Overfuelling problem

[martinpolley]

So the rocket powered earthen oven is pretty much finished, except for the dome insulation and prettification. I've fired it a few times now, and it works pretty well. It draws nicely, it makes the right noise, and after a few minutes, it burns nice and clean (unless I overload it, in which case I get black smoke).

But once it's been burning for a good while (a couple of hours), and there's a good bed of hot coals, it seems I can't put more than a couple of small pieces of new wood in there without it smoking like crazy. Which means whatever is in the oven gets all covered in soot. Not very appetizing (or healthy either).

I've tried restricting the amount of air going in the feed tube with firebricks, but it doesn't seem to make much difference.

I'm mostly using pallet wood and very dry cypress. The other kinds of wood I can lay my hands on are citrus and avocado. Maybe olive too.

Am I doing something wrong here? This is my first time building and using a rocket stove, so I don't know if this is normal…

[satamax]

That's usual, ember bed problem.

I have the same problem on some J tubes. So i guess it's a J tube! Tho, the smoke is unusual.

The cure is either to make an ash/ember pit, not realy usefull, as it just delays the occurrence of the problem.

The other usual solution is rock solid insulation, i mean soo much insulation, that embers don't form. Which is not that easy.

The last solution, increase the draft.

[martinpolley]

Hey satamax. Yes, it's a j-tube. I forgot to mention that tiny detail

I do have quite a lot of insulation, around all of the J. So as well as around the heat riser, I have insulation on all sides of the burn tunnel, including the top. I used perlite/clay slip, except under the burn tunnel—there I used vermiculite, as that was all I could get at the time.

As for increasing draft, how would I do that? The heat riser is a bit on the short side—any taller and it would have made the oven floor too high. Anyway, making it taller now is not practical.

The oven doesn't have a chimney, though I can add one if it will help increase draft. Currently the exhaust just vents to the air below the level of the floor.

So what do you think? Add a chimney? Or is it something to do with how I'm using it? Maybe I should be restricting air more? Will that keep the burn hotter?

My aim here is to try and get as much heat out of the wood as possible, as fast as possible, to get the oven up to temperature as quickly as possible.

[satamax]

Add an insulated chimney, with the intake bellow floor level, i think that should be good. For your insulated chimney, two pipes with rockwool in between should do the trick!

[wiscojames]

Martin -
I have been experiencing the same sort of thing with my 6" batch box pizza oven. Pretty sensitive to overfueling. Last week I experimented with a fan that I scavanged from a bathroom exhaust. I have a chimney at the end of a 3-4 foot bench that the hot air passes through after the oven. It's short, so it doesn't really improve the draft. But the addition of the fan worked amazingly - until the fan motor burnt out. I think the exhaust was getting a bit warm, and I left it on too long. I am looking for a replacement fan. It really helped get through those smoky moments. Of course, dragging an extension cord to your oven might not be the most convenient thing. I guess a small battery powered fan might work...

[martinpolley]

satamax Thanks for the advice! Is the insulation critical? How tall do you think it should be to get the desired effect?

wiscojames Yeah, I remember the chimney from the photos you posted. A fan sounds like a solid idea. Electricity shouldn't be a problem—anyway I need to run a cable there, because there's no lighting in that part of the garden.

I think I'll add a chimney first and see if that helps. If that's not enough, I'll see if I can get hold of a fan that can handle the heat.

[satamax]

Yep, having an insulated chimney is better. Even if it's two tubes separater by air, and the end between the two blocked.

I'd shoot for 3m (10ft) above the oven floor.

[martinpolley]

Thanks Max, I'll see what I can get hold of around these parts

[martinpolley]

wiscojames Just a thought: maybe a fan from the automotive world would work. Something like a radiator fan. I think some use a pulley and belt, so the motor could be off to the side, away from the heat. Could be worth a trip to your local junkyard

[martinpolley]

After adding insulation to the dome (about an inch and a half to two inches of perlite/clay slip--I will probably have to add another layer), I did another test burn yesterday. It's still problematic. But one thing I did notice was that with the oven door open, smoke was much less of a problem. This gives me hope that a chimney will indeed help.
But in the meantime, I can either run it with the door open, which means I can burn lots of wood at once, but most of the heat goes out the door, or run it with the door closed, which means I can only burn a few sticks at a time, so it doesn't get hot enough that way either 


I'm not sure I can afford insulated flue pipe. Not many people have wood stoves here in Israel, and all stove-related stuff seems to be imported, which makes it pretty expensive. So I'll go with the cheapest stuff I can find and see how it goes.



(BTW, if anyone's interested, I have a bunch of photos of the build on this Pinterest board.)

[satamax]

Martinpolley, cheap way to make your chimney. Two HVAC tubes, gathered from the scrapyard or else. Filled with whatever is convenient. Sand, rockwool, glasswool. Gaps closed at the ends and insulated only by air. (the gap should be about 3cm for best insulation) I did an insulated chimney by wraping rockwool around a tube once!

[martinpolley]

satamax This sort of thing? (It's galvanized steel.)



(From here.)

[satamax]

Yep, i've used that kind of stuff.

Some spiral too!



And also aluminized stove pipe.

en.wikipedia.org/wiki/Aluminized_steel

Whatever you can find in a skip.

Even galvanised sheet metal bent to form a tube does the job. Hold it with wire. If you can't bend it round, bend it square with two pieces of wood, clamps and a hammer.

[martinpolley]

Thanks Max. I know they sell the spiral stuff here, but I don't remember ever seeing any in a skip. I was thinking of bending a steel roofing sheet into a rough tube shape if I couldn't find anything else—I know where I can find some used sheets. But today someone told me about a place in the fields not far from here where there's a whole bunch of discarded metal pipes (most likely irrigation stuff). I'll go and take a look in the next day or two and see what I can find

[martinpolley]

Today I found a bit of old irrigation pipe in the fields. Which is all well and good, but we didn't have an elbow to connect it to the oven. So my brother-in-law Yair chiseled a hole in the end of a CMU, which we put on top of a second CMU and a few bits of wood to get it to the right height so that the exhaust goes into the side of the CMU and out the top, where the pipe is sitting.



I slapped a bit of mud into the gaps and fired it up while Yair held the pipe in place. (It's really heavy—the steel is about 3mm thick, and it's lined with another 3mm of concrete!)



The result: despite the pipe having a smaller CSA than the rest of the system, it was a huge improvement! I crammed as much wood as I could into the feed, and there was not very much smoke at all.

So next we need to get hold of some more suitable (and lighter!) pipe, and a proper elbow. Maybe even some insulation