Batch box pulsating sound out of main air port? Help?

Satamax Antone wrote:
The bottom of your second bell should be warmer than it's top! Try, even with touching, this should be feelable.  

It is very similar honestly.  Where the bottom seems to only ever reach about 20% hotter at max than anywhere above.  Its "feelable" that the draft of the system seems to willingly take a good amount of hot air to the bench.  Then as you go up it cools and then goes up again.  So the difference is more like 10% or less as its running top vs. bottom.  So you are correct there is not much of a bell there, but there is a little one.

Satamax Antone wrote:
Another question, why didn't you go to the bottom of the first bell, to transition into the second bell?  Your ISA is considered ISA only above the ports, bellow the heating is only marginal. Well, part of the first bell, from the top of the port between the two, and above the port of the second bell, should become as warm as the second bell. But this, in turn, can create a layer barrier. Like in a weather depression, hot and cold air don't mix. There is plenty of aspects which need to be thought about and may be changed in your heater. Not an ultimate answer, of enlarging the primary air. And that will do. Get my point?

I think i am following you.  The only reason i left the "pit" there in my first bell was because in my first bell heater i had an issue where flue ash formed large piles after a good amount of winter.  Cleaning these areas out of fear my draft would be constricted is something i wanted to worry less about.  So currently there is no way by the time the bench comes along there should be much ash.  So i can expect zero maintence down the line after the bells.  Maybe after 10 seasons or more from what i can tell how it is collecting.

I could cut the slit down between the bells further and that would not hurt a thing.  It would likely get that lower "pit" to collect less cool air and allow more free flow from one to the next bell.  Thus also likely allowing more hot air down the line.  Can you draw a representation of this layer barrier?  I get the concept of low and high pressure air, but i am curious on your thinking in relation to my bells setup.  Do you think making this "pit" smaller would minimize this barrier possibility?  Also i am getting the barrier is something not wanted?  Likely due to turbulence that could be created?  Like when a warm and cool front collide and storm kind of thing?

Byron Campbell wrote:velocity stack

Can you provide an image.  Im not sure my visualization is correct.  I am intriged...

The reason why is that it fits my other latest suspicion.. i noticed a few days ago my air port is about 2 inches up the door.  The bottom of the door is flush with the bottom of my box.  Which puts it pulling air in about 2 inches higher up the v-channel instead of along the bottom of the batch box.  So basically my thinking is if it were lower it may direct air better at the base of the fuel instead of 2 inches up it.  I may be cooling the flames while still feeding the coals a bit if that makes sense?  I get the captivation thoughts.... This in my mind sets up a scenario where the fire reaction is in flux because its starving from burnign fuel hot and being cooled at the same time.  The pulsing keeps it in a rotary effect where it cycles back and forth from trying heat up the current air, then its cooling off with each pulse of new air, and it continies until the demand for fresh air becomes too low to matter.  Can anyone confirm placing an air port too high doesnt hit the fuel properly to burn right?

I haven't tested in between designs. But i know a bell works well when there is more than 2.5 or 3 feet between entrance and exit. Where entrance and exit are at approximately the same level. Mind you 2.5 for a six, i would say. 3 for an 8.

Matthew, on this pic, in the first bell above the blue line, is that a cleanout?



If yes, i would take the chance to punch another hole at the very top of the bell, between #1 and #2, i mean as much as i could. To make it act as a single bell.

About your ash pit. 5 10cm  (2 to 4 inches) should be enough. My 11 inch workshop heater is 2,something heating seasons old. And i haven't bothered cleaning it yet.

About your air inlet in the door. I have had a door with air placed pretty much the same as yours on the green machine. With no adverse effect.

Velocity stack photo attached.

From what I've observed in my own stove, pulsing occurs from a momentary pressure increase inside the firebox, when the burning wood is producing more combustible gas than the draw (draft) can pull through the port into the riser to feed the secondary heat riser's combustion zone. The greatest tendency of pulsing is when all the wood is catching at once and out-gassing like crazy. A portion of the excess gasses cause a flareup inside the firebox, firebox pressure increases briefly and that pressure simultaneously pushes gasses towards both the port and the primary intake. The velocity stack mitigates this effect by encouraging improved firebox throughput, especially handy for burning hardwoods that tend to "coal" heavily.

Satamax Antone wrote:
Matthew, on this pic, in the first bell above the blue line, is that a cleanout?  

No, its just a piece of sheet metal sticking out that i layed across the top middle to help support the 2" caramic wool top.  That will be encased in mass when it fully ready to be.  The square at the bottom is a cleanout so to speak.  That brick eventually will have a hinge and handle....

There is still pulsing... I basically cut down to the bottom of the door so there is basically a bit more than a double sized air port that is rectangular(i will add pics).  There is some metal flashing cut that i can use to adjust the port smaller for testing purposes i have been messing with too.  Opening the port fully or almost full seems to take away the pulse.  My burning is still not perfect though.  I still see smoke out the top of my house!

I also piped from my ending 8" duct directly into the chimney eliminating the 6" reducer.  The smallest gap is right there still meauring about 8"x7", because the chimney is a 7" stainless liner(it may be a tad larger due to me fitting it).  I have good draft.  My exit temps also still about 150 regularily.  Other than making my bells function better at catching heat(which is less of my concern specifically) i am not sure what to do next.  I dont want smoke...

Have you tried to knock as much of the wall between Bell 1 and 2 as you could?

Did you remove the ramp?  I am thinking the ramp between heat riser ans first Bell is really causing the problem.

Satamax Antone wrote:Have you tried to knock as much of the wall between Bell 1 and 2 as you could?

Did you remove the ramp?  I am thinking the ramp between heat riser ans first Bell is really causing the problem.

The ramp is gone.  I have not knocked out the wall between one and two yet.  That seems to be my next move...

didn't you say that originally you had configured the bells that way to avoid excessive brick cutting? It would seem to be  some sort of sick cosmic humor to have that be the final solution, after going through so much trial and error to come back to simply making the bell bigger.

I haven't said much lately but I'm still rooting for you and learning a bit as I watch.

bob day wrote:didn't you say that originally you had configured the bells that way to avoid excessive brick cutting? It would seem to be  some sort of sick cosmic humor to have that be the final solution, after going through so much trial and error to come back to simply making the bell bigger.

I haven't said much lately but I'm still rooting for you and learning a bit as I watch.

Yes, a bit sick, lol.  At this point i am thinking no matter what i do it will not stop pulsing. Haha.  I will see when i can try out the "final" mod/solution.  According to all our suggestive efforts that is the last ditch currently.  We will have to see....

Here is a pic of the door mod.  I use the flashing to adjust the size.  Where it is now seems to keep a steady non-pulsing burn.  Just for testing anyway.  

So a slight update..  i have been running the unit a bunch and i am finding it is running quite clean if i leave the port open all the way(about 2.5"x4.5").  Barely a wisp out the chimney within minutes of the burn as of late.  Its seems again... if i give it a smaller load it works very very much better.  I get a clearer exhaust as long as i mind my reloads more often than not it seems.  I will continue testing and watching my chimney each burn.   The pulsing seems very minimal or nonexistant with the air port wide open like this.  

There was also questions earlier about my fuel source a while back, and i have now yet another hypothesis in relation to my pulsing issue.... i feel since i am using very small wood ranging roughly 1"x1" up to 1.25"x4.5", and about 5"-12" max in length(also birch burns hotter and faster compared to other hardwoods i am using)... that i am naturally getting a faster hotter burn in the box itself.  My thoughts is that this type of burn may also be requiring more air than the described optimal burn method(proper clearances, stacked from small to larger in the box, and what not).  Since most of my wood is the same it might flare up quicker this way.  Just a thought.  

I do plan to open the two bells into one main big bell.
Soon here i hope...  It may take me a bit to secure the proper mortar method for this.  I have been using high temp clay for most of the joints.  Sometimes i have used ceramic wool and top with a surface bonding cemet, but i am out of most of all that currently too.  I will make sure to let you guys know how that turns out.   Thanks again for all the talk everyone!

Matthew, might be a good idea to test with proper bigger wood. Too small wood, leads to faster offgasing, thus overloading the stove.

Three four inch logs.

Yes, I'm working with too much smoke and might be able to decrease the smoke with an increased primary air opening, but I'm trying to figure out the secondary air placement to really get that ram's horn going past the port.
I think my supply tube is about the right size, but the joint is just held together with a couple screws, so I may need to rework that.

I was looking for the height the secondary air supply should be from the bottom using the walker set up. I have yet to try it from the top. It is currently positioned about mid point of the port.

bob day wrote:ram's horn going past the port.

Do you have something that could show me what that is supposed to looks?  Just curious.

sorry, I don't remember the thread, I think I saw it both at the proboards Donkey forum as well as someplace here on Permies

It's a beautiful tongue of flame going straight from the port to the back wall of the riser and then curling around the circumference  on both sides back toward the port.It is the ignition of the wood gas from the batch box with the secondary air supply.

When the port comes in tangential to  the riser, instead of the double ram's horn there is a tornado like turbulence going up the riser.

I hope Satamax see's this and chimes in, I'm guessing he could add to or correct what I think I know about it.

...also this is my p-channel from the outside.  I used 1.5 inch galvanized steel water pipe pieces to supply air down to the top of the riser port.

Here is a video of a burn that was a bit more than half way done.  The wood on top are the largest pieces of fuel i get.  Bob, you can also kind of see my p-channel over hang is just slight.  It seems to do the job well.  I get a good glow in back when its burning right.  This whole burn also was a crystal clear exhaust from start to finish.  I just loaded it carefully, did not over load at all, and minded all my clearances(maybe even generously, but its finally working!).  Apparently just right and this thing gets kickin quite nicely.  About 150 degrees F max on the exit temp this whole burn.  Also this one was zero pulsing.

Also, i am now remembering reading and viewing something about this "rams" horn a long while back.  Do you have window to see that on your unit?  I am thinking a snake camera in my p channel could be interesting.

Hey all, haven't kept up on this for awhile due to life being busy, nor ha I read every comment. So, maybe this was already discussed or I'm remembering incorrectly. But aren't the ports between chambers positioned wrong ?

The drawings show port from chamber 2 to 3 being higher than 3 to exhaust. So stratified gases are trying to backflow instead of easily continuing thru the exhaust path.

Shouldn't it instead be, each successive port should be higher than the preceding, so gases reach the next chamber entrance before the preceding, so no backflow ?

Interesting point.  I am not too familiar with this backflow  concept this way.

As a result, the gases could be pushing back and forth at that port, ebbing and flowing as pressure builds back and forth. Ultimately, the added draw from the chimney helps the gases escape in the positive direction. But, it could be leading to the pulsation Till you give it the extra air flow it needs to push thru

Excellent info from Peter Van derBerg himself. Way more complex info elsewhere on net to explain in greater detail, if I can find it  http://batchrocket.eu/en/building#belltheory

This is a new design being worked on, but has some neat pics of different flame patterns after the port here   There's a nice shot of the double rams horn effect, even though it's not forming in a riser in this, the basic picture is the same

I think in a round about way we have been talking about the size of the bells being too small to adequately allow stratification of gases and possibly causing interference in the smooth movement of gases. When a bell becomes too small it may act more like a simple exhaust pipe, but if exits and entrances are structured like a bell (with exits at the low point) things get strange.

I lit mine again today, and while the  positive exhaust fan was running everything was good, but turning off the fan caused a small amount of smoke back into the room.  I won't always have warm days when I can open the house up when doing a trial burn, so this is a pretty important issue to solve. Although maybe the exhaust fan could become a permanent addition during the development phase so I can use it when needed, and experiment without it when I can vent the living space. still way too  much smoke outside too.  I'm pretty sure I need to revisit the port and secondary air supply. I may not be getting that afterburner effect in the riser due to lack of O2- seems a testo might come in handy to help me analyze things-- I liked the graph of co for the walker stove which used a floor channel, but maybe when I cast the new batchbox top I should include a space for a second secondary air supply

Ram horns

bob day wrote:
I think in a round about way we have been talking about the size of the bells being too small to adequately allow stratification of gases and possibly causing interference in the smooth movement of gases. When a bell becomes too small it may act more like a simple exhaust pipe, but if exits and entrances are structured like a bell (with exits at the low point) things get strange.

I think you nailed it on the head.  This also aids in explaining this "backflow" terminology quite a bit.  


..For now i am finally building extremely clean fires the last few days.  I got it down so to speak.  I have zero pulsing, 150-160 degree F exit temps, and we are warm.  My chimney puffs a light white wisp when starting for the first 10ish minutes during the initial fire, but reloads are 100% clear from minute one to minute complete.  I think i am slightly underfiring this thing now, but i do belive the bigger bell idea(when implimented) will give me better room to fill the box a tad more.  

Just to give an idea of my heating efficiency for now... i am currently using about 7 or 8 lbs of wood per batch.  Half and half oak and birch.  Temps are 30 or less(about 18ish here and there), windy, and wet outside.  Inside we are 68-72 during the day, stays warm all night, 64-65 in the morning, and the inside of the firebox is still pretty warm. I have been using about 32 lbs or so of wood per day max, or about 3 to 4 loads.   I live in about a 1200 sq. ft. 100 year old solid log home with a quite a bit of draft.  Just figured id throw out some more numbers.  Not sure if its great, but we are warm as i said.  

bob day wrote:
I lit mine again today, and while the  positive exhaust fan was running everything was good, but turning off the fan caused a small amount of smoke back into the room

I am sorry to hear of this trouble.  It seems you may be worse off draft wise than I at this point.  I will have to say the double sized air port helped me loads in getting to a clean chimney.  Although i do feel i should be able to load it heavier, and get no pulse from a standard sized air port(someday soon i hope).  So i am still far from perfect it seems.  I must say i have not had any smoke back at all during all my trials and errors though.  i have to ask.. are you direct porting outside or up a chimney?  You just seem to be up on the theory... im just wondering why your having smoke back?

Also i will add that i had done some experiments with the air port after i cut it too.  I had that flashing piece that could be slid down to test enlarging and closing the air supply... i did this to see if i could use less air than full open, or less air at times and more at others kind of thing.   I also used the velocity stack concept and no matter what i did i got mix results.  Honestly messing with the air intake at all no matter how i did it seemed to causes a slight smoke smell to leak out the flashing configs.  It just didnt seem to fit well the way i was trialing with it.  I abandoned the tech for now and now i leave it all the way open always.  I currently have no issues other than i feel i should be able to load it more densly.   Again the bell rework is the idea at some point to aid in fixing that.  

Anyone have a wieght of wood used for a single 6" batch?  Im curious how light i am able to fill mine versus something considered to be operating more at spec.

Quick update / correction... i have been able to load about 11 or so pounds of fuel successfully.  I am getting a good clean burn all the way through start to finish.  It seems how i am now stacking the wood(cris-cross/zig-zag) i can fill it to the top without having this overloaded smoking issue.  So i can confrm this operator error in the past.  I got an exit temp close to 190 plus degrees F off of a single load!  This is the highest yet off one bactch!

Matthew, Peter told me to avoid the criss cross fuel stacking. If i do this, with as small a wood, as yours. I run into overload straight away.


Please please, try to get your hands on log and split hardwood. Ready to burn.

One try you could do too. Stack your actual fuel, so tight. That air can't get in between. And do a 3/4 liad, lit from the top, near the port.