Reluctant to rocket

Filled around the feed tube/tunnel with perlite

No miracles to report

I was hopping some other regulars would chip in but …..
The roaring noise associated with the rocket stoves name sake, happens due to the strong draw being pulled through the fire box and burn chamber.
So when the stove is on song, you will get the roaring effect, the higher the temperature the more likely to hear the roar.
In order for this to work well, we need an open and free flowing route for the flames and then the hot gasses to travel through.
So the more complex the route, the more bends and right angles you install the more restrictive the route becomes.
That is why a bypass works so well because it offers a more direct route while the system heats up  the chimney and warms up too assist the draw.
In your case you have a quite difficult  route for the hot gasses to travel through.
To aid its travel, we need to get the very best out of the fire that is burning in the fire box and burn tunnel. We want that area to be very hot so the gasses can really shoot up the riser without any restriction and allow the hottest part of the system,  that will be at the base of the riser, to do its job.
Your thick dense bricks will take some time to saturate with heat but, more importantly we need to keep that heat from passing through the bricks, that is why the insulation is so important.
Different folk have different ideas but in my own builds I use a lot of insulation, I think you will get much better overall performance with 2” of insulation rather than 1” but in any case, the whole fire box, burn tunnel and especially the base of the riser will benefit  good insulation.
In order  to keep the flow going we need to keep the route as clutter free and open as possible, (piped systems are really quite an old designs compared with the much more free flowing ‘bell’ designs) so the first big restriction in the gap at the top of the riser.
There is not really an upper limit as even four feet above would work well but we need a bare minimum of 80mm. Personally I would go for 100mm as a minimum.
The gases have to spread out and travel down, while at the same time shed off heat that radiates out of the barrel, this area needs to be free flowing and not restrictive.
Next the gasses need to travel through the pipe, if the pipe is covered in mass and sealed completely, the the gasses will keep going and slowly heat the mass. If the mass is not sealed the heat will dissipate like the radiating barrel.
Ultimately the gasses reach the vertical chimney where they can successfully rise, further helping by pulling the gasses through the sytem.
The end of the burn chamber and the base of the riser will be the hottest part of the system so insulation is very important at the point.
In summery it is really difficult to trouble shoot from an arm chair so we must rely  on your input but hopefully some points made will help you work out some improvements.
I also notice you are burning quite large pieces of wood, small dry kindling  will heat the stove up much faster, I control the heat largely by the size and type of wood I use

Thanks fox - good morning

What is your analysis of this …



Is that a “roar” ?

I can control the ferocity as you say, by using smaller wood and/or choking with firebricks on the intake

I burned a moderate fire, 2 or 3 wrist sized logs at a time for about 3 hours last night, barrel temperatures were between 100 and 150c , there were always bright flames, and the now insulated feed tube got very hot

There is a minor choke point at the very bottom of the riser, where I cut the ‘polo’ which supported the inner former .. it’s a delicate operation but I might try and flip the riser over this morning .. possibly even removing its jacket

Insulating the sides of the firebox and end of the burn tunnel makes sense too, although the rate at which heat is lost through 4” of silica brick I would have thought was pretty low

Meanwhile I’m on the scrounge for a big barrel

Fire bricks suck in heat and eventually allow it to escape from the other side, the thicker the brick, the slower it takes to heat the fire. you need the insulation to stop the heat exiting the back side of the brick! Without insulation the bricks will never completely saturate!
A good example of this is my own fire that in made from only insulating material, the barrel top will jump from ambient temperature to 400c inside of 10 minutes.

Good Day, Nick;
I listened to your video.  It is rocketing but not as much as it could.
Fox is spot on about insulating outside the firebrick core.  I used about 4", of a perlite clay mix around my J-Tube core.

About the top gap from your riser. There is no upper limit.  As a lower limit, I would say you are there at 60mm.
My 8" ran a 65mm gap and worked flawlessly.  I was constantly told I should increase that... I saw no reason to as it worked so well as it was.
The more top gap you have, the lower your barrel top temperature will be.

Indeed I was referring to how long a single load of wood would take to burn (45-60 min)  You mentioned that you are home most of the time, popping by once an hour to refill the dragon is a pretty easy thing to do.   The other thing is how easily a hot J-Tube will relight.  I have had wood (no kindling) reach 456F just sitting in the feed tube and spontaneously combust!

I still think enlarging the barrel and possibly the transition area will give you the roaring dragon you are hoping for!

Nick Ax wrote:Thanks fox - good morning

What is your analysis of this …

https://youtu.be/NafXx_gRxDM?si=xylQvHXD4p7YLABr

Is that a “roar” ?

I can control the ferocity as you say, by using smaller wood and/or choking with firebricks on the intake

I burned a moderate fire, 2 or 3 wrist sized logs at a time for about 3 hours last night, barrel temperatures were between 100 and 150c , there were always bright flames, and the now insulated feed tube got very hot

There is a minor choke point at the very bottom of the riser, where I cut the ‘polo’ which supported the inner former .. it’s a delicate operation but I might try and flip the riser over this morning .. possibly even removing its jacket

Insulating the sides of the firebox and end of the burn tunnel makes sense too, although the rate at which heat is lost through 4” of silica brick I would have thought was pretty low

Meanwhile I’m on the scrounge for a big barrel

I think Fox James has made a few excellent points (concerning insulation) and has also asked for a few measurements in order to help a bit.  To which perhaps I have missed the answers.   So I marked up two of your photos,  the riser one, showing two key measurements with the gap being most important.

The second one, is one you really can't do anything about, unless you install on your end cap, a plug extended into your tube to keep the heated gas from bouncing, this is minor, but in looking for best flow, it can cause a few problems. WE can hope for the heated (gas-air-combusted particles) to know the best route, but that is only hope, we have to help it too!

Back in my HVAC days, we would occasionally run into airflow bounce, where the air was traveling rapidly down a tube, run into a dead end and then have to reverse to change directions.  Thus bounce back to the next opening.  I am not saying yours is doing this, but it is a easy fix with the plug and won't effect your inspection clean out.

As two your roar sound,  I feel yes, we can have it right there in the fire box, and then up the riser, but then it falls back into constrictions well beyond the roar. From past experience, I can tell you a full out burn to a full burn with restrictions is pretty amazing  But if you have never heard or seen the full out one, you might not know exactly the intended sound or feel of it.  

So what I am saying, is we can help your drive this beast and a 8" is truly that, but we have to know what is beyond the engine in detail.

And removing the jacket of the riser? this is but .xx of a mm, we are talking far more than that for a restriction. And  it won't open the gap, this removal will it?

Forgot to attach these,

one last thing, when my stove is cold, it takes 6-8 burns to get it up toasty warm, but then, I can maintain this easily with 3-4 burns daily.  it is a different concept, but I wanted to point out that the mass heats slowly, but then can be maintained.  I know this still doesn't make your barrel as hot as you want, but once we get that done, the mass thing will still be a ongoing event.

Hi Scott, thanks for the reply, which I have read a few times

Removing the outer jacket saves 1.4mm .. like you say, not a lot, and it risks trashing my riser completely …
Flipping the riser, and removing the square ‘teeth’ formed when cutting the inner circle might be worth doing, theoretically that is the tightest point in the whole system

I plan to mix up some more perlite/waterglass/cement insulation and cake the burn tunnel and riser bottom where airflow will permit tomorrow

Bounce *might* be a thing .. there is a third clearout too … but I think it’s going to make sense to try a large barrel first -  I can make some plugs up from drywall I have handy to prove the point… possibly something more robust if it helps.

The thickness of the wall of my riser is 75mm (3 inches)

The barrel-riser gap is notionally (440-350) /2  = 45mm  .. it’s  actually not perfectly centered .. so might be 40 one side and 50 the other but the CSA is over 160% of the ‘system’ CSA (314.159 cm2)  (spot pi)

Maybe I’m just burning it wrong - if one is supposed to pack the firebox to 80% full with kiln dried hardwood, split to matchsticks, light the blue touch paper and retire to a safe distance, well - I’m not doing that

I’m burning a bright fire .. at times with white flames, with fuel filling 40-60% of the feed tube … that’s where I feel it ‘wants’ to be, but temps are low - around 170c in that scenario

3-4 burns a day sounds like a whole lot more than .6 cords of wood a season - perhaps my expectations are unreasonable

Honestly, I don’t care about absolute barrel temperatures.. lower would (in many ways) be better - but it’s my yardstick for ‘success’ in terms of clean burn

Also.. my mass is incomplete, uncovered and possibly still damp - so I am unconcerned about it falling to room temperature overnight … I’m sure the cat is enjoying  his new nighttime sauna - that is all solvable

If I can get another big barrel soon, with a removable lid I can drop it on and also potentially cut out a big window and borrow my old stoves glass so we can peer straight down the riser with it  fully burning, might even be able to get a cross section of the flow out of the top of the riser using the laser level beam through the glass - not sure it will tell us anything useful, but it would be fun.

Hi Nick, what did you build you stove on top of, did you lay down any insulation under the fire box and mass?

The core was built on a 30mm foamed cement ‘fireplace construction’ board ..

https://shop.vitcas.com/vitcas-fireplace-construction-board.html?gad_source=1&gclid=Cj0KCQjwqdqvBhCPARIsANrmZhNGXTFFMlg3bkX1cs12FY0HYY7yr8DZFwueKiPYas_-wPHuK5OWddQaAl04EALw_wcB

I am lucky enough to have solid (poured concrete) floors in a 60’s bungalow, it perhaps doesn’t have the U-value of a 2” perlite bed, but it seemed like a good option

It would be quite difficult to swap it out now