splitting the duct and reuniting it -question

Hei everybody,

I'm thinking of building a RMH.

I wonder if there is a problem when i would split the thermal duct ,(right behind the barrel),in 2 ducts,and re-unit them again at the point where the chimney starts.

My question is: if the 2 ducts differ in lengt, will the airflow choose the shortest duct?

thanks!

Patrick Duthoit : O. K., 1st day of school and you got the crabby old curmudgeon everyone warned you about, Some stuff for the blackboard >

A well made 6'' Rocket Mass Heater RMH,can flow your hot exhaust gases 30 feet Horizontally through Your Thermal mass!

This is 30 feet - 5 feet for every elbow that you use ! Use Two elbows in your Thermal Mass and two elbows to duct the flow of your gases to
your final vertical chimney and you get to have all of 10 horizontal feet of Thermal mass.

A well made 8'' Rocket Mass Heater RMH, can flow your hot exhaust gases 50 feet horizontally through your Thermal Mass!

This is 50 feet - 5 feet for every elbow that you use ! Use 2 Elbows for the thermal mass and 2 elbows for the final chimney and you get to have
30 horizontal feet of Thermal mass !

So now we can answer your question in part !

To split your system requires a "T" which we will treat as though It was an Elbow then two elbows to direct the flow of hot exhaust gases to two
directions, and at the other end two more elbows to rejoin the two pipes at the base of the final vertical chimney, this will require a further rejoin-
ing "T" and an elbow for the base of the Chimney ! Count them up and you have a total of 7 elbows / "Ts" this equals 35 feet!

One pipe will always be dominate, and often it is not the one you think it would be !


O.K., Just like College you are expected to buy the class text books After all even if you were to accept the idea that the instructor
knows everything (he doesn't) you will still have to read "The Book" to learn how to do it yourself !

Please goto Rocketstoves.com to Download your PDF copy of the brand new 3rd Edition of Rocket Mass Heaters this is "The Book",
With 100,000+ RMHs built world wide, this is "The Book" followed inmost builds, and 95% of all 1st time builds (that Worked ) were built following
"The Book"

Rocket Mass Heaters will quickly teach you the basic language to be able to understand the simple concepts and converse with your fellow
members as you describe the sizes, shapes, and orientations of the parts to each other and the whole.

Get the Book, read the book, and come back here often your fellow members enjoy creating New Rocketeers every day ! For the Good of the
Craft Big AL

Hei Allan,

First of all sorry for the late reply
But thanks for the nice and very good technical explanation. After some rethinking and redrawing i came up with the next:

I use a 8 inch riser, i have 7 elbow's and 15feet duct (= 50 feet of horizontal duct).So,

Can i place a chimney that is about 27 feet high, going insulated trough the top of the roof, after the horizontal part? or is this to high?

About the book, i have it, read it more or less, (i'm more of a "get the picture-man") and it's indeed THE book.
But the data you gave i can't find it anywhere in the book. (must be blind)

Also, i want the heater to take his air from outside. So,i placed a 8 inch duct under the floor.(about 16 feet long with 2 elbow of 45°)(coming from outside and goes close to the feeding tunnel)
At the top of the feeding tunnel i would place a glass door. If it's closed, the air comes in via the groundduct. When opened, while feeding the dragon, the air in the house is used.

any experience or thoughts in that field?

Greetings, Patrick

Patrick D. : Some people are book learners, and some have to have pictures, + video's, and then there are the ''let me see it, and hands on guys !
Adult Education is always a special challenge because you have to convince the other guy that he Needs to Know the next piece of information!

I was the guy who could always Psych out what the Answer was the Teacher wanted, often without studying, I'm the guy that Foxtrot-ed the curve for
everyone else !

At the top right of this page under the Permies Banner and above the Permies Video of the Week is the Permies Toolbox find the [MY PROFILE] tab
click on and add your profile information, look at your name space and then L@@K at mine, this will give us location, and climate information, so that our
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Neighbor!

While you are learning to use the Permies Toolbox check out our special Search engine that can search ether the Web or the 10s of thousands of
threads posted here at Permies. Click on the [Search] tab and on the next page enter ''Outside air for an RMH'' select your search to look in the Rocket
stoves forum threads and select a search within permies, if the subject does not come up in the bottom search field starting to type it in should fill the
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0.25 seconds should give you a thousand hits on your topic !

As you must have gathered the length of the vertical chimney works for you, not against you, be prepared to add to the height of your existing chimney to
clear the peak of your roof by 4', and to have a storm hood, practical experience will quickly show you if you can do with less !

Think like Fire! Flow like a Gas! Don't be The Marshmallow! Your Questions andComments are Solicited and Welcome! for the Craft! Pyromagically Big AL

Patrick, a chimney can't be too long, exept when it blows your fire off.

If you want two run of pipe in parallel, it's not possible, look into bellsThat will reduce your elbows too i think. About outside air, i'm bit concerned, as thoses tend to create a backdraft or smokeback when you open the door. What you seem to want is along the lines of a batch rocket.

I also have this question, so I go on this threat, to gather all answers under a clear title.

From the new Wisners' book (I got the pdf recently), I got the information that a 8" system can work with a split in two 6" ducts.
The total CSA is larger, and this compensates the frictional loss.

I did a search in the forum, and some answers are not as recent as the book (te next quote are from 2010), and this subject has not been fully dealt with, so what new and what has been done?

tel jetson wrote:is splitting the duct into several shorter runs a workable solution to too many turns?

In my case, I do reduce turns, and my system has to be underground because of path ways in a small place.
I do beleive that a 8" systems with 2x 6" pipes can be interesting to think about, and useful.

Ernie Wisner wrote:Splitting the ducting into smaller diameter is a very tricky thing to do. each split off handles a different amount of flow unless it is done just right and the flow is totally constant where you split it off.; laminar flow can and will bite you if anything is slightly off.

not generally something i suggest even at the best of times. I would not do it unless i had a really really good reason for it; introduces to many variables.

Glenn answered to my recent question https://permies.com/t/41193/rocket-stoves/Photos-process-RMH#324847

Glenn Herbert wrote:From all I have heard, splitting the ducts in a RMH seldom works well, or at least easily. One always predominates and takes more of the flow and heat. If you have some way of restricting the one that heats more, then you could fine-tune the system with experience. You might have problems with too much restriction unless you are very careful.

Thanks, that is clear and logical if I compare to water pipes: the flow is going preferably straight,
and does not turn unless there is some restriction in the more straight duct.

If I have a 8" system that splits in two 6" pipes, then I should try to have the same angle.
If there is a turn, then gases will tend to take the easiest path,
so should I put a key to regulate, as in systems with a by-pass?

I guess this is what you meant by "with experience"? -> Know how much I should open and close for flow regulation...

The parallel of the RMH system to a water system, can be useful at times. Just remember it is upside down: water flows down hill; heat rises up hill.

Using a vertical chimney also gains you some draft. I recall seeing a video somewhere where Ernie Wisner said you can add back in 15-feet (making up for the loss of three elbows). But I don't recall the height requirement of that. Perhaps someone else remembers, and maybe can add a link to that video. I have no idea where I saw it, I just remember hearing him say it (or more accurately, I recall part of what he said about it, heheh)

Hi everyone,

Thanks satamax for putting the concept "batchboxstove" in my mindbox.

After looking at some video's from that type of stove i was sold as a kittycat in a petshop.

Also, i'm coming back from the idea of splittin the ducts. (that's something i would like to testdrive (read: "play with") a little more first)

So, my plan is: a 8' inch riser type Peter van den Bergh with a 8' inch thermalduct, 7 elbows and about 15 feet long. The chimney is about 26 feet high.

But, i'm of topic now. It's about that splitting of ducts.

Here some thougths:

*The differ in flow between 2 ducts can be usefull too?i mean: the hot bench part and the milder hot bench part? catch my draft?

*What about the changing in the speed of flow if you're going from 8' inch to 6'inch or viseversa? (like the venturi of a carburator)

*you'll have more ducts to clean?


The comparisation water/ airflow is tempting but i'm not sure if that works all the way.

I'm thinking about the cilinderheads that are tuned on a flowbank. Polishing the intake and outlet too much is no good. Creating too much turbulence is also no go.
The flowing out of cilnderheads is a process of trying, failing, measuring,..trying again,..

Hope this is usefull

Greetings fellow firefreaks

Patrick Duthoit : Liquids, Gases, and Plasma all do seem to follow the same rules regarding Flow! Try this for an exercise, You know those long funnels for adding Automatic
Transmission Fluids (ATF), If you match the flow through that funnel with a funnel with a shorter spout (same internal diameter) The longer spouted funnel empties faster.

Now when people start comparing flowing water to electricity, I nod my head all through the careful explanation -and find out 15 min later I cant repeat what i just heard
maybe if the guy who draws the cartoons for the Operators Manual for the RPG could sketch it out for me- I might 'get it' then !

Any way,go and try it! We are a long ways from wringing out all of the discoveries of fire science yet !

For the Good of the Craft ! Big AL

allen lumley wrote:

Now when people start comparing flowing water to electricity, I nod my head all through the careful explanation -and find out 15 min later I cant repeat what i just heard

In my experience, one of the limitations in using the water pipe analogy to electricity, is one usually fails to start with the fact that the electrons never "fill up" the space, or flow through it -- they are already everywhere in the wires and components, and the flow is actually more like bumping, or playing crack the whip. Crack the whip is an American kids game, but the idea is easily seen by tying a rope onto a door knob, pulling a length of it fairly tight, and then whipping the other end up and down sharply; this creates a "wave" that moves from your hand to toward the door knob. (This is also sometimes used as an illustration of the energy wave moving along a transmission line for radio transceivers.)

I never played with fluid dynamics, and hydraulics, but I suspect there is similar correlation, at least in a contained system. So flowing water through empty pipes is not exactly the same as many of these other systems, and that these other systems may already start "full" changes some of the effects.

So too, our masonry heaters will start "full." One easy example is the "cold air plug" that can stop the draft from flowing out a chimney when the air inside it is cold enough. Of course, our masonry system is open, as opposed to a closed system (like hydraulics, for example).

I'm only guessing, but I would think that in a multiple-path duct system, the major factors contributing to air flow would include:

Temperature throughout the system
Friction of the air passing the walls, and laminar effects
Dimensions of the spaces / pressures