Making a plug for rocket stove intake

Hey y'all! First post.

So I have made two mistakes.

1. I did not find and sign up on permies sooner
2. As a consequence of mistake 1, I added a sloped fuel magazine to my steel mini RMH (more of a rocket stove RMH hybrid)

While I am extremely pleased with how my rmh runs, the fuel magazine is a headache. While it feeds reliably, smoke does draw out of the magazine and contributes to a less effective burn. It also deposits lots of creosote.

I have been testing some makeshift plugs (by covering with painters tape) and have found that my rmh runs VERY well when the fuel magazine is plugged so air can only come in through the intake.

I have also found that I can plug the intake and unplug the magazine to add more fuel and the stove will still run fairly well.

I've decided the best solution is to make two plugs for the 3"x6" tube. Any recommendations on how I go about this? Would a cap of some sort be better? It does need to seal fairly well.

I've attached a picture of a sloppy CAD thermal simulation from an earlier design to give an idea of the layout.

Thanks y'all!

Hi D; Welcome to Permies!
If I were to guess, I would say make caps.  We are not real keen on building metal rmh's here, but rocket stoves are more short term.
I will mention that if its making creosote, then its not reaching true rocket temperatures .
Where are you using your hybred?  Do you have a copy of the RMH builders guide ? Written by master builders Ernie & Erica Wisner. It is the go to book for all builders,  available directly from them or thru amazon.It is worth every penny!

Thank you for the welcome and reply, Thomas! What makes you lean towards caps? I was leaning towards plugs as it might be easier to get a tight seal. What advantages do you believe that caps would have?

thomas rubino wrote:
We are not real keen on building metal rmh's here, but rocket stoves are more short term.

I wish I could use a more durable material, but the application for this RMH is unusual: occasional winter use in a travel trailer. The flexing, and size/weight/temperature safety requirements made traditional materials untenable, unfortunately.

I've done the best I can with this steel but do have relatively low expectations for life expectancy. The top plate is removable for inspection, so we will see how long the inner burn chamber lasts so far so good after two burns, no noticeable corrosion. The walls in the inner chamber (which is insulated) are 3/8". I am trying to keep temperatures relatively low (about 1000 F) and burn times short to extend the life of the stove a bit. We will have to wait and see!

thomas rubino wrote:
I will mention that if its making creosote, then its not reaching true rocket temperatures .

Sorry, I should have been more clear. When both the fuel magazine and the intake are open, some smoke rises up and out of the magazine, and creosote is depositing inside the fuel magazine exclusively. The smoke tends to stall in the magazine under these conditions and condense. The exhaust (which is 4", on the back of the stove, through the second wall) has been relatively clean once the stove has been operating for about 15 mins.

The stove runs awesome after testing and tweaking. I have tested it both in CAD simulation and reality and found it to be efficient and safe. It is definitely not meeting the ideals of a RMH for operating temperatures, but then probably shouldn't in this application.

I leaned towards caps thinking they would be faster, easier to make than a plug.  With a cap you could use a ceramic blanket / wood stove gasket to make an easy seal.  Seems a plug would be more bother to fit and seal.

I suspected that you were using this rocket in a non conventional way.
Sounds like your on top of its condition and have it tweeked to fit your application.
Send us some photos.

thomas rubino wrote: ...wood stove gasket to make an easy seal.  

Great minds think alike, Thomas! I just tried a plug design using a woodstove gasket. I just tested it and am blown away by how well it seals and how well the stove works. I'll upload and link to a video of the install and a run shortly.

Thanks for your support!

As you've found your sloped fuel magazine is acting as an alternate chimney hence the smoke and creosote.  If you're going to use a fuel magazine, vertical is better than sloped generally as sloped can provide enough friction sometimes to stop the fuel from feeding.  Capping or plugging the fuel magazine depletes the magazine of oxygen so the flames then head to the intended chimney.  So, whatever most easily seals it against air ingress is the better choice.

Hi Allen,

I am no expert on these systems but I am getting more experience by the day with the projects I currently have in process.  I am guessing that the feed tube is air tight, and that you are using it to feed wood fuel pellets.

The problem is that you have in effect created competing chimneys.  The rule of thumb on j-tubes is that the feed tube should be 1/3 the height of the riser.  This incentivizes the smoke to pick the riser as its preferred exit route.  I know it seems like you have provided plenty of air with the inlet, but like I said you now have a primary chimney (riser) and a secondary chimney (feed tube).  You are going to need to effective add some more air inlet in the right location.    Creating plugs or caps is a bandaid and not a proper solution to the problem.  I would suggest on the top side of the feed tube to put some decent size (but of course small enough that the fuel cant escape) air holes slightly less than 1/3 the distance up the tube.  Try staring with 2 -3 holes and see if the problem is lessened.

I know it is going to be hard to drill holes in something that you currently want to plug up, but I believe it may just solve your problem. If it doesn't improve the backdraft, stop and patch the hole and go to plan B, plugs or covers.  If it does work, put a bunch more holes on the topside face of the feed tube.  They should not need to go all the way to the top of the tube up a few inches of ventilation from the 1/3 mark up to the say 45% mark on the feed tube should do it.  You may eventually want to add some screening to eliminate small pieces of fuel from falling through.

Best wishes

Drilling holes in the feed tube would just add more places for air to move through the system, weakening the force of the primary draft. You want only one opening for air to enter the core, and through the fuel load pulling smoke and heat downwards is the tried and true method. I suggest you block (plug, cap, whatever) the lower air intake and use that only as an ash cleanout.

The exception to "one opening" is a properly placed and sized secondary combustion air source, which would need to deliver its air right at the point of maximum combustion, around the top of the horizontal tube between feed and riser. I wouldn't worry about that for this iteration of your core, though.

One aspect of good draft in your core is to have the riser significantly taller than the feed tube, preferably at least three times taller. As mentioned before, if the feed tube is about the same height as the riser, it will compete to be the primary chimney.  Is your CAD model to actual scale and proportion with your current core? What dimensions do you have for feed and riser, and for the rest of the system (barrel height, diameter, and clearance to top of riser; exhaust duct size, position, and routing; anything else you can think of?)

It is pretty safe to assume that the drawing is to scale as it was used to simulate the design.  Your proposed solution does not seem to address the issue of the similar height of the feed tube and riser.

As stated it runs ok when the air intake is blocked and the feed tube is opened for refueling.  This is due to the fact that the chimney is already warm.  

As for reducing the draw, the draw will be identical through the burn tunnel.  I agree the large air intake is not necessary, but with the fact that there is limited air flow through fuel pellets relative to sticks, I think some additional air from below is not a bad idea.  It is possible that the reduced length thru the fuel pellets by my proposed holes might reduce the air restriction sufficiently.

So once you plug of the air intake how do you propose he address the competing chimney problem?

This was my hybrid J and L tube rocket dry stack test.  All you need to do is just cap the feed magazine so no air gets in or out.
You can't have both open otherwise you'll get smoke.

Ralph, you are likely correct about the scale, though "CAD thermal simulation from an earlier design" leaves a question mark. We really do not have enough information to know for sure, but if the OP states that he gets smokeback with both open, making more openings in the feed does not seem like a good idea to me. Again, fuel type is not stated, though pellets make sense in terms of a "fuel magazine". If fire does not rise in the magazine for him with the top blocked, then using the lower opening for air intake should work, while damping it down each time the fuel needs to be replenished.

D, please post more details about your system dimensions, lengths, parts arrangement, etc., and photos if you can, so we can give you the best advice.

D and Glenn and Graham,

I agree with Glenn, D.  Please provide us some more details.

To Glenn, if you read my first post in this thread carefully, you will feel the trepidation when I suggest that he should drill a few holes to see if it works.  ;-))

As for Graham and his claim that it will only not smoke when it is capped off.  He is running an interesting little experiment.  Why not try the experiment on your cobbled up stove.  See if you can make it work with vent holes past way up and fix the design.

Btw,  if you did drill a pattern of holes on the uphill side of the slope, that does not preclude making a hinged or sliding cap which could be open or sealed to prove out the design.

To some degree ask of these designs are experimental.  Even when we attempt to build a standard "Wisner" model from their book, we are not them and have not built nearly 1000 of these stoves.  Thus we are still experimenting in a sense.  If we approach it as such, we stand to learn more and can help advance the state off the art for everyone.  That is the beauty of this site.  We share and assist.

Ralph, I think it's a known issue.  If you have a J style rocket and add an additional air intake such as like an L tube, you'll get smoke.
So the answer is to close one of the air intakes off.  His feed tube is too tall so you can't close the bottom intake off, you have to close the top one to prevent it working like a chimney.

Ok, just found the relevant thread.  It's the F Styles Rocket Mag Stove ( mag for magazine I presume )

https://permies.com/t/54074/Rocket-Mag-heater-Rocket-Mag

Hi Graham,  

That is slightly different than what I was proposing.  I was looking as some folks that tried to feed pellets into a rocket stove and trying to create a design that would have feed tube that had perforations in it to allow air flow and thus shorten the effective height of the feed tube.  If you look back at one of my posts, I believe I suggested he might want to try capping off the lower tube and drilling the holes.  If I didn't post it I thought it and imagined that I had posted it.

What you are proposing or showing from F Styles is quite a bit different and I do understand the desire to create a larger fire so that you do not have to tend it as frequently as with a J-tube.

With the bottom intake capped, air holes partway up the feed could work fine. That wasn't actually written in your original post, which is why I questioned the idea.

I wonder if the fact that pellets are so much more tightly packed than sticks keeps heat from radiating up in the feed and igniting upper layers...

I can't see that the OP is using pellets. Was it implied somewhere?

Hi Glenn and Graham,

I read my old posts and you are right that I did not state sealing of the lower tube.  I was assuming that the magazine was fit pellets because of the rectangular shape of it and the similarity to one I had seen somewhere else which had used pellets.  It would be difficult to load anything but quite small sticks into those rectangular tubes unless they are quite large.  It is a different story if they are short. When they are long however, the sticks well more likely bins up because they are not straight and have intersecting branches, etc. If you were going to feed the stove with dowels go for it.  Thus I perhaps erroneously assumed pellets.  It would help if I had read more carefully.  Sorry for any confusion caused by this.

Fuel type was never stated, but "fuel magazine" sounds appropriate for pellets, and yes, the shape and size would make feeding sticks tricky unless they were milled dowels.

I don't have access to provide direct links, but look up Bigelow Brooks Farm on youtube, he heats a very large greenhouse and aquaponics system using a rocket mass heater that is fed (at least at one point, not sure if currently does) using a pellet feed system. It was a rather fancy setup from what I recall, as is the entire greenhouse and aquaponic system as he's growing food commercially with it. While the materials my differ and certainly the scale would change, you might find some helpful info regarding a pellet feed in there.

    I think this is what mark wanted to show.