my design of a custom rmh for my existing fireplace

here are my sketched for what im considering.. my fireplace has a ash tube down the back that also has a clean out door on the outside of the fireplace.. im thinking of building a box lined with cob and filled with granit rock and sand around the exhause tube which will then exit out the back side of the box then down into the ash pit and outside to a 5 ft section of stove pipe attached to the exterior of my fire place.

thoughts opinions?

Brian Hall : Welcome to Permies.com and a big Welcome to the Permies Rocket Mass Heaters and wood stove forum Threads, With over 19,000 fellow members world wide
you can expect to come here 24 / 7 and find someone who wants to talk about what you want to talk about, they will have widely ,and possibly wildly different outlooks on
your subject, because you can think outside the box I think that you will hold your own, and we will all remember your first few posts here at Permies and our sister site
Richsoil.com

The easy one first, Your Cob Bench can be a totally free standing monolithic structure, after you have tested the whole system out doors, it works, and you have constructed
the Rocket Burner part, and have figured out exactly where you want your stove pipe for your horizontal chimney to run, you want to seal your duct work with tape, paint all
of the duct work with Clay Slip and then apply the Cob to the Horizontal pipe to make contact with ALL of the pipe, this layer of Cob is what protects your pipe and transfers
the heat energy into your bench, hopefully with zero air spaces so no weak spots and great conductivity. Then as you find places for it the granite or other rock can be added,
and can form a decorative border for your Cob Bench/Built-in-furniture ! In effect you said it backwards from the easy way !

I really want to express to you that you will have a much happier build with more of a chance at success if you do not try to build any system smaller than a 6'' system, for a
first build that is almost mandatory, so my next question to you is how large is the clean out in the floor of your fire place and also the sizes of your interior connections and
the exit from the back of the chimney, and how many 90 degree bends will you have, 1 to go from horizontal to descending vertical ,1 to turn back to horizontal, and after
you exit the chimney base, a 3rd elbow to return the flow of your exhaust gases to a new vertical chimney, all in about 6'.

Then because these bends will act like a p trap under the sink, to use this system you will often have to go outside to remove a Clean out Cap at the base of your chimney to
build a small fire in the chimney to get it to draw ! Then return inside to start your fire in your Rocket Mass Heater RMH!

Just three more simple things to deal with and you can work on your sketch again, I want you to go to - ' village video.org/products/rmh/scenes/ ' -and watch every
video clip, paying attention to how they protected the wood floor from excess heat, the need to brace up the floor as you add 3 tons of cob bench, and in the video clip
about getting your house ready for your stove, the part about stack effect and what will happen with your short chimney when you open an upstairs window !

Think like fire flow like a gas, don't be the Marshmallow ! As always, your comments and questions are solicited and Welcome ! PYRO - Logically big AL

my fireplace rear vent is 4 in by 7 in ill have to go square.. ill stay square on my exterior piping perhaps aswell...

and i may add a thermostat controlled fan outside to push air up the pipe creating suction untill it comes up to operating temp.
and i dont have an upstairs...

Brian Hall : Ianto Evans found a special place on the west coast, with a very stable micro climate, in a valley that enhanced the very stable wind direction year-round!

Then he build his houses to that location, none the the structures build by Ianto in the Intentional community called Cob Cottage and built by him are over 15' tall, I would
strongly suggest your vertical chimney extend above your roof line aways ! If you plan on building it that way and budget for it, and if your location for your outside chimney is
perfect you too may be able to use less. It will take a couple of heating seasons to tell if you were that Lucky.

Obviously you want to make this build attempt, The standard recommendation is to build it full scale out doors, and when you can get it to work there, bring it inside and try it !
My strong recommendation, based on the large number of failed sub-six inch systems, especially in a First Build still stands, I personally have had no need for that small a
system! While borderline due to your systems use of elbows, it just squeaks through on interior Dimensions (as you have found and reported so far !)

Many people have reported good luck with using booster fans, especially if they did not need to run them continuously, Living in my location I find it necessary to have plans
for winter storms, and power outages, I have a plan and a back-up plan, and a back up plan to that plan, a 12 volt system with 2 car type batteries and a trickle charger
should easily get you through two days of forced off-grid use !

You Do not want to go square, an effort to make your interior passages as smooth as possible with a reline/patch of any rough places with a combination of a clay/sand mortar
mixed with Refractory Cement to allow you to have the smoothest walls possible, with the largest dimensions possible will put you inside the Constant Cross Sectional Area
that every one needs to follow. Whenever possible it is recommended to use round pipe because of the increased turbulence and friction inside your pipe that only increased
height of your external chimney can overcome !

Have you been to rocketstoves.com for a PDF Copy $15 of Evans' and Jackson's great book Rocket Mass Heaters there is STILL No other book in any language with more Rocket
Mass Heater information, (and I don't make a dime !) This will allow you to understand the need for Constant Cross Sectional Areas through out your build, and the rather strict
ratios that must be followed regarding the Depth of Feed Tube to Heat Riser height, And the Burn Tunnel/Combustion Chamber length and The Heat Riser height, And where to
measure from, it will also allow you to use a common language to describe what you have/see to other builders !

Good luck with your build, come back here often, All Of your fellow members would like to see another Successful RMH build ! For the good of the craft !

Think like fire, flow like a Gas, Don't be the Marshmallow! As always, your questions and answers are welcome and Solicited ! PYRO - Magically Big AL !

Hi Brian.

Check these links to read up on my design and implementation of a 4.5" system that I installed into my existing fireplace:

experimentation phase
you'll see in this one that I did a full build of the final system's critical components outside before I installed them into the fireplace.

implementation and modification phase


I simply directed the exhaust up the existing chimney.

nice chris!