Dual 6" duct in an 8" system?

Given that used 6" duct is about 1000x as available as 8" on the freebie market, and the former has about half the CSA as the latter, could one not just run two parallel 6 inch pipes ( simple, single run, no elbows) out of the manifold and through the mass, then both outside? Would the increased surface area necessitate a slightly lessened maximum duct run? Thanks!

With essentially identical runs, you could probably make this work well. I think you would need a slightly shorter run, not because of increased surface area so much as increased friction. A smaller diameter with the same velocity (since cross section is the same) will have more friction.

You will want to recombine the runs at the end into an 8" chimney. Just dumping the exhaust outside without a chimney only works reliably in very specific circumstances, and most of the world is not going to have those constant prevailing winds that are needed.

Derick Greenly : Past experience with identical ducting for delivery of cereal grains, ( Corn, oats, wheat, barley ) and the further handling of the resulting Animal Feed Streams

( dairy, horse,chicken, rabbit ) to bagging stations taught me that even with visibly identical ducting there will always be a difference in flow rate, this could be determined by

ear, or by time to fill identical bags at parallel stations !

Therefor I Can promise that regardless of intention, one run of ducting will always flow faster than its visibly identical neighbor! Exactly how this will present itself when the material

Flowed is only governed by Fluid Mechanics ( The flow of a gas through ducting can be considered roughly as having similar characteristics to fluids flowing through Ducting ! )

Will be determined by your D.I.Y. skills !


Much more important will be the way you create the two Manifolds or distribution boxes *1) where the rapidly falling hot exhaust gases turn at right angles to flow horizontally through

your Cob Thermal Mass Bench and then again 2) where you recombine the (2) 6'' runs of ducting back into a single 8 '' Duct ! Not only will what you do here potentially set you up for

Markedly different flow rates for your Ducts, but it will make the proper placement of clean outs that much harder. it will also increase the complexity ( Read Co$t ) of that portion of

your build, 6'' Elbows together with reducing "Ts" 6'' by 6'' by 8''are not cheap, and will also further reduce the distance you can flow your hot exhaust gases through !


Just my two cents - is this related to your earlier post regarding a RMH in a spawn house ?

*Your 1st Manifold can be created as a chamber placed along beside the bottom of the RMHs barrel, and there is really no size limit on the space that your Manifold can contain -at least

in comparison to its primary function of turning that column of vertically falling exhaust gases and channeling those gases out through your Horizontal piping(s). This and the manifold

at the final vertical chimney can both be freehand created out of Hardware cloth and Structural Cob ! For the good of the craft ! Big Al !

Derick, another cheap solution

http://s65.photobucket.com/albums/h228/mremine/NYC%20Rocket%20Stove%20Build/

Derick Greenly wrote:Given that used 6" duct is about 1000x as available as 8" on the freebie market, and the former has about half the CSA as the latter, could one not just run two parallel 6 inch pipes ( simple, single run, no elbows) out of the manifold and through the mass, then both outside? Would the increased surface area necessitate a slightly lessened maximum duct run? Thanks!

if you indeed have an bunch of or unlimited supply of "used 6" duct" and it is duct and not just sheet metal tubes i would say open them up at the seems where they snap together and combine two 6" ducts together to make a single 12" system or at least a single 12'' exhaust system and chimney stack if you still want to keep your 8'' system. no splitters, joiners or adapters needed, problem solved.

All very chewy food for thought. But two six inch pipes have a csa around 60. Is combining into a single stack a necessity?

Derick Greenly wrote:All very chewy food for thought. But two six inch pipes have a csa around 60. Is combining into a single stack a necessity?

best thing is build a mock unit outside and test it. sounds like you have already made your decision... go for it brother.

Satamax Antone wrote:Derick, another cheap solution

http://s65.photobucket.com/albums/h228/mremine/NYC%20Rocket%20Stove%20Build/

wholy 55gal barrels batman

If you can access the split to influence relative flows so you get equalish airflow, or if you don't care about the two 6" runs putting out equal heating, then you can just split the run as you described. But if you want a longer run, the two 6" ducts combined with the splits will have distinctly more friction than an 8" run or especially a "merged" 12" duct run.

F Styles wrote:

Derick Greenly wrote:All very chewy food for thought. But two six inch pipes have a csa around 60. Is combining into a single stack a necessity?

best thing is build a mock unit outside and test it. sounds like you have already made your decision... go for it brother.

Very astute! Head like bull.

charge ahead brother... good for you. let us know how you made out with your idea.

have you seen my NEW idea on how to build a RMH core? the proof of concept i build was just the outer part of my heat riser so i know it at least works for what i have done and very well. but i digress. The concept is like this put together standard stove pipe in your favorite RMH core shape like a J or L or whatever, then take your favorite high temp fabric like rock wool or fiberglass or other flexible porous material and soak it thoroughly into a Refractory cement slurry and do a thick paper machete like coating around the RMH heater core you created with standard stove pipe. then put a layer of flexible insulation like rock wool or such and then do another thick layer of slurry soaked high temp fabric over top of that to create your RMH core. the stove pipe will eventually burn out and leave your highly insulated hard refractory cement core in the custom shape of your choosing.

how that for an idea eh?

check out my pics of that idea here. first RMH cooks, bakes, and heats water custom design.