Rocket stove not drafting

I used an old 100lb propane tank measuring 14.25" internal dia. 6" pipe for the fire box and vertical. Used 2000*f header wrap as insulation. Exhaust is 6" out the bottom. Won't draft at all.. Had 1.75" clearance from the riser, figured that was the issue, cut .5" off leaving 2.25 clearance. Made a slight difference but not tons

Any ideas??? Pipe is .25" wall

More pics

Measurements

Nathan Timmermann : Wow, Mad Mad Metal fabrication skills ! I can totally understand why you would try to build a Rocket out of metal with your skills !

Anyway, Welcome to Permies.com and our Sister site Richsoil.com, And A Big Welcome to the Rocket and Wood stoves Forum Threads, With ~35,000~

Fellow Members World-wide you can come here 24 / 7 and find someone who wants to talk about what you want to talk about !

I rather expect that you have found a U-tube video where someone posted his creation and said that it worked - most likely it didn't as has been abandoned

We can go into great depth dealing with all the reasons that as soon as you upgrade the insulation around your Burn Tunnel and Heat Riser, and close off

the Horizontal 2ndary air channels You will create such High Temperatures that your metal heat riser will fail !

O.K. the exhaust - it is being constipated by being lower than your Burn Tunnel ( which should always be the smallest area for combustion gases to flow thru !)

And then further restricted by the 180º Stove pipe bend , and then further constricted by the Reduction in pipe size ! ( You were hoping for a Venturi effect here ?)

So with these obstacles we can only try- Immediately after your 1st bottom Elbow a-Add a short length of piping to allow the 2nd elbow to clear the side of the

tank and allow you to add on several feet of stove pipe say ten feet !

You will need to move the whole thing out doors, if you can get this going you will have temperatures at those elbows that will release a % of the Elbows Zinc

coating as an oxide gas - really nasty stuff It and the paint on the barrel are going to give someone a headache

Next I would try preparing your tinder and kindling and then using a simple hand held propane torch to warm up your Heat Riser- you are trying to create a

Natural draft !

If that fails , you can try using a portable multi speed fan closing off the Feed tube or Your 2ndary air to boost the flow of air !


You must have the final Vertical chimney in place as soon as you place the Tank over top of your heat riser !


After you have got your system working to prove it is possible we can all work together to build a more conventional Rocket Mass Heater that will work with

natural draft !


I hope this was timely and helpful, remember, we all started where you are today ! For the Good of the Craft ! Big AL

Nice welding job! Unfortunately, steel J-tube combustion cores don't last very long, plus all the metal in the flame path sinks away the heat leading to all sorts of performance problems. But attach a tall 6" diameter chimney system that exits several feet above the roof line of the building, and it will force your metal rocket to draft. Expect it to stall once it warms up really well and the heavy metal heat riser reaches I/O temperature equilibrium.

If you decide to build another, I'd use insulated refractory (masonry) for the flame path throughout the J-tube core. Metal can still be used but only as a casing for the J-tube core. HTH.

Edit, addendum: just noticed your additional photos, and Allen's post too. The exhaust reduction output at/plus the 180 bend, yeah, two additional problem areas as Al mentioned.

Awesome thanks for the quick response!
To summarize
-better heat riser insulation
-move exhaust parallel to burn tunnel
-taller exhaust
-YouTube lied to me lol

Questions
Exhaust size 4" or 6"?

Also during my test burn I closed off the feed tube and fed the stove with some oxygen off my oxy/acetylene tank. Also tried throttling intake and exhaust in multiple configurations.

Is there any hope of this layout working?

Thanks!

Typically, you'll want to keep the CSA constant throughout the system; 6" feed, 6" burn tunnel, 6" heat riser, 6" chimney system.

Nathan Timmermann : Your willingness to accept constructive criticism is noted ! -and makes our jobs much easier !

Stick with the 6'' Vertically fed J-tube style Rocket Mass Heater For your 1st build. Sub-6'' RMH units are a very tricky 1st build !

With less Thermal mass to hold heat over night ! A good 6'' RMH is versatile for all but the largest workshops or bitterest climates !


Will this build ever work ? May be try the fan and an extended final vertical chimney! I do not expect it to work without the fan !

For the Good of the Crafts ! Big AL

Thanks guys, I'll start by moving the exhaust off the bottom and shooting the piping nice and high, test burn and report back!

You guys don't think the spacing at the top is an issue??

If still no improvement I'll be cracking it open to insulate the vertical.

Your top gap is just on the verge of being too small.

Yep youtube is full of metal pieces of shit!

Check for spalling, or high temperature spalling.

May be you could save some of your build, by lining the firebox and burn tunnel with firebrick splits. Or fireclay. And the heat riser with a layer of superwool or something like this.

The 6" exit duct out of the bottom of the tank may not be helpful, but it may not be harmful either. Before spending time changing that, I would suggest first giving the unit a complete 6" chimney to above the roofline. (Temporarily moving it outside with a 10' minimum chimney to burn off zinc/paint would be good.)

Once you see how it drafts with an actual chimney, add some horizontal 6" ducting to be encased in cob or other thermal mass once you install it indoors. A 6" system typically can handle 30-40' of horizontal duct, minus 5' for each 90 degree elbow. I would keep it below this unless you find you have draft to spare.

How thick is the 2000 degree insulating wrap, and exactly what is wrapped? You want all of the burn tunnel very well insulated, though not so much on the feed tube. And a secondary air inlet at the base of the feed tube will cause problems with the fire coming up out of the top of the feed. That should be sealed while burning; it is not even needed for cleanout, as a 6" system can be reached for scooping ashes with a tuna can, and the ash volume will be small and easily handled.

Chimney will make that much of a difference?? I can't help but think the riser doesn't have enough clearance.. I'll definitely try the 6" chimney first.

Insulation is 1/8thick total, used 2 layers of the highest temp header wrap. It's wrapped right from the horizontal all the way up the vertical to about 4" from the top.

During my first burn I toyed with closing off the face and having it draw from the chute. But it quickly smothered. I had the best results covering the chute and just having the horizontal open. Not sure if this is valuable info or not.
I have little faith in this being able to do 30' if it can't draft with 0' currently.. Or am I really underestimating the potential of it being fine tuned?

Well, for your top gap, the ring projection is 42.41sqin

And your CSA is 28.27sqin

It would have been better to keep the first number about 2 times to 2.5 times the csa. But it can work. Some have been seen to work with 1.5 times the csa. Which is your case.

To get this straight. Ideally you want 2-2.5 x your csa?

Csa is 28.27insq

Current gap 2.25" w/csa of 42.41insq

2x csa would be a 3" gap @ 56.54insq

2.5x csa would be 3.75" gap @70.69"sp

In other words I need a slightly bigger gap?? Or am I backwards?

I'm new to this so I apologize for the learning curve lol, and I'm Canadian so when I see csa I keep thinking
"Canadian standards association"

Nathan Timmermann wrote:

Insulation is 1/8thick total, used 2 layers of the highest temp header wrap. It's wrapped right from the horizontal all the way up the vertical to about 4" from the top.

1/8" in rocket stove terms is effectively zero insulation. I.e. a fire brick splits (1.25 x 4.5 x 9 inch fire brick) constructed heat riser will typically be insulated with 1" thick ceramic fiber blanket -or - several inches (I use 3 to 4) of clay stabilized perlite.

What size space do you intend to heat?

Nathan, it's ballpark. 2 to 2.5 normaly. You have 1.5 times the CSA, which you could get by with, if you have no other problem.

Ive got a small 22x20' shop, insulated with 8'cieling. Shouldn't take much to heat even with our 40 below Celsius winters.

As for the gap and insulation- my thought process is if I'm going in to insulate the vertical better then I should also cut the optimal gap. So it optimal is 2-2.5 x then I'll make sure I'm in that range before welding it shut.

Thanks again for the responses.
Way easier the guess and check method lol

csa X2-2.5 is not the optimal
it's the minimum

OK on the shop size. Personally, I'd go a little bigger. That ~14" diameter gas cylinder would make a nice exterior casing for a home brew cast heat riser, like Matt shows here:

https://www.youtube.com/watch?v=AqvTdbRSlwk

The inner 6" tube is just a temporary form that's expected to burn out. HVAC air return pipe or even cardboard sono tube works fine.

Build the J-tube feed and burn tunnel from normal size fire brick or fire brick splits, with an opening or channel size of 5.5 x 5.5 inches square throughout. Could be insulated (clay stabilized perlite) and encased in the traditional "cob" style, or with standard brick on the outside, or insulated with 1" thickness of ceramic fiber blanket, and even encase in metal kind of like Matt Walker's ready made J-tube and batch-box combustion cores:

http://walkerstoves.com/aluminum-series-stoves.html

For the exterior heat exchange barrel, I would use 55 gallon drum for lots of radiating surface, since you've made no mention of adding any thermal mass etc. A 2nd 55 gallon drum could be cut in half, and set below the full drum, and used as a base and exhaust manifold. Band clamp the two drums together, use fiberglass rope gasket (replacement wood stove gasket) held in place with high temp aluminum duct tape, for the gas seal. This system has several advantages, like being able to remove the top drum for inspection and cleaning.

Of course the final component, a 6" chimney system as mentioned previously.

So I'm looking at Adding 2" of high temp insulation around my vertical. So total dia of internal riser would be 10", with an ID of 6".

Anyone see a problem with this?

Csa of 28.27insq (6“dia) flowing into a area of 75.39insq.

Tank dia is 14"
Internal vertical is 10"
J tube is 6"

Or would a greater difference be best? Such as a 9" vertical?

Is better insulation more important or space?

Thanks - Tnt

A 14" ID tank and a 10" OD riser leaves 2" space all around, which for a 6" system is adequate. Better insulation is of prime importance, as long as other factors meet the minimum.

the insulation will kill the metal over time

How would it kill 1/4" steel..? I get maybe over 50 years it might but i can't see it doing anything before then. Please clarify

how a mice can kill 1/4" of cheese?
more insulation=more heat
high heat+high oxigen flow = high spalling.

Satamax can tell you first hand about destroying a 1/4" steel vessel in one season. Good insulation will let the steel get so hot it softens and sags, maybe before it blisters and corrodes from the hot oxygen-rich atmosphere.

High oxygen, eats the carbon of the steel. Steeel and iron have different expanding rates, so the metal starts to blister, which is called spalling. I did an experiment with some hydraulic tubing, 10cm external diameter, and a gas bottle "cotton shaker" cyclone. All insulated with 4" of vermiculite. It died within 32 hours of burning.

The hydraulmic tubing delaminated badly. It looked like 1 inch puff pastry by the end of the 32 hours of burning. And the top of the gas bottle, which was about 1/8 thick sagged under it's own weight, since it heated up above red, around white, which is iron melting temp.

If people don't believe me, go on guys, try it. I mean, make a rocket which is burning properly, and see if your metal can cope. I'm ready to make a bet on the failure of such a contraption.