New Rocket Stove - not drafting too well, Help!

Hello!
I could use a little advice on my rocket stove.
Attached is a drawing of my current construction.
The drafting isn't working so well. My guess is that the Flue Pipe is not big enough.
Please comment!

Thanks much!!!

Chatch

I have been researching rocket stoves for a few weeks now in preparation to build one. It looks to me from your drawing that you are correct. Your secondary burn flue is too small. The best designs that I have seen specify a secondary burn chamber that is BIGGER than the firebox. So if there is a way to decrease the size of your firebox I think that would help. Of course, I haven't built one yet so I may not know what I'm talking about!

The tunnel should be not smaller rhan every other dimension in the system. But the most likely shortcoming is probably the top gap, the space above the riser. The riser is quite close to the barrel side, out of center. This could very well work as intended but the top gap is too cramped so you'd better make the riser a bit shorter.

Another potential problem is the riser which is as I see it made of stovepipe with insulation around. This stovepipe will fail miserably when the stove is working as it should. Any metal in the core is doomed as Satamax, a prominent meber of this forum would say.

Hi charles; Do you have a copy of ianto evans book rocket mass heaters -version 3 ? If not you should buy or download one. Looks to me that youre J tube is sized to be an 8" and you have a 6" riser and mass pipe. It will not work that way. Scott is also correct in that your horizontal transition area is too small. How dry is your cob ? Cob can take weeks to dry & heat up. We need more information & pictures. Add pictures by clicking the attachments button in the lower left next to options, then open your file and add one picture at a time.Three are allowed.

Your firebox cross sectional area (~56 in^2) is bigger than your cross sectional area of ducting (~38 in^2).

The firebox cross sectional area should be the smallest in the system. Perhaps you can bricks in half to get the small cross sectional area (4.5 x 4.5 =>20 in^2), maybe put the bricks on their side and then bring the sides in to only use 7 inches of a full brick (4.5 x 7 => 31.5 in^2).

Hope this helps.

Scott H. Please take a look at the comments I made to John Littles thread First Rocket Build - very pleased in the Rocket Mass Heater Forum Threads

Building from the sketch you show, you will have Similar potentially dangerous consequences!

here is the Link :

https://permies.com/t/41349/rocket-stoves/Rocket-Heater-Build-pleased



I would also plan on making the Transitional area bigger with a larger opening into the start of the Horizontal piping of the Thermal mass, gradually tapering down to that size
pipe

This is a common trouble area and bigger , even 3 times bigger is better, this area is shaped free hand and can use scraps of chicken wire or hardware cloth to get the curves
needed for this space !

Hope you find this timely and helpful ! For the Good of the Crafts Big AL

Thanks to all responders!
A few comments:
1. I'd like to try to keep the firebox as-is if possible. It's really built like a tank. Outside of the Cobb layer surrounding the fire-brick is an extra layer of reinforced mortar mixed with clay and wire mesh. All of this has been curing since early October. I didn't try firing up the system until 11/14.
2. Hopefully I can focus on adjusting the flue and the piping which passes through the thermal mass to the outdoors.
3. Also, I plan on getting the 3rd edition book mentioned and doing a little more studying.
So far I've been working on this project for 6 months - an hour here or there, when time allows.
From my understanding is that the flue pipe could be enlarged to to match or exceed the cross sectional area of the fire box. Right now it's at 28 SI and the Firebox is 56 SI.
Also, the distance from the top of flue to top of 55 gal drum is 2 inches - this gap needs to grow? I suppose I'll find that in the book.

The steel double wall flue is comprised of two pipes - 6" & 10 schedule 40, w/ a donut ring welded together top and bottom. On top, there is a small opening to allow the interstitial space to be filled with vermiculite.
There is no doubt that eventually the steel will degrade. However, at approx. 1/4" thickness, I think it would last for a while.

Charles; To keep your firebox as is ,you will need to replace your riser ,(the metal was doomed faster than your thinking)consider a cast fireclay/perlite riser. enlarge your transition area and increase your piping size (look into half barrels as bells ) The top gap at 2" is a good place start. Ernie & erica wisner (Leaders of this forum) are about to print a new builders guide soon ,you may consider getting a copy of that as well as ianto's RMH book.

Charles Hatch : Many people prior to you have built up around the firebrick of the combustion core with a wide range of insulating materials before adding a Layer of
Structural Cob and had success with it,. Your particular system will come up to maximum temperatures and maximum efficiencies slightly slower than one that has a
layer of insulating materials between the Fire brick and your outside Structural Cob ! It will be interesting to see what temperatures you can record !

All in all, you will probably have to go with the size of your final vertical chimney ! Any reshaping of the internal dimensions of your Burn tunnel should favor taller over
wider !

A good plan for rebuilding your Heat Riser is in your future, The iron/steel used in your build will fail so rapidly you will think it willfully committed suicide !

You may be interested in doing a google search for ''High Temperature Hydrogen attack'' and/or Hydrogen/steam Embrittlement !

Good luck ! Big AL