Hi Martin, First off, stupendously documented project-- bravo, good sir, bravo! But when I was watching your videos... and you go from totally psyched in the first one, to completely depressed in the second, to desperately pleading in the last... my heart goes out to you, man. I want to try and help you fix your problem without all of your laborious intent becoming moot or discounted.
It is from that place that I ask you to bear with me as I am going to propose a radical solution to your system, a slightly less radical solution to your system, and then a low-brow solution to your system, but before I do, I want to try and bring you to a place of understanding about what's happening in the second half of your construction that's hindering your first half from rocketing right along. Believe me, if you could just burn your J-tube/riser, even without the barrel, it would easily shoot flames out the top in a matter of minutes. I've built J-tube dry stacks that draft properly just to illustrate the principles of 'sideways fire' to people and they work beautifully... as long as your geometry is right on, it's flame on!
Before I go any further, I must again commend you for your excellent video and photo documentation throughout the construction/test process, for it is that attention to detail that provides enough information for me to completely agree with Andor's assertion that your baffled mass is the major issue at hand. You don't have to test it with a baffle blocker because I'm going to explain to you why it's causing the problem and you don't need to waste any further effort at this point because I think you've already got some more work ahead of you to make this thing right. But not to worry, it is still a construction that will be beneficial in the end. Right now, however, it's a 'flow death chamber'. Disclaimer: short but relevant anecdote-- many years ago, I bought a (then) 16 year old BMW sedan from a friend who was moving out of town. He had purchased it from an Air Force major at the local base who had originally bought it in Berlin, hence, every word that was printed, cast or engraved on the car was in German. When my friend was handing me the keys and going over everything on the car that he had translated so far, we had the hood up and he pointed out this odd looking, multi-bubbled plastic chamber on the firewall that had a hose coming out of opposite ends-- one hooked up to the intake manifold and the other disappered back into the firewall. He said that the rough translation of the label on the side of it said' "flow death chamber" but that the intended meaning was something more like' flow-deadening chamber'. It was essentially a series of equally sized chambers that were interconnected to each other by a single opening on either side (sound familiar?). The car had an elaborate but effective vacuum-powered control system for all of the vent, A/C and heater dampers that was fed, like most vacuum systems, from a tap on the intake manifold. The cluster of chambers would allow the system to have a reserve of the required pressure differential to provide the user with the ability to make multiple climate control system operations without compromising the negative pressure in the intake manifold during low RPMs. It was a tank that could be taken from on the output side without the 'loss' being felt on the input side. That's exactly what you have with your multi-chambered mass-- it's a big flow death zone. That's why, as you mentioned earlier in this
thread, having a fan on the other side, even if it was just a 4" pipe, seemed to provide no worthwhile improvement to the draft of the system; because the chambers are deadening the flow by making the gases do work to get in and out of each successive chamber.
Take a moment to visualize the fluid dynamics going on inside your system as the hot gases try to travel along the intended path. Your J-tube and radiant exchange are golden. Beautiful. But after your gases leave the bottom of the barrel, that's where your trouble begins. Your slab-topped clean out box is your fist flow restriction. As the gases leave the barrel, they are still quite hot and want to stay at the top of this first chamber, but have to flow down the end wall and then out the first triangle at the bottom. This area is not such a big deal as is the comb chamber, and will work to your advantage in the end, but more of that in a minute. Your chambered mass starts with a 'first' vertical dead end chamber that is larger than the rest, then continues with a series of smaller, yet equal, vertical dead end chambers, all with openings that have CSAs larger than the triangular opening at the bottom where the exhaust is expected to flow into the next. As the hot gases enter this first chamber, they want to flow upward and fill the chamber before they try to flow back down and out the next hole, all the while having to push out the gases that were in there in the first place, only to crash into the gases flowing behind them that are trying to flow up-- this whole process repeats itself in each of the other
narrower chambers, By the time your exhaust has reached the end, it's done-- all the velocity has been turbulated out of it. It's like some kind of Heisenberg's molecular death march.
Okay, let me pause for a moment and apologize for that last remark-- that's a bit cheeky, and the Heisenberg reference might be a bit obscure, sorry, but I'm trying to add a little levity to the fact that you've done an excellent job of constructing a giant passive flow damper and you're going to have to bypass it in some way if you ever hope to get the glory you deserve of listening to your beautiful front end rocket up to temp. Really, Martin, brother, I understand... the chamber... it's your brainchild; your baby; a geometric inspiration that is elegant in its implementation and carefully constructed with durability in mind; you cut every one of those damn slabs that you bought expressly for this application and spent hours upon hours mortaring the whole #*$&! heavy thing together and you'll be damned if it's the problem... dammit! (<-- levity) But, I've been there; when you see this beautiful solution to a problem or an idea that just has to work and when you spend hour after focused hour on a singular vision that has seemed so right all along... but in the end, it either just won't work, or it actually winds up compromising something else in the scope of its dysfunction. ...you just have to let it go and re-invent what you have into a new brainstorm.
So, listen, I've got a few options you could consider to kick your system into overdrive, and all require cracking into your masonry in one way or another,
and all will require a bit more work, but all of them focus on using what you already have... what you've spent so much time and effort putting together... just using it in a different way, and without it having to be a complete 'phoenix from the ashes' type of rebuild.
To begin with, I want to propose an initial change that will not only improve draft, and will mean much less labor spent on maintenance in the long term, but would be required in order for you to implement either of the first two options I'm going to describe in the next two paragraphs: you would need to re-configure the slab-topped clean out box such that it exhausts horizontally out of the top instead of the bottom. Pretty much like that photo that you posted where the camera is looking right under the end of the slab towards the exit from the barrel...right there is where the gasses should exit. This is the direction that they naturally want to flow. Plus, by doing so, you will have transformed this chamber from a once-every-year clean out box into a once-every-
ten-years clean out box. To do this, in such a way as to be set up for the next options, you should: remove those two large stones in between the end of the slab and the beginning of your chambered mass, reach down inside and mortar off the hole at the bottom, dig out a little in the area where the stones were, and use some ducting to make the 45 degree turn remain level as it exits from under the end of the ash slab and meets up with the top of the first slab of the chambered section. As a component part to this process, I would also recommend that you cut a hole in the ash slab and just put an end cap in it or a smaller slab on top as an inspection/cleanout hole so that when you eventually do have to clean out the
pit vault, you won't have to chip away mortar to do it.
Now, I'm gonna warn you before I go into the three possible 'chambered chamber' solutions; all involve removing one of the cap slabs that cover the ends of the chambers. You'll need the access and it's the most straightforward way that results in the least amount of work. Now, on to...
Option #1: This is going to be the most painstaking of the three, but I think it would give you the best results... cut a cradle into the the top of each slab so that an 8" duct can run through the top of the chambers with as much direct contact as possible, and the cap slab fitting back on flush. This would, of course, include cutting a new hole in the heavy end slab, which looks to have already suffered some damage...eek! Then, before re-mortaring the cap slab back on, fill the chambers with perlite/vermiculite/sand/whatever. This is going to transform your flow damper into a rather sophisticated thermal battery. You can then run your exhaust out your exterior wall without the 3-4 foot rise being a problem.
Option #2: Okay, so you don't want to have to go through the hassle of cutting a circular notch in the top of each slab... well then, how about cutting a hole in just the first slab, digging out the triangular passage a bit, and then plumb ducting through by coming in the top of the first chamber, 90 degree elbow down, 90 degree elbow into your triangular passage, and then out your existing hole in the end slab (no more futzing with a compromised component). You would then back-fill the chambers with perlite/sand and mortar the end slab back on. While this solution involves less
concrete cutting, getting the ducting all shoved in there and connected could be a bit of a frustrating pain in the ass... but it may be your best option as it would allow you keep your existing exhaust along the floor in case you want to cob that in and increase your mass-- which you may want to do since you are going to see big jumps in temperature when you finally do get that 7x7 feed tube blasting away on its own.
Option #3: So, let's say that you don't want to hack into the masonry of the first chamber... fine. Then just keep digging out your triangular passage so that you can shove a piece of duct in there... but you still need to fill the airspace within the chambers with perlite/sand to get the same multi-plate thermal mass battery core benefit of the first two options.
So there you have it, man. I hope you can see the possibility of one of these solutions working out for you, and I at least hope that I've helped you understand a bit more about fluid dynamics.
But if I haven't made anything clear enough or if you've got a ton of questions... just ask.
Chris