Questions from 17 Days 1-3 hours outdoor testing

OK. Hello everyone. I am new to this forum and on my way to a RMH for our house I reached a point where I need to have some questions answered, and I hope to find these answers here.
I bought Ernies and Ericas amazing book as a pdf and red it at least twice. So much knowlege gathered there. Unbelieveble! I also rented their vimeo movie to add moving imagery to the tons of text in the book.

So I spent the last weeks testing my J-tube in my cortyard night by night and went researching for all meterials and parts needed around our villages here in Portugal and all looks good.
So If you would be so patient to read through my long quenstiair catalogue I would be more than thankful. I will be numbering the questions to mae answerin easyer. And feel free to also answer only one question, as all input helps at this point of my travel. So here we go:


First here is the CSA of my system: Our chimney pipe is 12 cm inner radius, wich resembles about 43/4‘‘, so I calculated everything fitting to this. I am also aware of the proportions 1:1.5:3 which in my case is 1:1.4:3.3 due to my refractory brick sizes. In measurements this is: 34,5cm x 48,3cm x 126,5cm. In inches this would be: 14‘‘ x 19‘‘ x 50‘‘.

1: My RMH burn unit is assembeled stacked / without mortar in the cord yard and insulated with rock wool and produces high amounts of soot in the heat riser during my last 10 days test firings. I am wondering why.  Could it be that the small amount of air entering through the very tiny joints / through the wool cools the burn down so that the combistion is not complete? The outside temperatures are around 17°C. Or is it maybe a fuel problem? Im am burning 100% stone pine, with the botanical name Pinus pinea, also called the Italian stone pine, umbrella pine and parasol pine. Is this fuel unsutable? All I know that this wood has lots of rasin. I was doing my test burns with dead wood of our stone pine trees, that where in the Portuguese sun without rain for the last 6 month and are probably dead for years so I dont think its wet.

2: For rocketyness, if we take as measurement how big of a part of the wood is actually burning, measured from the bottom of the feed tube upwards as all fuel is standing vertically, what resembles an optimal draft / burn / rocketyness? With me often more than the hight of the burn tunnel burns and I have the feeling that this is too much wood burning at once. SO my thought is, if there would be a stronger draft, the burn would be more restricted to the tips of the fuel, is that right? How rockety should the rocket be?

3: Is my other thought right, that when testing only the J-tube in an outside environment, that it should more likely overdraft, as it does not have all the friction that will be produced by the final setup with all the elbows through mass and out the chimey?The heatriser is the motor of the whole system right? So the mtor should be working really hard without and friction / resistance..

4: After being shure through outdoor testing that the overall sizes, measurements and dimensions of the RMH are right. Does my thought make sense, that on building the system indoors, the only direct and quick access to finetune the system is to adjust the gap between heat riser and barrel to control the rocketyness of the heater at full opening (overdraft vs. too little draft)?

5: What happens exactly if I see a splendid burn in the burn chamber, yet out the heat riser comes thick black smoke, linke an oil pipeline is burning? I can say, that one specific piece of fuel causes this smoke. Whats going on there? Later, operating the heater in our house I can not check the chimey after every piece of fuel I put in the heater, could one of these unknowingly "bad" pieces (and this one is quite a chunk) cause danger for us in the house or harm the heater?

6: With barrel I observed HEAVY condensation on the inside of the barel on my outside testings (here HEAVY means litte puddles and dripping water). Ok it was 17°C which intensifies the effect of condensation, yet I am wondering how harmful this water would be in extreme cases for the cob that is used for great parts of the heaters construction. Any thoughts on that?

7: I read Erniess and Ericas book, that a RMH basically is a device, that produces a controled chimney fire in the heat riser for total combustion of the fuel. Also in diagrams you see very often, that the heat riser is marked as the part of the RMH, where the gases are reburned. Here my question: Should there actually be flames all the way to the top in the heat riser? On my outside tests I had only two cases out of 17 test fires so far, where I could see some strange purble-red "soft"-looking flames come out of the heat riser (no barrel was used) and I thought: Heureka! I made it!. Yet, its 2 in 17 and all the other times the flames come some max. 10 inches around the corner of burn chamber / heat riser and they are of "normal" fire color, meaning that the flames burn much like in this great open side burn tunnel video of Ernie and Erica at min 0:37 ( [youtube]https://www.youtube.com/watch?v=30rPCRB5e-s)[/youtube]  Do I burn wrong, or is the controlled chimney fire / secondary burn overrated? How does the secondary burn look like or is it invisible?

8: I need to make a quite compact / slim design for various reasons in my mild portuguese climate. Therefore I thought to use a tall gas bottle like the tall one on the left of this image (https://ipemsp.files.wordpress.com/2010/04/botijoes.jpg?w=187&h=261) with longer dimensions and rather thick metal surface compared to a 50 gallon drum with big surface and comparibly thinner metal. The surface area of the gas bottle is about 56% of a 50 gallon drum. What effects could that have on the functionality of my design? Any hints?

9: What do you do, if the heat at the end of a burn is mot strong enough to turn the last big chunk of wood into red coal and it starts the smoke into the system, rather than burning clean into it?

10:  Why is my RMH test burn unit creating black coals almost at every burn? They always show up under the white ashes on clean up?


So you see there are a loooot of questions and most likely more will follow. Also I will open a new thead soon with detail drawings of my RMH plans from feed to chimney exit with all leghts and dimenstions. Stay tuned.

All the best and thank you for your patience.
Moritz

Hello Moritz, welcome to permies!,
It has been a while since you posted this so I am going to bump this back to the top in hopes that someone will try to answer some of your questions.

1) You could be getting air leaks which could cool your small system too much. That happened to my first mockup of an 8" J-tube, which is much more powerful and forgiving. The rockwool will have no noticeable effect on the seal.
The very resinous wood could also cause issues. If the system gets hot enough for total combustion, it might be okay, but any less and you would see lots of smoke.

2) No matter how strong the draft is (within reason), the height that is burning depends more on the loading style and character of the wood than on the draft. If you have good draft, the wood can be burning nearly to the top of the feed, and the smoke and flames will be sucked back down.

3)Yes, the J-tube alone should roar if it is working right. It will be much more subdued when covered with a barrel. You will get a much better feel for actual performance if you put the barrel over the riser, with a vertical chimney to add some draft. It has happened that a J-tube that seemed okay alone did not work well when brought inside to a full system.

4) Adjusting the barrel to riser top gap would probably be the easiest structural change, but you can partially cover the feed tube with a pair of bricks to adjust the draft on the fly. Too much draft is a less common problem than too little.

5) A smaller system is more susceptible to individual pieces of fuel causing issues. As yours is close to the practical minimum of 4"/10cm, you may just have to accept this. If your core is well insulated and formed right, you will have fewer problems. Really high temperatures combined with good turbulence and long enough time spent in the hot core will cause full combustion. This is harder to achieve in a small system.

6) The outdoor temperatures will certainly increase the amount of condensation inside the barrel, but a hot enough burn (and indoor conditions) will overcome this once the system is heated up. Lots of condensation is typical for a new cob-built RMH while the mass is drying out. Once it dries sufficiently, there is said to be a change in the behavior of the system, starting easier, getting hotter, and having less condensation.

7) I don't think the flames need to reach the top of the riser for full combustion. If there is more fuel burning at a certain time, you will see more flame. I tested a tiny 4" batch box mockup, and it got flames out of the riser a few times, but during some extremely clean burns (clean enough to comfortably breathe the exhaust), the flames were not coming near the top of the riser.

A standard 55 gallon drum is appropriate for an 8"/20cm system, so a smaller barrel/vessel should be fine for a smaller system.

9&10) A small system will have issues with the fire getting hot enough; I don't think there is any way around this except to use highly insulating materials to minimize heat loss. Dense firebrick next to the flames may, once it warms up, retain heat enough longer to help the last coals burn. This would require extreme insulation around the firebrick core.

Hi everybody.

Moritz, may be a good idea, with such a small  pipe, would be to use a batch box, for a little more heating power.

http://batchrocket.eu/en/

WOW! Somebody answered!
Thank you Glenn for your patience.
And thank you Miles for the important bump.
And thanj you Santamax for your batch box idea.
I already gave up to wait for answers and as ironically as it seems, my little mini RMH got done yesterday. Yes, yesterday on the day these beginning questions where answered. Isn't that a strange coincidence? This morning the house still has a humid smell, from the water that evaporated out of the bench last night. And our little dragon went insane! Its almost to rockety.. It went from could, first start to clean smokeless burn in 15 minutes and the bench became nicely warm. If you're interested I would post a few images and tech-specs.

All the best.
Moritz

So glad to hear it! Please do post pictures and description of your build. A successful small build is an important thing to have documentation about, as many people want to do it but often fail the first time.

Glenn Herbert wrote:So glad to hear it! Please do post pictures and description of your build. A successful small build is an important thing to have documentation about, as many people want to do it but often fail the first time.

OK Glenn. I will try to put together a nice summup with images and specs. Stay tuned.

Glenn Herbert wrote:So glad to hear it! Please do post pictures and description of your build. A successful small build is an important thing to have documentation about, as many people want to do it but often fail the first time.

Sorry Glenn. I still did not find the time to make a little write up of my build. There was work, christmas, new years and the start of my online PDC with Geoff Lawton. Exciting times!

For now I have this foto to share and two lines to say:
After one month of heating with our new stove, we are using 1,5 buckets of these buckets you see on the foto full of stickfuel and quite tiny chopped wood per day. One small fire in the morning to get cozy and strted (1/2 bucket) and one longer fire at night (full bucket) to charge the battery for the night. The living room has a constant temperature between 20,5 and 22 °C and even upstairs with an uninsulated ceiling (1,4 cm thick woodplanks and very airy terra cotta roof pans over the attick) the temperature never drops under 17°C after a long cool 9°C night outside. Our dragon could not work better!

Glenn Herbert wrote:So glad to hear it! Please do post pictures and description of your build. A successful small build is an important thing to have documentation about, as many people want to do it but often fail the first time.

Hello Glenn. Sorry it took me some time, but here is the full post on our RMH built:  https://permies.com/t/63422/RMH-gateway-Permaculture

All the best.
Moritz