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Rocket stove not getting hot enough

 
Posts: 9
Location: Eureka, NV
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I built my first stove using a propane tank about 36" tall, 15" in diameter. I've got a 4" feed tube, 4" fire tube and 3" heat riser that is surrounded by a 6" pipe with Perlite in between. The top of the insulated pipe is about 1.5" from the top of the inside of the tank. I'm getting excellent draw and the fire is burning very well. However, after burning for three hours, it just doesn't seem to be throwing much heat. I've seen other stoves that get cherry red on the burn tube and heat up rather quickly. Also, I'm burning scrap lumber, mostly pine flooring and 2x4's that I've split into smaller pieces.

Any suggestions?
Rocket-Stove-002.jpg
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Rocket-Stove-010.jpg
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Hello Mike. From what I've been able to deduce from all the posts and articles I've read, I would guess that your problem is an uninsulated burn tunnel. You would have to do some modification, but if you could surround your existing tunnel with a larger sized tubing and then fill that with perlite, you might find it improves the performance. Nice job, by the way!
 
Mike Pop
Posts: 9
Location: Eureka, NV
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Gary,

I've read differences of opinion on the insulated or uninsulated burn tubes. It's a good place to start but I'm hoping for an easier solution. The biggest hurdle is draft which I don't have a problem with. I'm thankful for that.
 
Posts: 126
Location: Springfield, mo
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Mike
Using that tank is great for a rocket heater. I imagine that it came together with parts on hand. Here's a link to a plan that is very similar to what you have ZeroFossilFuel He give's great detail on planning the design. I got started on my design after being inspired by his rocket heater.
You should have used a 4" riser and maybe an 8-10" insulator around it. Yes you do really need to insulate the burn tube. You could try surrounding it with perlite and another larger metal tube and fill the bottom of the tank with perlite too. To get the super hot temps for maximum combustion the heat needs to be contained, not allowed to radiate off of the metal burn tube. If you insulate it will necessarily increase heat and hasten the deterioration of the metal. If you haven't read it already metal is likely doomed to failure in a relatively short time. Unless of course it's cast iron.
You might also consider adding an ash pit area at the bottom of the feed tube, lower than the bottom of the burn tube. All that said your tank is a lot thicker than a metal barrel and therefore would need maximum burn efficiency to get it radiating good heat.
Lastly what size it the exhaust hole on the tank? It too should be 4"
 
Mike Pop
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Location: Eureka, NV
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John,

Thanks for the tips. I will make those modifications. The feed and burn tubes are 4" and the insulated pipe is 3". Exhaust is 3". It's all I had on hand. All of the steel is 3/16" thick so I would imagine it does take longer to heat up.

Much appreciated!

Mike
 
John Adamz
Posts: 126
Location: Springfield, mo
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Mike Pop wrote:John,

The feed and burn tubes are 4" and the insulated pipe is 3". Exhaust is 3". It's all I had on hand. All of the steel is 3/16" thick so I would imagine it does take longer to heat up.



The thicker tube metal is good, but I was talking about the tank thickness reducing the radiant heat. Enlarging the riser, exhaust port on tank, and the flue duct from there will help significantly with flow and heat output.
 
Mike Pop
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Location: Eureka, NV
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I made upgrades to my stove:
1. Increased the insulation around the up pipe from 6" to 8".
2. Insulated the bottom of the tank, up to the burn chamber.
3. Added an ash pit and screen.

The results:
Stove gets hotter but not hot enough.

My hypothesis:
I think the steel tank is just too thick. The stove is working as it should but taking forever to heat up the tank. My solution may be to go to a thinner walled tank.

Overall, it doesn't burn a lot of wood, drafts nicely, and puts out very little exhaust. I have it vented through a window in my garage.
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gardener
Posts: 3466
Location: Southern alps, on the French side of the french /italian border 5000ft elevation
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The answer is, what do you expect from a 3 inch riser and 4 inch feed tube all metal stove?

I'd say zilch!

You say it's working properly, but to say this, have you ever built another rocket stove which worked?
 
Mike Pop
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Satamax Antone wrote:The answer is, what do you expect from a 3 inch riser and 4 inch feed tube all metal stove?

I'd say zilch!

You say it's working properly, but to say this, have you ever built another rocket stove which worked?




No need to bash me. This is my first stove and I am learning. I would appreciate feedback that is helpful, not insulting.
 
Posts: 112
Location: Groton, CT
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I don't see that there is any reason why this won't work.

There is only one problem with it. You say it is not heating up, and thermodynamics are law; therefore the heat that is generated in the burning wood is either insufficient for your uses or you are wasting the heat.

My vote is for the latter. My guess is that the exhaust coming out of this pocket rocket is very, very hot. Too much draft is the dead giveaway. Move the rocket into the center of the space with a longer exhaust pipe. Most of the heat from the rocket will radiate from the exhaust, not from the riser. (The differential temperature in the riser/downcomer region is only there to create the natural circulation needed to make it rockety)

Experiment with the length of the exhaust. The goal should be to get condensing action near the end of the exhaust. You should get water vapor at 90-150 degrees, not steam.
 
Satamax Antone
gardener
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Location: Southern alps, on the French side of the french /italian border 5000ft elevation
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Mike Pop wrote:

Satamax Antone wrote:The answer is, what do you expect from a 3 inch riser and 4 inch feed tube all metal stove?

I'd say zilch!

You say it's working properly, but to say this, have you ever built another rocket stove which worked?




No need to bash me. This is my first stove and I am learning. I would appreciate feedback that is helpful, not insulting.

Mike, it's not insulting.

We see about once a month, someone comming with one of thoses metal rockets, yours is a smidge better than the usual one. But still; and i've been there and done that; 4 inchers are not realy intresting. May be a cast core from Dragonheaters works better. But the average joe's usual rocket in four inch taste isn't heating anything usualy. Your water tank or gas bottle could be reused. But try to make a six incher in there. Vermiculite board or stainless steel pipe for teh riser, insulated. Half bricks for the feed tube and burn tunel/ beggining of the heat riser. All insulated. This should give your better results. Only prob, if you are into direct heat, your bell then will be too small.

Dave Turpin wrote:I don't see that there is any reason why this won't work.

There is only one problem with it. You say it is not heating up, and thermodynamics are law; therefore the heat that is generated in the burning wood is either insufficient for your uses or you are wasting the heat.

My vote is for the latter. My guess is that the exhaust coming out of this pocket rocket is very, very hot. Too much draft is the dead giveaway. Move the rocket into the center of the space with a longer exhaust pipe. Most of the heat from the rocket will radiate from the exhaust, not from the riser. (The differential temperature in the riser/downcomer region is only there to create the natural circulation needed to make it rockety)

Experiment with the length of the exhaust. The goal should be to get condensing action near the end of the exhaust. You should get water vapor at 90-150 degrees, not steam.



Dave, this is no pocket rocket. It's a rocket with a bell.
 
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Mike..
Am no expart in this nor have i ever built anything or project yet, but am very much interested in what you have done. I wish that one day i could build a rocket stove like yours.

From the pictures you made, the first thing i noticed was that THE SECONDARY BURN(where the heat/fire travels Upward is TOO TALL. The heat generated is not able to cover the whole Burn hence not heating the whole room well.

I believe that if you manage to cut the BURN PIPE to a half and then cut the Insulator as same as you cut the Burn area will help the heat to travel faster and you will use less time and fuel for the same project.
Heat generated must be greater than the Secondary burn and the Outside Temperature, for it to heat and be Stored in the stove

Again it's just my opinion not from experience speaking.. please don get me wrong...
 
Rocket Scientist
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Location: Upstate NY, zone 5
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The heat riser can hardly ever be too tall. Extra height beyond what is needed for full combustion simply adds to the draft generated. It could be possible that the tiny 3" diameter riser is creating too much friction. The system size for heat generating purposes is pretty much determined by riser cross section, so you effectively have a 3" system which cannot ever be capable of generating enough heat for your needs. I believe replacing your current 3" riser with a 4" riser, and insulating around the burn tunnel, will do a lot to improve the output of your rocket. Increasing the insulation on the riser without putting any insulation on the rest of the combustion core will do very little. I think that a 4" system will be enough cooler and less intense than a conventional 6" or more system that the metal components may last several years. The better it burns, the shorter will be its lifespan.
 
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3 inch round pipe = 7 square inchs

4 inch.                    = 16.6   almost double

6 inch                   = 28 .8  almost 4 times larger
Go bigger. 6 inch will still fit into your tank and allow big pieces wood. The riser  size determines the size of your stove. 3 inch system is about half the size of 4 inch.
 
pollinator
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My thought on this would be that you would get more heat without such a thick walled tank. Since you are trying to extract heat immediately instead of move it to storage in a mass, the quicker that heat gets through the metal the better. My guess is that most of the heat is going out the chimney. Since it burns well and draws well you know it’s making heat, so where is it going?
 
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