Why the smoke

[mayapinion]

I received the Evans & Jackson book on Rocket Mass Heaters Wednesday, read it, got all the parts together Friday & Saturday, and built a mock-up today (the book suggests assembling the masonry to see how it works out before actual assembly).

Even when it began to draw the fire into the burn tunnel (quite impressive), smoke still came back out of the feed tube. Do you suppose that is because of all the little leaks (dry assembly) and lack of barrel, insulation, etc?

[Johnjmw]

If I understand the system (I will be building one too soon) there is sometimes an issue with the feed tube not having enough draw. The solution I thought was to partially cover the feed tube with a brick or two to see if it corrects. There was a suggestion as to why this is happening but I cannot remember what. Something like the difference between the heat riser hight vs the feed tube hight.

Good luck

John

[Donkey]

There are a couple sizing ratios to keep in mind..
Basically, the burn tunnel and feed tube should be as short as possible and the heat riser should be as tall as you can reasonably get it.. The minimums set in the book seemed like a good idea at the time and as far as I can tell they still hold.. They are a good compromise, meant as a guide, for the "should work every time" recipe for the first time builder..

Likely your mock-up was leaky (as you've said) and cold. Probably a bit wet as well. Try sealing it up a bit with handfulls of sandy mud. Then build your fire and just give it a little time to dry properly.
Yes, insulation will help but a simple brick assembly should work fine (by it's self) without any extras as long as it's all sealed well and dry. If that won't do it, THEN change sizes, lengths etc.

Oh... and, if it won't work without the barrel, adding one will only make it worse.

[mayapinion]

Hi Donkey, thanks for the reply. I actually figured out what was wrong before I read your useful message. At first, I put a brick over the edge of the feed tube to try and cut down on smoke, and this made things worse - because - it pushed the wood away from the mouth of the burn tunnel. Once I moved the brick and packed the wood a little tighter, it worked perfectly. Once it heated up, I actually tried to make it smoke and couldn't.

The sound when it got going was great, and when the smoke emissions cleared - to almost pure heat - I was grinning ear to ear.

No pun intended - I was stoked. I really wanted this to work. It is a beautiful idea. I've been a mason for twenty plus years, built many conventional fireplaces (most recently, the Rumford type.). What I love about this concept is the simplicity, light weight, and of course the efficiency.

I'm planning a futon bed unit. I'm putting the rocket at the foot of the bed behind an 8-10" thick masonry (probably adobe) wall, and running the guts in three channels through the bed. I am wondering about building the flue with red brick and mortar - like a Russian stove; then covering the top with earth material.

Any thoughts would be appreciated - Like... do you think I would get a vastly different heat from side to side if I made the 18-20' a continuous run? If I Y'ed the flue in three parallel runs - tripling the volume - would that slow down the movement and provide time to absorb more heat?

I read somewhere the suggestion that y-ing the flue in a floor might be a good idea - so I was just considering this possibility in my plan.

[Donkey]

Hmm...
Well, a continuous run pipe does tend to create a fairly uneven heat. Easy to predict the pattern there..
I know of a few branched systems. Some have worked well (quite) though one that I can think of didn't ever work properly. Unfortunately we never figured out what killed that stove, weather the branching or something else.. ??

Up to now the rocket rule has been continuous cross sectional area throughout. It seems though that there are exceptions. There are pressure/volume tricks that are beginning to show themselves. Ernie has a trick where he opens the volume just behind the barrel. Apparently, it can create an increase in flow. I've touched on tapered heat risers with encouraging results.
Though I've also killed a couple test stoves by putting too large of a widening in the wrong place, mainly just before the heat riser.
It could be (cause it sounds right) that what you say is a good idea. More volume in the storage area(s) will slow things down there and should allow the heat to sink in. As long as the slowdown doesn't propagate backwards and slow the whole thing down. You will most likely need to reduce back down to your original volume on exiting the storage area.

Good idea, worth the test. Do it and let us know.