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

There are all sorts of possible choke points that could cause the resonance/ pulsations , but i agree that the fin seems like something that could be causing the wiggle problems, and if it's easy enough to remove it, that might be a good place to start. That 8x8 opening also seems a likely suspect, along with the bigger bell.  So maybe the best thing is to do the easiest changes until things are working smoothly.

Agreed bob, and thanks again satamax for your trusted influence and understanding to boot.  You have guided me correct before, so the fin and 8x8 are going to go.  I feel good about this strategy, and I hope to get to it within the next few days.   I will update as soon as i am able after these mods are complete.  Wish me luck for results.  Thanks all!

Well, reasoning behind my advice. There might be  not enough room between that fin, and the opposite wall.

Also, it creates a sharp corner for the gases flow. Not too keen on that.

In the bell/channel, where the 8x8 opening is. The gases go down, then have to take a sharp corner again. So there is lots of drag and turbulence. Impeding the flow i think.

Another point i haven't paid attention to.

The ash ramp at the top of the heat riser?

Is that an 8x8 inch hole again? I would get rid of that ash ramp if possible. It might create a turbulence right behind it. Like on a plane wing stall.

Look here around 6.30 and 8 50.



Or here, around 4.15.

Satamax Antone wrote:

Another point i haven't paid attention to.

The ash ramp at the top of the heat riser?

Is that an 8x8 inch hole again? I would get rid of that ash ramp if possible. It might create a turbulence right behind it. Like on a plane wing stall.

Its closer to about 8x13, but that may be an underestimate...

Just an update on my calculating before i do these "mods"...  firstly i did an ISA calculation below.  A few notes on how i did these measurements....  i did not include the area of the bottoms of the bells or bench, nor did I include surfaces below any of the port holes.  I did not include the ISA of the 8" chimney pipe or the chimney either.  So here we go...

Approixmate ISA for my 6" batch box system

Above riser
Left wall, top, side.01, side.02
1×1+1×2+1×2+1×2
=7

1st bell
Left wall, right wall, top, side.01, side.02
2×1+2×4+2×1.66+1.66×3+1.66×3
=23

Second bell
Left wall, right wall, top, side.01, side.02
1×3+1×4.3+1×1+1×4.3+1×4.3
=17

Transition
Right wall, top, top.p2,side.01, side.02
0.75×1.166+2×1+1×1+1×1.166+1×1+1×1
=7

Bench
Left wall, right wall, top, side.01, side.02
4×0.75+6×0.75+6×1.33+1.33×0.75+1.33×0.75
=17.5

TOTAL ISA=71.5 sq feet

So, it looks i am well above the max ISA of about 53.8 sq ft.  Making a difference of 17.7 sq ft!  Likely also a contributing factor.  As you notice I did not include my "fin" SA as well because I plan to be removing that tonight or tomorrow.  Also, i am thinking of removing the second bell altogether as i need to remove these bricks anyway in order to remove the fin.  So in one fell swoop i lower my ISA to just about what is considered max.  What do you guys think of this plan?  Do you all think being close enough to this number is enough?  Just trying to gauge my logic...  Also are these ISA calculation a strict science enough to make this call?  Id love all your feedback.  

...another side note... i noticed my chimney is drawing enough air to feel it a bit at this alternate 6 inch port to my stainless liner.  Currently i have it just stuffed with wool insulation, and it seems to not be air tight enough.  My plan is to seal that tight tonight to eliminate all question.

Also i am going to go ahead and seal below the port to my heater in my chimney too.  I dont want any drag or draw lost to air from my basement clean out door being drawn up easier than my system needs.  I really think this is where i am finding more air is helping the fire burn better.  Its just i am not getting clean enough burns.  My thoughts are now leading me to believe it has to be in likely a large part from weak draw along with these free gass "choke" points.  What do you kind folks think of that reasoning?  I am looking to increase my success in modding by eliminating all questionable areas right now if I can.  Again, feedback me if you have the times amd thoughts to do so.  Thanks!

Again, I will be watching replies of real experts, but I think max surface area refers more to keeping temps high enough to effectively draw out the chimney, I think they like over 150F as a minimum exit temp.

But I don't think that would be as big a cause of the pulsations as  the other issues. granted a stronger draw might lessen them, but I doubt that would be your biggest problem. Whether that could be an alternate problem down the line is another matter, and the strength of the draw you mentioned might have an effect to allow for a greater ISA than the average.

The other changes do sound positive, keep us posted.

Well, it's not exact science. Several bells extract more heat than a single one. But usually you don't account the partition walls. Mind you, it should be part of the total isa when the stove is cold. A good way to test if it would fix the pulse, would be to make a bypass on top of that bell, before dismantling it.

bob day wrote:...over 150F as a minimum exit temp.

My exit temp has maxed so far at about 160 F.  It averages aboit 130 F until the wood all reaches "hot coals" status.  Then it shoots up to 150 plus till it finally cools back down.  This i would imagine would increase the averages with just widening the port before the bench and removing the fin.

Satamax Antone wrote:...make a bypass on top of that bell, before dismantling it.

So maybe I should do the two above mentioned mods, and work on sealing the chimney stack before getting too hasty about removing the secondary bell...

I think I remember seeing where someone in the Donkey threads mentioned having an exit temp of 130 and being told they were really pushing the minimum, and also a discussion about optimizing draw on Peter's stack by installing a cowl that maximized his draft, which eliminated some of the smoke issues he had been experiencing. So smoke back would likely be a primary indicator of trouble related to low temps.  but I don't recall any mention of pulsing from the lowered draft.  After the mass you add dries out, I think that will act as an insulator and the system should warm up quicker.

I'm waiting with bated breath to see how the mods turn out

I have the "2nd" bell off and i have adjusted two main areas.  Which would include the port to the "2nd" bell by 3 inches deeper... also from bell 2 to the "transition" that goes to the bench.  For the transition(my original 8x8 ) i widened the hole by 1.5 inches, and 5.5 inches for the height.  I noticed my "transistion" is an odd place, lol.  This definitely needed work.  I found an extra brick and a dish sponge in there too!  My kids may have been the reason for the extra "stuff", or my tired brain trying to get work done late as i do sometimes.  Anyway i think i have much better sizes now.  See what you think  these measurement are more exact and 100% current.  My mod diagram...

...also i wanted to mention my batch box is a tad off on the measurements according to spec, but maybe negligible.  We are pretty close yet.  I actually built this last year and simply did not remember there were a few small items slightly off from spec.


My box itself(reference the 6" spec for comparison), here is what i found...

-It's .75 inches too tall
-1 inch too long
-The width is perfect


My port is...

9.5 tall (this is spec)
2.375 wide(3/16 too wide)
Depth is exact to spec too.

My riser and main port is exact.  Also i have the p-channel adjusted to by very close to spec now.

I just though I'd add more so you all can call me out on more of my mistakes, lol... although i feel my box is not too bad.  You be the judge..

...btw the pic is peter's specs sheet...

Another video of mine...  Please excuse my mumbling for i have a sinus cold thing going on.  Also, i was trying to be quick because my kids are 3 and 6 and impossible to be quiet for more than about 30 seconds, lol.  Sorry for all the posts all.  Its been sooo cold here.  Not going to lie it has been a tad stressful trying to get it going right.  For propane heating my home is extremely costly, and RMH's are not do much if working right... which is a mode of survival for me at this point.  Thanks again for all the contributions!

https://m.youtube.com/watch?v=FzcL-WAIcSE

Matthew, your dimensions are good.

Hi Matthew, my batch-box (firebrick combustion unit, IFB heat riser, 6" system size) had the identical pulsation to your stove, at exactly the same frequency, with small flame bouncing out of the primary air. Likewise, slightly opening the firebox door stopped the pulsation and the stove behaved itself. The stove is going into its third heating season and doing very well, after a bit of fine tuning. Each heater build will naturally be a little different; construction materials, flu path, chimney draft, wind effects on the chimney, fuel-wood burning characteristics, cold start-up time, and etc.

I'm burning mostly ash and beech, and the primary air opening ended up needing to be 3.125 x 3.125 inches to correct the problem. I got an idea of how much larger to make the primary air intake by observing how much, on average, that the door needed to be cracked open over the course of the first heating season. In windy conditions that significantly increase chimney draft, I'll close the primary air by one third to one half.

Byron Campbell wrote:Hi Matthew, my batch-box (firebrick combustion unit, IFB heat riser, 6" system size) had the identical pulsation to your stove, at exactly the same frequency, with small flame bouncing out of the primary air. Likewise, slightly opening the firebox door stopped the pulsation and the stove behaved itself. The stove is going into its third heating season and doing very well, after a bit of fine tuning. Each heater build will naturally be a little different; construction materials, flu path, chimney draft, wind effects on the chimney, fuel-wood burning characteristics, cold start-up time, and etc.

I'm burning mostly ash and beech, and the primary air opening ended up needing to be 3.125 x 3.125 inches to correct the problem. I got an idea of how much larger to make the primary air intake by observing how much, on average, that the door needed to be cracked open over the course of the first heating season. In windy conditions that significantly increase chimney draft, I'll close the primary air by one third to one half.

Interesting to hear at this point.  I will keep your words in mind very closely.  Thanks for chiming in!

Since you mentioned type of wood i figured i would too...  My main fuel is mill "waste" wood consisting mostly of birch with some oak and maple mixed in.  I have been cutting down similar cedar pieces as my kindling to get things hot really fast as well.

I am curious how quickly you achieve a clean burn?  I am having a hard time putting full faith in my gauge for this...  For instance in my older j-tube system i had no smoke after about 5 minutes nearly every time no matter the wind.  So far in this system it seems i have smoke as long as there is flames in the batch.  When i achieve hot coal status in the burn(about 20 minutes in) i get a clean chimney output.  Also, so far it seems to be fairly consistant after a relatively cold start or a full on hot one.  Just curious since your experience resembles mine with the pulsing and flame licking port hole.

My current state is... I got all my tweaks done last night that have been mentioned except i left my "ash ramp" in... i did not remove because i know ash will collect at the flat spot there at some point anyway, and will create an ash ramp of its own if you follow.  I want to see where i am at first too.  My chimney has also been sealed up nice and proper also btw.  I plan to fire it up tonight after the 24 hour mark hit so that i can hopefully count on my mortar being set.  We will see how these changes will affect my issues then.  Stand by for my results.  Discussion till then is welcome too 😜..

Isn't it fun to watch theory and practice come together!

Good luck

The mortar was more than dry so i gave it a quick burn.  First thing i noticed was heat... A noticible upgrade in temps immediately!  My exit temps have risen to about 160 F or so after about 15 minutes!  They held at about that for most of the burn too.  So it seems my heat is not being pushed out the chimney too much yet as i have higher exhaust temps.  A good thing no doubt.

I also noticed the whole unit got to just about the temps it was running at after two burns in just a single batch this time!  This is all really exciting...  The output in general seems to be better by a good margine.  Also my chimney was outputting much cleaner smoke much earlier in the burn than i have ever noticed, but still not perfect....

There was still pulsing.  There was no outward licking out the port though, which is good.  So even if we have achieved a better burn it looks like i still need improvement somewhere.  How about this idea to widen my main port much past peter's spec?  Peter, if you are out there what say you?... im just curious at this point.  I will do another burn a bit later.  Possibly a video to show the "progress" too.  Stay tuned...

Sounds good Matthew. Leaks indeed cause a performance hit.

I don't notice any smoke when burning seasoned wood, not after startup, only steam that vanishes within 4 to 6 feet of the chimney cap. The door has a viewing glass and I can see clear back through the port into the inside of the base of the heat riser. When secondary combustion is occurring, the inside the heat riser is illuminated in golden yellow light. Pretty cool, er uh hot.

My usual start-up routine is to first burn a handful of kindling to verify the draft and warm up the fire box a bit, using wood of about 1" diameter and thinner. All my wood is cut to a length that provides the recommended clearances between the fuel and the primary and fuel to rear port (about 2"). Once the kindling catches the secondary combustion inside the heat riser begins in less than a minute or so, noticeable by a change in sound as the rocket roars to life. After burning down to gently flaming coals and the secondary combustion subsides, I'll load in a full batch of 1" to 2" diameter sticks first followed by 3" to 4" diameter ones, filling the firebox to 3/4 full. Secondary combustion lights off within seconds of the firebox door being shut. HTH.

Seems like the things you haven't done yet are to close up the ash pit or enlarge the first bell. But it does seem like things are dramatically improving.

Let us know what's next.

And get the ramp after the riser out. This ought to make a verry bad turbulence.

matthew boersma wrote:How about this idea to widen my main port much past peter's spec?  Peter, if you are out there what say you?... im just curious at this point.

Most of the time, say 99.9%, the cause of pulsing is a restriction somewhere downstream in the heat extraction part of the heater. Opening the door a crack so more air is coming in cools down the fire, deminishes the off-gassing of the fuel so there's less volume of exhaust gases. The cause of the pulse behaviour isn't solved, only the symptoms are treated.

My own heater never pulse, the last build of my design won't pulse either, I am pretty sure of that. It even started right after the build was completed, soaking wet and all and without heating up the chimney by whatever means. See http://donkey32.proboards.com/thread/2364/rocket-heater-build-peter-mallorca

Matthew, in my opinion your problem is all related to your door and air settings.  I do not believe the pulsing has anything to do with restriction downstream in your case.  I saw your door video from the other day.  Is that still how it is sealed?  That is your problem in my opinion.  You need a great seal on that door, and then you might need to restrict primary a bit from there to tame the pulsing.  Seal your door before you chase this any farther if you haven't yet, is my advice.

Here is my lastest burn report as of this morning.  I loaded using the "75% full" reference and minded my clearences greatly.  I did once again notice it burned better than ever.  Very little white and wispy smoke out the top of my chimney that seemed to evaporate within 5 feet or less.  No darker colors at all this time.  So apparently i have been jamming it too full amongst all my other failures, lol.  There was a slight pulse again as we know.  That also was a bit better since the fire was not as starving due to smaller load.  My exit temps werent as high as last night(about 145F) but it was much cooler in here to start.  Here is the quick video...

https://youtu.be/h-ufQIxISXg

Another video....  you can clearly see the secondary glow from what i can tell anyway.  I suppose I should ask.. does this match what this is supposed to resemble?  My exhaust from this was extremely clean.  Very little white smoke even more so than my previous report from earlier.  You can still hear the pulse ever so slightly in this video...

https://youtu.be/3-6GTvxX3lU

Also, im taking a lot of notes guys.  You all havr me thinking and turning my rocks in my head...  

(??)*

bob day wrote:close up the ash pit or enlarge the first bell.

* How, why, and what do you mean?  Can you explain to me why the below ash pit is a big issue?


(??)**

Matt Walker wrote:You need a great seal on that door, and then you might need to restrict primary a bit from there to tame the pulsing.

** How air tight do i need? ...the reason i ask is that i have a decent gasket of ceramic wool all around the door.  There may be a milimeter by about an inch of leak space due to the door scraping out the wool on the bottom as it opens and closes.  I do plan to etch in a valley into the door edge to fit a classic rope gasket that will be much more durable in the future(hopefully soon).  Unless you have a better material suggestion?  Im mostly just curious on how would you explain the importance of this in relation to my "pulsing".  I do understand the air direction has a role in the burn efficiency, so i do understand it is important in a general sense no doubt.  Just wondering if you would expand that idea for me if possible?

In general the "??" means im unsure, but honestly i am a bit stumped.   Although improving my possible restrictions seems to have improved things already... All in all I am open to all your suggestions guys.  Also I maybe willing to try them all in the end as i go.  I guess it depends on what works.  This will be interesting...

I was echoing Satamax's suggestion about the ramp, I just got my terms mixed up.
Whenever a smooth flow is interrupted by a backwater  or any obstacle there is going to be some turbulence.

Congrats on getting most of the pulsing and the rest of the system working better

bob day wrote:I was echoing Satamax's suggestion about the ramp, I just got my terms mixed up.
Whenever a smooth flow is interrupted by a backwater  or any obstacle there is going to be some turbulence.

Congrats on getting most of the pulsing and the rest of the system working better

Oh i see.  Thanks!  I plan to take out the ramp later tonight.  Also i have one other mod too that i thought to do.... after the bench there is black 8" pipe to my chimney, and there is two 90 degee elbows before it reduces to 6" right before the chimney, and each 90 is going to cut draft in half... so i am removing one and instead adding some left over 13x13 flue pipe to angle from my bench on upward.  This will then pipe strait up with only a singular 8" elbow before the 6" reduction.  I know its mostly 8", but i think since its "a mess' in there that should help the draft a tad.  Plus i could use a brick backing to my bench to lean on instead of that metal pipe anway.  So it seems worth while while im at it.  I will report again later tomorrow on that one...

I took out the "ramp"...  The area from the riser now enters from a port that went from about 13x8(may have actually been more narrow it turns out) to about 12 x 13, and it made the very first bell quite a bit more open and a bit bigger too.  I also modded my output and succesfully eliminated the "extra" 90 degree elbow for good measure.  It seems awesome right?  Well i know i have increased draft.  Exit temps are up on average again too.

The big reveal.... drum roll.... the pulsing is still there, and possibly worse.  I dont mean to be a downer, but for the first time ever the unit kicked smoke back out the main air port as it pulses.  I had to crack the door to get it to stop even!  I will do some more tests later and in the morning.  I will be back later for a bit to see if i can add a few things from my findings... In the mean time any ideas?  

My thoughts and experiences... there are a few angles that have been rattling around my head.  I will start by explaining my successes versus my failures, and then lead into my hypotheses.

During my first modding session i removed a "fin" in the first bell after the riser connection point.  I widened a port to the successive bell.  Also I widened all the way through the bench transistion area..  This significantly slowed the pulse and seemed to help move more heat into my bench and upped my exit temps a good 20 degrees.  

So, if we dial back i did two things simultaneously...

1. Widened possible "bad" turbulant causing restrisction points that is thought to cause pulsations as a general rule.  

2. REMOVED internal surface area.


During my second modding I removed my ramp from my riser connection point.  I also removed a 90 degree 8" pipe elbow after my bench exit, and replaced it with essentially a 24"x12"x12" flue tile.  It now ports out the top perfectly strait up into an 8" pipe to the chimney(that is reduced to 6", then 90s right up the chimney).  This seemed to increase the pulsations and increase my draft through the system.  Also this has seemed to increase my average exit temps about 10 more degrees.  

So if we dial it back yet again basically i did two more obvious things as well.

1. Widened possible "bad" turbulant causing restrisction points that is thought to cause pulsations as a general rule.

2. INCREASED internal surface area.



...My hypotheses are not final, but i am leaning toward three ideas...

1. ISA may be having an unknown role.  So attacking this would include removing ISA to lower it as needed.  This one i hold least stock in, but i dont rule it out.

2. How hungry is my system? Would a bigger air port be all i need(since opening my door seems to yet be my goto band-aid solution)?  I just hate going that far out of spec.  I have seen a lot of peter's numbers, and simply i like them, and would love to trust them.  Seems like a possible simple solution, but i am not into "band-aid" fixes if you follow.

3. The port to my chimney is the cause(my last restriction possible)?  My thinking on this last one is since once again there is essentially a 90 degree as it enters my chimney that potentially cuts natural draft 50%?..  I just figured most 6" inch systems pipe in with a 6" at a 90 degree typically at the actual chimney port.  I am thinking i could have overlooked that little detail.  Please correct my assumptions if you can.  Maybe an 8" all the way till i am at the 6" liner?  Or bigger?


My brain hurts a bit, lol.. what do you gurus think?

That's weird.

Matthew, do you have pipe and elbows on hand?

Now, it's time for experimental  troubleshooting.

IIRC, you said that you were already high in ISA; And you added some with your latest bit of flue element. Tho your exit temp went up? But what if the whole temps went up?




What i would try now.  Get some rockwool or glass wool. Open the first cleanout. Shove a bit of pipe in there, airtight  it with the insulation you found. And pipe that direct to the chimney. Repeat for second cleanout etc!

I can't remember, what did you say was your top gap? 12 or 13 something like this.

Satamax Antone wrote:
I can't remember, what did you say was your top gap? 12 or 13 something like this.

What do mean by "top gap"?  Also i am short on stove pipe.  So that troubleshooting option may be costly currently. So, again im not sure.

I can only watch as you make these mods and hope to start to understand some of the dynamics of this new(to me) system. Likely as I get to that part of my own build ( today maybe) I will start to see my own set of issues.

Some idle thoughts, what I think I've learned about primary air flow. Too much Primary air increases primary combustion in the batch box and reduces wood gas combustion in the riser.  not sure where to go with that idea, but since  it seems to solve the pulses--the band aid- perhaps it is important. Perhaps the wood gas decreased combustion in the riser reduces the overall speed of the gasses in the rest of the system? Would rising heat reflect that decreased speed-ie less heat going out the stack? Or is there simply more heat because of the decreased surface area from the first mod and this last increase of ISA still doesn't take it back to the original.

Do these pulses seem any different as far as frequency?

If they are farther apart it might indicate greater distance between the restriction and the riser.

I understand your reasoning with the 90 at the stove pipe connection, and that idea occurred to me before I actually read your analysis.   Let us all know what you find.

matthew boersma wrote:

Satamax Antone wrote:
I can't remember, what did you say was your top gap? 12 or 13 something like this.

What do mean by "top gap"?  Also i am short on stove pipe.  So that troubleshooting option may be costly currently. So, again im not sure.

Top gap, distance between the top of the heat riser, to the top of the bell or barrel.

As for pipes. I have scavenged a lot in skips, dump yards  etc. So you might be able to do the same.

It's the only way i can think of, to narrow the problem now.

A daft idea, do you have  a window nearby? Shoving the tube in the window, instead of the chimney, could tell you if it's the heater itself, or the chimney.

bob day wrote:
Some idle thoughts, what I think I've learned about primary air flow. Too much Primary air increases primary combustion in the batch box and reduces wood gas combustion in the riser.  not sure where to go with that idea, but since  it seems to solve the pulses--the band aid- perhaps it is important. Perhaps the wood gas decreased combustion in the riser reduces the overall speed of the gasses in the rest of the system? Would rising heat reflect that decreased speed-ie less heat going out the stack? Or is there simply more heat because of the decreased surface area from the first mod and this last increase of ISA still doesn't take it back to the original.

Do these pulses seem any different as far as frequency?

If they are farther apart it might indicate greater distance between the restriction and the riser.

I understand your reasoning with the 90 at the stove pipe connection, and that idea occurred to me before I actually read your analysis.   Let us all know what you find.

...to finally get back on this again, and answer a few questions. The frequency has been the same speed no matter the intensity.  I like your thought on this though, but i have figured the earlier reporting of what i am now calling potential false positives was due to the fact i started building less packed loads.  Thus i found less oxygen demand, and then less captivation as a result.

My next plan is to fix that 6" 90 at my chimney port with in the next day or so.  It just has to be the cause.

On another note... IF my draft is not providing enough pull for a spec sized air port would widening the hole to compensate be in the same realm of pyrodynamics?  As long as i has enough air(not too much) would the unit work properly?  I just am curious if speed and quantity of air has been pitted against eachother.  Like would it be possible to get enough oxygen with increasing the size but because draft could be lower it would not cool the combustion zones at all decreasing off gassing?  Maybe a question for peter?....

Matthew. I would say, to test the draft. If you approach a lighter to the 6" hole in the chimney, and it blows it off when you are at the very edge. Your draft is good.

Your design is soo convoluted, with channels, rather than bells really. It owes to have restrictions.

Your first bell 20x24, is 17 times the CSA of the heat riser. So, that's fine. Tho, you have more of a downdraft channel than a bell in this case. since the entry point is on top. Mind you, this is the case of most single bell heaters. Let's not make a fuss of this.

Then you have your second bell. 12X12 if i have understood well?  So that's 144 sq/in. And i haven't retracted the corners. That's 5 times the CSA of the riser. Peter said to me, a bell, in order to work, needs 4 times the CSA of the riser.

So far, i have tried 2 in that range. And i never managed to make these work well.

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

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?

Hth.

MAx.

Matthew, I agree with Max that your stove more resembles an ever decreasing-CSA flue run system. BTW, that is very similar to my own stove. If your first bell was much larger, then it would be more like Peter's stove.

One thing I noticed about my batch-box is that there is much less tendency of pulsing when the burn is occurring just in front of the port to no further forward than about 6 inches in front of the primary air intake. Through experimentation I also discovered that a velocity stack added to the primary air intake, to promote straight-line primary air flow into the base of the burning wood, to encourage the burn at the back of the firebox to just in front of the port, significantly improves performance under all conditions from a full firebox load to the coaling stage of the burn.

The velocity stack protrudes through the firebox door's primary air intake into the firebox by 2 inches, thus directing primary air straight into the firebox's V-floor. As a test I made a temporary experimental velocity stack from coffee can tin, formed into a square tube tightly fitting the primary air cutout in the firebox door. The velocity stack is flush with the outside face of the firebox door, with a half inch wide 90 degree edge-lip to keep it from being pushed through the primary inlet while using a poker to tend the fire (shove embers and coals back away from the primary air intake). Anyway, maybe give this a try and see what effect it has for your stove.

Satamax Antone wrote:Matthew. I would say, to test the draft. If you approach a lighter to the 6" hole in the chimney, and it blows it off when you are at the very edge. Your draft is good.

Your design is soo convoluted, with channels, rather than bells really. It owes to have restrictions.

Seriously, how big does a bell need to be to evade this "channel" status?  Just a quick and dirty number i am curious...

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.