Completed my first rocket stove today and the fist burn was disappointing. The fire never seemed to 'take off'. The wood burned slowly and was always smokey. I experimented with the baffle in different positions but temperatures out of the flue never exceeded 400 deg I'm hoping you might have some tips or suggested modifications. Attached are pictures of the stove.
3" Square tubing. 30" tall flue
3" tubing welding into a 7" wide old fire extinguisher filled with perlite.
Angle iron pot stand, (plan on trimming to increase airway)
Welcome to the boards and welcome in rocket heater land. Satamax waked up earlier so he was able to point out metal isn't a good material to use in a well-running rocket heater. There are more reasons why you little stove won't work now and it's an open question whether it ever will.
1. It is made of metal, in a properly running stove this will burn away surprisingly quickly. Even stainless won't hold out for a full season. The steel will not melt but due to a hot, oxygen rich and carbon frugal environment it will show a phenomenon called "spalling". Every burn flakes will form which can be peeled off by hand once the thing is cooled down.
2. It's tiny, I've done a 4" J-tube myself and only with a couple of bells and whistles the thing could be brought to good behaviour.
3. The horizontal end sticking out in front of the feed tube shouldn't be there, and the air inlet at the end is utterly wrong. I'd suggest you cut that off and close it tight.
4. The feed tube (the short vertical tube) is far too long for such a small system, I'd suggest you cut that in half.
5. The heat riser (the longest vertical tube) is also too long. Try to do the length proportions feed : tunnel : riser something like 1 : 2 : 4. Measured in the heart of the tube, I hasten to say.
6. The bottom of the feed tube should be insulated as well, just for experimenting you could use rockwool for example.
When you've amended all this, you will see a stove which is working the right way all of a sudden. But... the ash build up is quite large probably and the metal will be eaten away at an alarming rate. In case you want to know more, go to rocketstoves.com and download the third edition of Rocket Mass Heaters book. This book is the absolutely must-have when building these things for the first time. Also the second, third, and fourth time for that matter.
I have nothing to add but that RMH...if they have to be called that...really need to be looked at for what they are and have been for over 4000 years...a horizontal run masonry heater with some up drafting...all of which are never (nor should or really can be) built from metal...
posted 5 years ago
Wow! You guys are fast!
Yes, you have caught me red handed copying the yahoos on Youtube. Having a welder and scrap metal laying around left me vulnerable to their evil ways;)
I can get really wordy on my posts so I kept my original plea for help short and concise by skipping the full story. This stove is intended to be portable and for cooking food only. It will probably get 10-20 hours of use a year. I'm fully aware of the oxidization steel goes through when subjected to red hot heat (1500-1700deg) for prolonged periods of time. This contraption is by no means a long term solution.
Peter thanks for your detailed response. Your thoughts were exactly what I was looking for. I suspected the vertical feed tube was too tall but wanted to consult with more knowledgeable people before I start cutting. It's concerning to hear you had only marginal success with a similar yet bigger design. Can you explain further how my air inlet is all wrong? I figured that if my feed tub was jammed with wood there would need to be another route for combustion air in to enter...? My first test run burned slightly better when the baffle was open 1/3. It feels counter intuitive to block that path for air. Is there really such thing as too tall of a heat riser? That definitely goes against everything I know; should of bought the book.
All the air for combustion need to come from above. When a rocket heater is running well and air is coming in low you run the risk of fire which is creeping out of the feed. The lower part of the feed ought to be insulated in order to get it to work the right way. And yes, there's always a possibility of a riser which is too long. Longer risers tend to contain more mass which in turn need to be kept hot, so the heat used for that isn't available for somehing else. The ideal rocket heater core is extremely insulating with no mass at all.
I forgot something about your lower air inlet. Even if you close the valve real tight there's still a negative effect. The air stream is running into the feed tube to the tunnel, bypassing that extra horizontal end. The velocity of the gases will create a slight vacuum in there so it will act as a drag factor. This will limit the velocity of the gases passing by, something like stamping down on the accelerator while the hand brake is still engaged. That's the sole reason why your contraption appeared to run better with the valve slightly open.
Again, please cut that end off and close the hole, this air inlet will cool down the fire which will result in emitting smoke. A well running rocket heater won't emit smoke, at all.
posted 5 years ago
Followed your guys advice by chopping things down and prepared to insulate the entire feed tube and horizontal section. See Pictures.
Did a test run with no insulation and the stove produced twice the heat as before with some not so dry pieces of 2x4. The entire assembly got very hot so hopefully insulation with increase performance further.
I filled up the entire lower portion of the stove with a castable refractory-ish insulator. 1 part furnace cement, 2 parts clay and 8 parts perlite. It will be several weeks before refractory is dry. I will later fill the top half of the stove with loose perlite.
Will keep you guys posted and thanks again for your help.
would rock-wool insulation hold up in there? it would be lighter than the refractory mix. i also think the steel expansion would crack the harder mix. kind of questions/suggestion hybrids, since im still wet behind my rocket ears.