Rocket stove backdraft

Guys,

Did my research and built my stove with refractory and perlite. About a 7" burn stack with 1 3/4" gap from barrel head. It's in my basement so 6" pipe vented out an old window to 2' above ground level.

First fire had serious backdraft before it started to rocket. As the fire died down, I had black smoke bellowing into my basement. Wife was not happy and I was very sad after all the work I put into this!

I read you could have a vertical vent, so wtf happened!? Also, if I don't open my exposed basement door, I get no draft.

Oh! Had an old 4" dryer vent I'm using for supply air, also have clean out opening in front of feed chamber now covered with Fire brick.

Any suggestions on draft other than a 50' chimney will be much appreciated!

From my limited knowledge Gained from experiments and a lot of reading: "Once the barrel goes on, the chimney must go up" Simply stated, you can build your stove 100% as per specs, without a chimney it wont work

Dan, so many variables it's hard to say without seeing the rest of what you did when you had it burning. No mass? Dry internals or wet? Temperature and weather outdoors when this was going on? Location of stove? Etc, etc, etc, etc....

But, to start with what we do know, the 4" air inlet and lower clean out are both going to cause that type of scenario, and together they are a recipe for disaster. I would permanently remove both of those features, tighten up the core in that area, prime the chimney with a small fire in the base of it, light your J, and restrict the top of the feed by about 1/2 by covering it with a brick. I bet it will work well.

The smoke when it went out indicates you have strong reverse draft due to the install location. I imagine your space is drafting better than the chimney, so the chimney is acting as a fresh air inlet. That's not a stove problem, but you will have to solve it before things will work well. Good luck man!

Thanks guys!

I used perlite in the refractory mix which is still moist, so just did a couple short burns.

I did cut off the intake 4" supply and narrowed the feed chamber and it did rocket very well. Before starting the fire there was serious backdraft.

Guess I'll have to go with a chimney... Thought I could avoid it but way too much backdraft.

From the firebrick splits at the base of the feed tube, it looks like you have a 7" x 7" square feed tube and at least 7" diameter riser. These will not work with a 6" exhaust unless it has exceptionally good draft; with poor to no draft, it can't work well. The feed tube and burn tunnel need to be the smallest cross section if there is any variation in system diameter. You need to either cut down the combustion core to 6" dimensions or increase the chimney to 8".

The refractory still being wet will also reduce your performance; at least that part will improve as you burn and dry the system out.

Ok so bring the feed and combustion chambers to 6" and go with 8" exhaust pipe? Not sure that will fix backdraft but it does rocket as is...

Looking into 6" chimney liner to vent stove and hot water heater. Just not sure of moisture in chimney?

Here's more pics

The size modification is not both, it's either/or. What is the diameter of the riser? It looks like a sonotube form, and I haven't seen those smaller than 8" nominal or 7 1/2" actual. Unless you have a very modest heating load, you are probably best off handling it as an 8" system and getting chimney material to match. A tall well-insulated 6" chimney might work if there is little horizontal run and very good flow patterns, but I would hate to advise it from long distance.

the potential problematic things-

wood burner at the basement
chimney shorter then the building roof
chimney run outside the building
chimney not insulated
the dryer air inlet enter the burn tunnel
the leaky cleanout
wet materials
chimney diameter too little
narrow manifold
narrow top gap
opened windows (or other leaks) at the upstairs.

maybe you can go with some of them but obviously not with all of them

Forget about the fresh air supply. That's known to give more trouble than what it's worth.

Look into "transition area" and plenum.

Increase your chimney height!

All good points guys!

Did some searching on drafting issues with rocket stoves and came across this exerp... verry interesting!

ROCKET STOVES – CONTROLLING DRAFT
http://stoves.bioenergylists.org/stovesdoc/Ogle/stovedraft.pdf
(Variations in chimney height and air flow
openings in a modified “rocket stove”.)
by Damon Ogle
CONCLUSIONS AND RECOMMENDATIONS
Controlling draft at the exit point of a “rocket elbow”
is much more effective than trying to do so
from the point at which
air enters the elbow.
It is possible to produce high temperature exit gases (above 1200 F) and maintain their
temperature through the length of a long combustion chimney.
High temperature and “dwell time” by themselves
are not sufficient to produce dependably clean
combustion. It may be necessary to further mix the exit gases (either by turbulence or by other means) in
order to aid combustion in rocket stoves.
Higher temperatures and cleaner burns are sometimes obtained by partially blocking the flow of
gases through a rocket stove. It may be possible to use mixing devices or jets of air to achieve this
blocking effect and improve combustion at the same time.

I presume you noticed that this was for L-shaped rather than J-tube combustion chambers. The J-tube, aside from being self-feeding, has been found to be more efficient in combustion. Its design produces the turbulence noted as desirable, especially when more recent developments like the P-channel and tripwire are included.