dual benches ? is this possible?

I have just recently been interested in learning how to make a roacket mass heater and was wondering with a big enough system could there be 2 warming benches in 2 directions and if so will it hurt the draft to have 2 vents i plan on practicing out doors with a hot tub, sauna and if im sucessfull i was thinking of making a home heater as well currently run off an ashley wood stove burn about 6 cord of wood per season heating a 1965 trailer with full lenght addition. i would also like to say i love your site and the things you are doing this is so awesome and interesting i wish i had discoverd this 10 years ago and again thank you for your input. joe in maine

While I have zero experience with RMH, I do have some experience with drafting and air flow. That said, one problem that I would see with dual benches is that you can't really have two exhaust ports, because even if they are on the same side of the home, any kind of wind eddy or pressure imbalance would disrupt the parrallel flow of gases. The two exhaust channels would have to meet back at the center, having followed mirrored paths with identical lengths, before exiting the home.

Joseph F.: Good idea to do a practice build outside with each and every build ! Have you been to rocket stoves.com to get your copy of Ianto Evans; great book "Rocket Mass Heaters"
PDF download $15.ooU.S. ? Also you might want to check out ernieanderica.info ! I'm guessing that your rocket build in your trailer/addition will be on/near flammable materials .

Properly built you can run your ductwork through a thermal mass the size of a large twin bed,( or a little bigger ) turning your duct work back on itself making a second pass through that
thermal mass then extending past your rocket stove to run through a second thermal mass on the other side of your stove before before being vented outside ! A bench and a bed!

Good Luck! Be Safe, keep Warm, and share your experiences on the Permies pages ! Pyro magically Big Al

excelent idea and yes i did purchase the book as it was mentioned or recomended in every video/web site i think i went to. I did plan on placing the unit on fireproof material and in maine wood burning stoves cant be installed in trailers but if the trailer has an addition it can be in the addition part but must have a seperate flue from the trailers flue. Currently where my wood stove is now is perfect lay out for a rocket mass heater thers lots of room its in the addition part which is down at ground level and well supported underneath and i could use the existing chimney for my vent. Im so excited to get started with my outdoor project. I have been telling all my friends about this.

We installed two benches, but on two different floors maintaining air flow in one direction.
Our RMH is new, first firing was in October 2012, but so far really good. The upstairs bench doesn't get nearly as warm as the downstairs bench, but it does bring heat upstairs and it is still a cozy place to sit. For our purposes we are pleased with the functionality of it.

Jasmine Maurer wrote:We installed two benches, but on two different floors maintaining air flow in one direction.
.

I'd like a better description of how you did this. The thought of a RMH on one floor and a heated bench on another is intriguing.

J. M. : Yes, Please ! Jasmine, this deserves its own thread, I for one would very much like to see this in operation, but pictures,and/or simple diagrams
would be very much appreciated !

It seems to me that directly after the 'magic' that occurs within the Heat Riser/Heat Riser-Barrel interface (super efficient combustion ) the second magic
occurs when the hotter (hottest )gases start driving the lower barrels exhaust gases horizontally through the thermal mass. I have used the phrase
' heat pump ' to describe what happens here !

Sometimes people find that they are heat extraction limited by the lay-out of their house, and the need for traffic routes thru/past their Rocket stove and
Thermal mass. This results in high exhaust gas temperatures after their 'Stovepipe' leaves the thermal mass. Often this is compounded by a local ' Zoning '
requirement that the 'Stovepipe' must exit the house through an insulated chimney pipe. Some people are reporting that a second heat riser/barrel at this
location (Counter-intuitivly, this additional barrel is often allowed ! ) can extract the extra heat needed to get the exhaust gas temps down to 200F or lower
before the chimney!

Being able to use the Rocket Mass heaters ability to 'pump heat' to a remote location, here to a second thermal bench, is an under explored wrinkle in the
Rocket system, and needs more 'research', please let us 'hear' from you on your experiences after a winters heating season !

Be safe, Keep warm, Pyro magically - Big Al

I'm building what's basically going to be my first COMPLETE RMH build, and I'm wondering about the dual bench potential.  

It IS called a manifold, so it seems likely to be possible to have multiple outlets.

If Outlet One goes right, and Outlet Two goes left, and they meet up again at the barrel where they T into the chimney, that seems like it should work to me, but what do more experienced minds have to say about it?  Also, what if the two outlets are at different heights? One directly under the barrel, and one about 8 inches lower?

It IS called a manifold

Manifold is a general misnomer in this case, and I think is confusing to many in this regard, but it is simple to figure out why this term was used and there is no reason for not having a manifold, if your system is designed for it.  It is called a manifold (and could potentially be), but should be called a unifold since that is how it has been built in most RMH systems.  The term manifold as it is being used here, comes from idea of the space of the unit where the gasses are exiting the combustion/thermo-siphon area, and are exhausting into the flue/bench/mass area, like when exhaust exits many engines in a true manifold.  

I don't have any links to it, but I have seen videos where this (a true-to-form properly named manifold going to two different benchs) has been done.  

One of the issues, from what I have gathered, is that the standard build has been given a magical max length for it's bench pipe of 20 feet.  This might be wrong.     I'm not sure why this 20 foot number exists... and the Tipi build at the Lab seems to defy this with Paul's super long underground extension... so I'm not sure.  

If you are stuck to twenty feet total of horizontal duct before you go vertical, doesn't give a lot of length for two benches unless there is a diversion 'valve', which closes one bench's flow, forcing all the hot gasses to the other bench.  I figure, with a good valve system, a person could have as many benches as he/she wanted, but only one bench would be warmed up at any given time.  Again, this might be wrong.

I'm still in the research phase of this and do not have either I Evan's book or the E and E Weisner's book.  Sorry, maybe I'm not the best one to answer this.  I hope what I wrote is somewhat helpful.  

Also, what if the two outlets are at different heights? One directly under the barrel, and one about 8 inches lower?

 From what I have learned, it is best to have the outlets 4 inches or so above the floor of the manifold/burn area.  This allows a place for ash to settle (and be cleaned out) and not go into your bench pipes.  You could have two over each other, above that 4 inch height, but it might be better to have them side by side or on opposite sides.

I know that there are some experienced RMH builders on this forum, but sometimes these questions go unanswered.  It would have been nice to see some follow up in this thread by the OP and the lady with the two floor system.  

I'm pretty sure the woman with the two floor, two bench system has not been around here in three years, so it's unlikely we'll get any more info from her. What she describes is probably so straightforward that it could be replicated by anyone who can build a working RMH, though.

There is no 20' maximum bench length dictum. Different size systems do have a practical maximum amount of effective duct length. An 8" system (all 8" diameter components or the equivalent) can support up to 50' of horizontal duct, minus 5' for each 90 degree bend. A 6" system can support around 35' of duct, again minus 5' per 90 degree bend. Larger or smaller systems are uncommon and do not have published rules of thumb.

The maximum duct length is based both on effective heat transfer (not too much to leave cold exhaust, nor too little to leave hot exhaust) and on the friction of airflow through the duct.

Multiple simultaneous duct paths always run the risk of unbalanced flow; I have heard from people with experience that one branch always dominates unless all conditions are truly identical. If you can put balancing dampers in each branch, you can probably get reasonably balanced flow. You would want to ensure that the dampers cannot totally stop flow, or somebody sometime may play with them and shut down the system completely causing smokeback or carbon monoxide backing up into the house.

Thanks for joining the conversation and clarifying, Glenn.

My design has shifted a bit, and I'm looking at a system where two ducts leave the manifold, one to the left and another to the right.  The left one leaves the manifold about 8 inches lower than the right one.  Also, the left one has about 14 linear feet of ducting before returning to the chimney pipe next to the barrel.  The right-side bench has about 28.3  linear feet of ducting before returning to the chimney pipe next to the barrel.

After reading the comments here, I've decided to implement controls by which I can regulate which bench receives more airflow, and originally, I had planned on having both benches join up to the same chimney pipe at the base of the barrel before exiting, however, now I'm thinking that it may be even better to have dual chimney pipes next to the barrel which rejoin each other somewhere near the ceiling before exiting the building.

I recall that placing the chimney next to the barrel creates additional efficiency, that the barrel-heated chimney creates an updraft which PULLS exhaust fumes through the benches, supplementing the PUSH of exhaust fumes by the barrel. I'm thinking that dual chimneys will augment this effect, and further, if the chimneys are joined at the bottom as well as the top, and at the bottom are controls to regulate which chimney (or both) are pulling air from which bench (or both), this will mitigate some of the effects created in the difference in the bench height and lengths, and be a better solution overall. Thoughts?

I think you are getting needlessly complicated with the chimney(s). I would join the two ducts at the base of the chimney stack with a single flapper damper to regulate the relative flow in the two paths. That would give all the control you need. I would not put separate dampers in each path as then both could be shut off choking the system.

You might have misunderstood me: I'm not talking about blocking both at once, and in the design I described, that would be impossible. I mean to install a damper between the two chimneys, that when closed, will allow the left chimney to draw only from the left bench and the right chimney to draw only from the right bench, but when open will allow both chimneys to draw from both benches.  Additionally, I would install a similar control near the manifold which would allow me to choose from three options: only left, only right, or both. No option for neither.

Okay, you have considered safety issues, but I still think all the complexity of multiple chimney stacks and multiple control points would not get you better results than a simple dual path with a single adjustable damper at the rejoining point.

I'm not looking to be contrary, but it doesn't seem any degree more complicated to me, just a couple extra pipes configured differently.  I guess I'm looking for additional insight from a few different people on how much difference two chimneys pulling from the same bench will produce compared to only one chimney.

If a design only had one bench, but it was 20 feet longer than most designs recommend, would two or more chimneys make possible what only one chimney could not accomplish?

My feeling is that two chimneys rising next to the barrel might create more draw but only because there is more surface exposed to barrel radiation. I think you would get the identical result by using a wider duct as it passes near the barrel, or even making a closer/stronger connection between barrel and chimney.

Also, when it comes to airflow in ducts, simple really is better. Each bend, split, or junction imposes a measurable restriction on flow diminishing the benefit of the reheating.

when it comes to airflow in ducts, simple really is better. Each bend, split, or junction imposes a measurable restriction on flow diminishing the benefit of the reheating.

 Restriction = friction, and that is not what you want.  You want flow, and heat absorption in your mass, as much as possible.  As I've had it explained by Peter V Berg, a chimney next to the barrel does heat up the pipe and thus increase flow, but it also results in heat loss from your efficiency since it is absorbing radiant heat from your barrel and allowing that absorbed heat to flow out of your home, as well as increasing the velocity of that which is lost from your mass up your chimney with the increased velocity.  Thus, from the way I see it, it creates several points of inefficiency.

i think recombining into one chimney is better than two chimneys, what fer the blowin wind not able to try differnt fingerins on yer smoke flutes

I don't disagree with you, Abe. It's more a question of WHERE to recombine them.

In your illustration, the two come together forming an upside down T, and originally, I planned on using a T joint at the base of the barrel. But if instead, I put that same T joint somewhere above the barrel,  with two vertical pipes leading up to it right alongside the barrel, I get pull from two chimneys. If, at the base of those chimneys, instead of elbow joints, I have two more T joints connected to each other, then it would not be limited to left chimney pulling from left bench only, right chimney from right bench only, but both chimneys could pull from left, or both from right.

I understand there's some heat loss, but how significant is that? In my mind, it's a negligible amount.

Chad, the question is, why do you need to do this?

Have you seen this one?   http://mha-net.org/docs/v8n2/wildac15f.htm

Motives:
1. curiosity/innovation, since I don't find this idea anywhere else.
2. concerns about the length of the right bench and
3. what effect the left bench will have on it.
4. control over different variables, like Paul's install with a vent out of the lower wall, out of the wall a bit higher, and out of the ceiling.

There's probably more than these four points.

Chad Sentman wrote:I'm building what's basically going to be my first COMPLETE RMH build

Chad, how many incomplete builds do you have behind you?

I don't want to diss or anything. But even if i'm a tinkerer, and i try things. If it's a durable install i will rely on for my heat. I'd rather keep things simple. And be sure that they work. You were describing a siphon earlier in the post. I can tell you that when cold, such an arrangement is no fun to start!

if you want to quote me, then QUOTE me, and there find your answer. It's disheartening to read "Chad Sentman wrote:"  followed by NOT MY WORDS.

Here's a true quote:

I'm building my first completed/second started RMH at the moment.

I started a build a year ago in a location I thought would be okay, got 80% finished and was told I had to uninstall it.  The parts had been languishing away in someone else's garden until this year when I got a new location and rebuilt using the same materials.

I'm testing every step of the way, but don't know what to expect moving forward from here with site-inspired innovations that nobody seems to have tested but nevertheless has an opinion on. If I build what I've described here and it fails, it will be a lesson learned and not uncomplicated to undo or redo until it works.

Um, Chad...

Chad Sentman  posted Nov 19, 2016 · 06:19 PM
I'm building what's basically going to be my first COMPLETE RMH build,

I grant that you also wrote the other sentence, though it was in a different thread.

Sorry for your location misfortunes. I hope this build succeeds for you, and if you can afford to redo it if necessary, have fun experimenting! I suspect that you won't get failure, simply not the advantage you are looking for, and you won't be able to tell whether it's better or worse than a simple system without trying both.

Ha ha, Sorry. I searched briefly for the statement in this thread and couldn't find it, so I thought it must be from the other. My apologies, at least I'm good humored about it.  You also bring up a fair point about the nature of experimentation, thank you.

Chad. See, first post that you wrote in this thread.

https://permies.com/t/23699/dual-benches#513469

This forum attracts all sorts of tinkerers. Some are good with their hands, who can think. And still, we don't see many completed projects. Lots of them having failed due to lack of perseverance mostly.

Some very good tinkerers on the other hand, think they are better than most. And don't have to follow the rules. Sometimes they have laughable fails.

Some, quite rarely, come up with something new, something of interest.

What i was saying, is; are you really sure you need a dual bench? With split exhaust and all. If yes, go for it. If not, if it's just the urge to tinker. Well, my opinion is, make something which functions well first. Then see what you can improve. There is no shame to follow a beaten path.

Besides this. I'm a strong advocate of bells. Benches take up space. Are time consuming and labor intensive.

Tho, for a big bench, Matthew Walker has created one which always tickles my fancy.


http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/#/user/mremine/library/NYC%20Rocket%20Stove%20Build?sort=3&page=1&_suid=148048700735906756329280480526

To respond to the original questions:

Yes, dual benches have been built.  Symmetry appeals to a lot of people.  There have also been multi-duct exhausts, with a true "manifold" dumping exhaust into as many as 7 or 12 separate pipes fanning out under a floor.  Some of these systems re-gather at a single chimney, but others just dump the exhaust outside somewhere with no chimney advantage.  These multi-flue systems might be similar to some of the k'ang or hypocaust layouts, which often had a proto-chimney or chimneys embedded in masonry walls, rather than a single chimney.  It is almost impossible to achieve totally symmetrical passive flow, just as it is highly improbable to find a major river that "splits" and does not re-converge; usually, one path 'wins out' under any given conditions, and becomes stronger flowing, and therefore gathers other advantages to itself (in the case of the river, a deeper channel; in the case of the chimney, a hotter flue).  In both cases, the less-traveled path tends to silt up, further reducing its advantage.  
However, it's perfectly possible to artificially tinker with the flow, and even it back out to any practical degree.  The old sub-floor method might involve a random brick or two placed to partially block the over-heated flue or channel.  A pivot damper will give you similar control with less soot, but possibly greater impediment to cleaning unless you plan around that.  There's a school of thought among masonry heater builders to avoid sliding dampers, as they jam up easier than pivot ones; all good designs will anticipate and make it easy to remove fly ash (floating small particles of mineral ash and soot).

Whether or not you will get the necessary draft advantage for the system to draw properly depends on a LOT of factors, some of which I will detail below.
The big question is whether your two chimneys will become as warm as a single chimney.  The flow equation for chimney draft is given in one of the appendices of our book, and it's also available from engineeringtoolbox.com.  Briefly, the temperature, area, and height of the chimney all affect draft.  Area has the biggest effect, but that has to be balanced against the available heated exhaust from your particular firebox.  If temperature (or more accurately relative density) becomes too cold/dense, and especially if you get below the condensation point for the water in the exhaust, your chimney(s) will draft backwards.

Yes, benches longer than 20 feet have been built.  There is no hard and fast limit, and it varies with system size.  A 4" system rarely has enough spare heat or draft to push exhaust through 4" channels; some 4" j-style projects have been successful using a bell and a 5" exit chimney.  They are fiddly.  5" systems can have a short bench using the pipe layout.  6" systems are good up to about 20-30 feet of horizontal pipe, assuming an average chimney after that.  8" systems can push up to 30-50 feet of horizontal pipe, with a decent chimney.  There have been systems built 60 feet or longer, but these often turn out to need special conditions for success (fans, downward-flowing exhaust on the lee side of a building, or some magical-seeming natural advantage with the building structure or prevailing wind direction).

There have also been systems built in the "standard" ranges (20-30 feet of pipe) that failed for "mysterious" reasons, most often relating to the building height, chimney design, etc.

If you have a 14 foot bench and a 28-foot bench, you will have dynamic draft differences due to different amounts of heat being removed (or added) by the different benches.  You will always be adjusting for wind direction, indoor and outdoor temperature, etc.  The total length of both benches, assuming they both have some bends in them, is pushing the limit as far as the length of pipe that one 8" firebox can typically heat.  You may find it's simpler to run one bench until it's hot enough, then the other.  As a consequence, the part of the room heated directly by the barrel may become unpleasantly hot before both benches are at full comfort temperature.  

Another consequence, if you use dual chimneys, is that the same available heat will need to prime more surface area.  (Surface area is generally a drawback: total surface exposures contribute to heat loss, friction/drag, and other costs that detract from the system's total energy budget.)
Your exit chimneys will logically be cooler than they might otherwise be, and therefore draft more slowly/poorly.  I'm not sure whether you plan to size them each to the system size (for example 8" diameter), or slightly smaller so the total area is comparable to the system size (say two 6" chimneys for an 8" system).  Since you may want to run either side alone, the full 8" size would make sense I think.   I don't see any advantage to running two parallel 6" chimneys, and some disadvantages.
The bends where you connect the chimneys represent some added friction/turbulence/drag, so two dogleg chimneys are not going to flow as easily as a smoother Y or single chimney.  

I would guess that if you run the 14' side of the bench while the 28' side is mostly shut down, there will be plenty of heat to overcome these drawbacks, and you may want to start the system this way at the beginning of the season or in any marginal conditions.  
If you run both sides simultaneously under marginal operating conditions (cold start where the benches are relatively cool; unfavorable winds or outdoor temperatures; cold chimneys; etc) your exhaust may become too cold to rise, causing parts of the system to run backwards.  In a linear system this would choke the fire on its own smoke (and the occupants); in this combined system there are more degrees of freedom, and perhaps one side of the bench would run backwards or stall while the other side ran; or you might see slow or delayed choking, or priming problems.  You can get very weird bounce effects during chimney stall conditions even with a linear or bell system.  

Under ideal operating conditions (both benches and chimneys significantly warmer than outside air, very dry firewood, house height and building draft dynamics working with you not against you) you may be able to run this system with both benches drawing simultanously.  It's not outside the realm of what's possible, just beyond what's been reliable for predictable success in the past.  

The added complexity might also make it a one-operator stove - something that the original builder can operate and love, but that other housemates or future tenants would struggle to operate successfully.  That's because the different settings for different conditions can make it hard for a new person to learn the stove.  If you always do the same thing to light the stove, and it always more-or-less works, you can have a wider range of people operate the stove successfully.  Rocket stoves are already upside-down and sideways from "normal" woodstoves, and even the batch box rockets take more care to operate properly than a regular box stove.  So adding complexity has costs, not just in extra parts, but also in extra difficulty for future operators.  
There's also the intrinsic risk of taking a proven method and changing it - chances of failure are higher with the change than with the proven system.  
(Having seen a few other dual bench prototypes, some of which were scrapped and cannibalized due to never working properly, I would say this experiment has something like a 75% chance of working worse than a single/linear bench.  I have seen more that failed than succeeded, but I have seen a few that succeeded to their owners' satisfaction. If that 25% chance of a ground-breaking result is worth the effort, and you can proceed safely and have the time/energy/resources to re-do a couple times if needed, then go for it.)

The most likely benefit of this experiment that I can see would be to discover in a direct and practical manner whether a 14-foot bench, a 28-foot bench, or a 42-foot dual bench is the right size for your situation.  Since you may find that the dual bench is too long, and remove one or the other extra bench for better heating balance, I would anticipate the possibility that you might only need one chimney at the end of the whole game.  

If you hope for the experiment to be useful to other people not just your own situation, the most popular experiment would be a symmetrical dual bench, with the same length, pipe size, and number of turns in each wing.  This configuration is interesting and relevant to more people (based on past questions), and in my opinion has a slightly higher chance of being able to balance the flow in both benches.  

I do think you're wise to bring the chimneys back by the barrel (for self-priming); this is my favorite configuration, especially for longer benches.

Without getting into the building height, climate, etc, that's most of what I would consider on this project.

More important than all of the above are safety considerations.  Especially with experimental systems, where the chimney temperature is less predictable, and you might end up running the firebox longer to heat both benches,  I would recommend following all relevant guidelines for safety clearances, material thickness, etc.  Chapter 6 in our Builder's Guide goes into detail on these.  The basics are: allow at least 9" from any pipe to a combustible wall, with at least 4" of that as an air gap, and 5" masonry thickness around the bench pipes.  
(The proper distance for single-wall stovepipe is 18", but this can be reduced to 9" in most areas by using good heat shielding or double-walled pipe.)
For bare metal, or masonry thinner than 5", or un-certified wood heaters of any type, some areas call for 36" to combustibles.  Depending whether they recognize modified clearances, this can sometimes be reduced to 18" with good heat shielding, or as little as 12" with very excellent, double heat shielding with double air gaps.  If you put any bare metal less than 12" from a combustible wall, you must heat shield that area, and check your work to ensure that the heat shielding is effective (the combustible wall's surface (paint, wood, wallpaper, plaster over wood, etc) should stay below about 160 F, or cool enough to touch).
Parts of your system will heat up more at different times, and the dynamics will change as you adjust flow and so on.  So please allow excellent safety clearances so you can be surprised intellectually, rather than catastrophically.  

Because the system may have irregular draft, your chances of a chimney fire are higher.  The word "rocket" does not confer magical protection against chimney fires, nor does any individual element of rocket-type design, especially when tinkering and experimenting beyond well-trodden paths.  The whole system has to balance itself in order to produce the smokeless fire that provides most of the safety margin against creosote buildup in the heat-exchange flues.

So again I would caution you to install the chimney(s) completely properly, according to manufacturer directions, including appropriate fasteners, fittings, insulation, shielding, etc.  Minimum 3 screws per joint, for single-wall.  Pipes warp with heat, and may be damaged by repeated re-configuration.  Sections that go through walls, roof, chimney, or are exposed to outside air should be properly insulated class-A chimney, and note the manufacturer's recommendations about whether to add further insulation by building a 'chase' around exposed outdoor sections.
Many people under-estimate the dangers of a cool chimney, which can collect creosote when fire conditions are not optimal.  When the draft of a rocket stove changes substantially, the risk of smoke and creosote is higher.  And once a creosote fire starts, there may be significant damage to even the best chimney.  House fires suck.  Please take all available precautions to prevent a chimney fire from progressing into a house fire.

Yours,
Erica W

p.s. Ernie comments that if you have that much space, and parts, why not do two separate stoves, with separate fireboxes.  The one firebox may never properly heat both benches, 42' is a lot of duct.  

(This also depends on your climate, heat loss rates, etc, but I would tend to agree with Ernie.  The cool end of a long bench is generally cooler, we don't see many of them anymore except for things like greenhouses/freeze protection.)

It may be worth considering if your configuration now would leave room to add a second firebox later.  Might be a small change, like shortening the 28-foot run, would let you retrofit a second firebox and barrel if needed later on, separating the original dual bench into two completely separate systems.  However you would not want to feed one firebox the exhaust from the other, or give the chimneys the option of dumping cool exhaust out a second hole into the room instead of up and outdoors.