combusting gases which are expanding and rising due to being much lighter than ambient atmosphere. This certainly pushes on the downstream contents. An isolated, insulated J-tube with barrel over it will, once a draft is established in the riser, push the exhaust out the open bottom of the barrel, not very vigorously but definitely. No one disputes that the rest of the system where gases are cooling does not push; depending on details of the path the internal pressure may be more or less than ambient at various points.

A rocket mass heater system is so sensitive to environmental conditions that only a comparison with one in the same location can be valid and instructive. If properly instrumented with a multi-thousand dollar Testo recorder, you can make comparisons which may have validity provided conditions of the other systems including weather can be verified to be very similar. You also need to use the same instrument to test each system, or ensure that all instruments are precisely calibrated by a lab to match.

I'm just gonna hang around until I get to see this new thing-a-ma-jigger and if it turns out to be all that and a bag of chips, I'll happily take my slap in the face.

Again “It may seem the riser is pushing but it’s the downstream cooling effect that is pulling (“contraction”) the flame to behave the way it does in the J-tube. Even with the addition of the by-pass that is used to draw the combustion first horizontal then vertical up then vertical down to “PRIME” the system... to heat it up to establish a pull.”

Third law... in every action there is equal and opposite reaction, that would render the RMH “stalled” without a draft due to contraction. Think the contraction is the opposite and equal reaction to the combustion process?

Change the way you look at things and the things you look at change.

Newton’s Third Law is not absolute!

Glenn Herbert stated five days ago that has 7 likes and an apple, I think you got them in an hour: “An important function of riser insulation in a traditional RMH with steel barrel around the riser is to keep the exterior of the riser from getting very hot and making the barrel space hot. That would tend to reduce the temperature differential from riser interior to barrel, and reduce the "push" to encourage gases to flow down and into the mass. People have found that a system with an uninsulated riser can stall out after a while as the riser heats through.”

The whole paragraph was quoted where if we read the last sentence “People have found that a system with an uninsulated riser can stall out after a while as the riser heats through.” STALL due to re-expansion of the process medium within the bottom of the barrel creating back pressure and or ‘smokeback’ at the feed tube. This re-expansion stopped the contraction of gases pulling on the riser, burn tunnel and feed tube. I’ve also contended that overfiring causes the same issue, impatient operators pushing their systems.

Tipton, you’re going to be waiting for at least 18 months. First we drill the water well in a couple of months to the upper uncontained aquifer with water depth at 400’ with a 500’ casing for nice pure water at 450 TDS. Second the garden shack for sleeping, third the shower building complete with a clothes washer and modified Trombe Wall heating the floo so helpers can get cleaned up. Once the place is livable friends and family will help build the 400 sqft tiny cabin with the RMH heating the native soil floor within.  

   

18 months ain't nothing Joe,  folks around here have been waiting longer than that to see me put up or shut up.  To be sure, the putting up has been going mighty slow but it's getting there.  I've changed my mind about a dozen times or more about what I want to do with the space I have and there has been an enormous amount of learning along the way.  I like to do things my own way, so I'm taking the best of everything I've learned, and trying to put something together whereas I still get to experiment on the way to completing it.  I love to invent, but tinkering is sometimes expensive.  

Best of luck to you on your construction plans.  That sounds like a mighty deep well.  I think mine only goes about half that deep, but to be sure, we never run out of water.  Please consider documenting your constructs and sharing with the rest of us.  You are an innovator and I respect that, but to be sure, extraordinary claims demand extraordinary evidence.  So don't be offended if you run into a doubter or two.  For the most part, experimentation is valued by most folks around here, except when it comes at the risk of life and limb, but you don't sound like the reckless type.  

Joe, I find it odd that you can say that contraction of gases as they cool can be a pulling force, yet imply that expansion of gases as they heat cannot be a pushing force. Why can both cases not be true?

Simple, draft (draw/pull) is needed first to direct the flame (combustion) sideways in the burn tunnel at the feed tube/chamber: sometimes needing a bypass for starting up difficult builds. Once in the riser it continues expanding (in volume, given room by down stream cooling/contraction/draw) as it hits the cooler bottom side of the barrel lid that immediately starts cooling (contracting-condensing) the hot gasses - draw. It further contracts as the combusted hot gasses scrub the cool barrel sides as it enters the next transition the heat sink (battery) that further cools - draws the exhaust stream. Finally it reaches the bottom of the vertical exit flue where hopefully there is enough heat to rise (buoyancy) creating further PULL at the FEED TUBE.

How to assist the 'process': correct flue cap, free unrestricted combustion air, a bypass, correct fuel size fed at the correct rate, cooling at the bottom of the barrel and a little help from inertia.

Thomas T. you will get your documentation written and with pics of the build and startup.

Joe Danielek wrote: “It may seem the riser is pushing but it’s the downstream cooling effect that is pulling (“contraction”) the flame to behave the way it does in the J-tube. ”

I don't get it. If it was relying only on the downstream cooling effect, then how would the hot air rise up a normal chimney?

Is this just a semantics thing, like arguing about whether a horse pulls or pushes a cart because although he's in front he's attached to it via a collar around his chest so from his point of view he's pushing?

I can't figure out if we're just talking about the same thing in different ways or if you really don't understand how a chimney works?

I can't figure out if we're just talking about the same thing in different ways or if you really don't understand how a chimney works?

How do you put out a chimney fire? Serious question.

Okay, then if the contraction of the gases downstream of the riser is "pulling" the air into the feed, why does a bare J-tube core without barrel, connected to nothing but the atmosphere, burn so fiercely upward from the riser?

Hi Joe;
I wanted to compliment you on your brick oven build.  
Rereading this thread I realized no one has commented on it!  Outstanding Job, it looks great!

Rather than continuing debating pushing or pulling.
I, like our friend Thomas T , am happy to wait for your build.
Thomas Edison was looked at like he was crazy when he talked about electricity.
If your ideas pan out to be a "face palm moment" then I am looking forward to hearing all about it!
If they don't work out as planned then you know what to fall back on.
We really are just trying to help, not give you a hard time.
Keep us posted as you find the time! .

thomas rubino wrote:Hi Joe;
I wanted to compliment you on your brick oven build.  
Rereading this thread I realized no one has commented on it!  Outstanding Job, it looks great!

Rather than continuing debating pushing or pulling.
I, like our friend Thomas T , am happy to wait for your build.
Thomas Edison was looked at like he was crazy when he talked about electricity.
If your ideas pan out to be a "face palm moment" then I am looking forward to hearing all about it!
If they don't work out as planned then you know what to fall back on.
We really are just trying to help, not give you a hard time.
Keep us posted as you find the time! .

Thank you Thomas R., lots of cuts on hard heavy fire brick to keep the weight bearing refectory mortar joints to a minimum thickness. Interesting build, again I didn’t follow the norm: there are three terminally isolated components, the oven floor, the dome and flue entry assembly that can expand and contract independently of one other. Also the dome bricks were stood on end rather than flat to allow brick shaping for tighter mortar joints and free handing the dome build without a form.  

Joe;  What can I say but, OUTSTANDING!
I'm jealous! I love brick work and you did a fine job!
Thanks for sharing!

Glenn Herbert wrote:Okay, then if the contraction of the gases downstream of the riser is "pulling" the air into the feed, why does a bare J-tube core without barrel, connected to nothing but the atmosphere, burn so fiercely upward from the riser?

Why doesn't the fire take the path of least resistance and burn verticle up the fuel standing upright in the feed tube? Buoyancy in the J-tube riser more specifically your flame temperature in this unrestricted free burn expands the combustion mixture 4.86% @ 2000°F (at 1000°F 2.86%). Its basically an 'explosion' (at free burn) peaking somewhere within the Burn Tunnel - Riser. Within the riser there is contraction 'implosion' occuring drawing/drafting the combustion mixture. Add a barrel over the J-tube, contraction of the process... when hitting the top of, rapid contraction as the process stream wipes the barrel sides (contracting) being pulled downward, further contraction in the horizontal runs untill "buoyancy" is realized at the vertical exit with residue heat. Where there is no residue heat at the exit the bypass came to be, an accident?

The 'explosion' expansion in the Burn Tunnel isn't enough to overcome down stream resistance without the 'implosion' drawing it.

Some builds work, some sorta work and some 'smokeback' that is the result of resistance within the process path or luck or more so an eye for creating a smooth path.

I think you have a typo - the screenshot says the expansion is 4.86 times at 2000F, not 4.86 percent. So it is a large difference in density, thus buoyancy. If a chimney causes draft by buoyancy, does not a J-tube riser do the same? I suppose you will say that the exit from the chimney cools down and pulls air up the chimney...

The textbook definition of buoyancy is that less dense fluids are displaced by more dense fluids which push them up. More specifically, the upward force on the bottom of the less dense slug is greater than the downward pressure on the top of it, with net upward push. https://physics.info/buoyancy/summary.shtml

Typo it is, multiplying it is... makes the implosion greater, thanks for confirming.

How much wood is required to get it hot enough to cook the Neapolitan pizza?  And it's not 6 hours of burning wood to get it to this temperature is it?

Graham Chiu wrote:How much wood is required to get it hot enough to cook the Neapolitan pizza?  And it's not 6 hours of burning wood to get it to this temperature is it?

Takes more wood than one would think to heat a pizza oven up to temp from ambient. Than again it depends on the total mass designed into ones oven and its intended purpose. We use the DROP (heat stored in the bricks released during cool down) to slow roast other things like beef, foul, soups and bread. Our oven cooks a cold stuffed 18 pound turkey to perfection in 10 hours at a starting drop temperature of around 300 with a kindling fire to finish the glaze after heated up to cook 25 to 35 12" pizza's. End of cook temp is right at 180... again that is after enough heat has been stored in the brick mass for cooking pizza's.

Cooking Neapolitan pizza requires the floor to be almost as hot as the doom that is at 700°F, I like the floor at 600°F. Cooking the pizza requires a flaming fire to radiate that heat down onto the pizza, thats half the cooking process. The other half is the floor or deck temperature you place the pizza on and when your cooking 25 to 35 pizza's it takes time to transfer enough heat into the floor so it doesn't cool down while cooking those pizza's in rapid mode as in a pizza every 5 minutes. That is where the long pre heat fire time comes from, to heat the deck.

Also the long pre heat time occurs as the oven gets above 400 degrees wood gasification accelerates to a point the off gassing is greater than the amount of combustion air that can circulate into the oven. Over charging the fire with wood being in a hurry only wastes the available hard wood btu's.

How much wood, hardwood, about 2 to 2.5 cubic feet provides the cooking conditions I like.

Wood fired ovens do not necessarily take 6 hours to fully saturate with heat, so much depends on the size and amount of mass.
The ovens I build are 32” internal diameter, 3” fire brick floor and 2.5” cast dome with full 4” ceramic insulation, they take around two hours to fully saturate the mass and around three days to return to ambient  temperature.

I didn't read the whole thread so didn't notice if the question were answered but a rocket stove by definition has an insulated riser.  That's according to the inventor Larry Winiarksi.
So, all those metal things you see around there aren't rocket stoves by definition.

Fox James wrote:Wood fired ovens do not necessarily take 6 hours to fully saturate with heat, so much depends on the size and amount of mass.
The ovens I build are 32” internal diameter, 3” fire brick floor and 2.5” cast dome with full 4” ceramic insulation, they take around two hours to fully saturate the mass and around three days to return to ambient  temperature.

At 36" I have a lot of mass on purpose, specifically the floor that has a double layer of bricks as wanting a large heat sink. The dome fire bricks are installed upright (on edge) with each receiving custom multiple cuts to minimize mortar joint thickness as well as accomplishing a free hand dome build. Wife wanted a pizza oven, I wanted a wood oven, we got both where we plan out usage of the stored heat during the temperature drop for cooking all kinds of stuff in the days following a pizza party. Again I take 4 to 6 hours to heat for a pizza party from a cold oven using the floor temperature as a gauge of being ready: usually an hour after the floor first reaches a temperature if 600°F, dome at 7 to 800. Than again if cooking more than 20 12" pizza's more time is taken to allow the deck to store more heat to assure pizza crusts are cooked correctly. I really like cooking porterhouse steaks in the oven, the radiant heat off the dome does a fabulous job. The pizza oven build was a trade, she gets a pizza oven and I get a water well on our 36 acres.

Graham Chiu wrote:I didn't read the whole thread so didn't notice if the question were answered but a rocket stove by definition has an insulated riser.  That's according to the inventor Larry Winiarksi.
So, all those metal things you see around there aren't rocket stoves by definition.

Why is there a need to insulate the riser when the velocity of the burning flue gasses is such that any affect heat loss would have is zero? You can't transfer heat through any material fast enough to make a difference.

Think insulating the riser has more to do with rocket stoves designed for INSTANT heat rather than focusing on storing heat in the mass for slower release. Using for instant heat requires intermittent firing so heat stored in the riser between firings aids in establishing a draft easier for the following burn cycle.

Graham Chiu wrote:How much wood is required to get it hot enough to cook the Neapolitan pizza?  And it's not 6 hours of burning wood to get it to this temperature is it?

This is likely why pre-industrial civilizations often had a "bread oven" that was used by the community (which is why the nursery rhyme says, "mark it with a "B" - so you know which loaf is yours ). I know so many people who built pizza ovens that rarely get used. People need to be very organized about their menu to benefit from that stored heat over multiple meals or days, and often need to be feeding many more than the typical current North American family size. It's also why I'm holding out until I can do serious infrastructure building that will allow more of a Rocket Cook Stove type build - I'm hoping that before I get to it, someone will have designed an efficient combo of cook-stove top with oven. I'm sure I'm starting to see signs of that development???  I want to cook my bread and make my soup at the same time.