Questions On A Large Rocket Stove (with a twist)

Well hello all…

I’m having some difficulties with a RMH I’ve built for an aquaponics green house I’m constructing. I’ve got so many figures in my head and I want to only bother you fine folks with the facts you need to properly analyze my situation. Of course I’ll be happy to answer any questions you may have about my use and build as they are posted.

I purchased the book by Evans & Jackson a couple years ago and have largely tried to base my construction on the “science” in that book. I have obviously strayed from the main design in that book as I have some rather special features I’m needing to incorporate. It would appear however I’m missing or have yet to incorporate some important aspects of the RMH in my design.

I intend to use this RMH to heat water in an aquaponics greenhouse. The exhaust is buried under a 1500 gallon sump using an 8” pipe. The water will not only heat the fish tank it's pumped to but eventually all the grow media rocks used in the four 4’ x 20’ grow beds. I’m sure it will take a considerable amount of time to stabilize all this mass. But I’m hoping it will also hold onto that temperature once it has captured it.

I’ll post some pictures and try to briefly explain…



Above is a CAD of my basic design. The exhaust or horizontal flue sets 2’ under water. It runs under a 24’ long body of water before rising 9’ feet and then through the wall. At the moment the flue stops there just outside the wall under an overhang outside. I have played with the idea of placing a fan out there to help the draw. The heat riser is made of an 8” steel pipe 1/4” thick. It is insulated with the vermiculite/clay mixture described in the book. The insulation is 3.75” around the heat riser. There is at least a 2” space between the heat riser and the top of the capping barrel, there may be more. The heat riser is also completely offset in the barrel to reflect the heat toward the inside of the greenhouse. It also directs the flow strait down into the ash pit area. It then moves over to where it will go down 2’ below the water. When the heater is running well the pipe at the other end is cool enough to put your hand on. It’s only barely warm with the smoke wafting out lazily.



Above is a view of the system as it is now. In the distance the 9’ pipe up the wall can be seen. Coming forward is the 24’ sump of water. In the foreground is were the main RMH can be seen. The ash pit and 8” x 9” (the 8” Wide ID label at the bottom of the CAD picture should read 9”) square channel where the exhaust goes down under the water is covered with a 1/4” steel plate.



The above picture is a view with the 1/4” steel plate removed for clarity. The narrower fire bricks seen in the middle was where I planned to introduce another exhaust pipe should I need to redirect heat if the water in the sump was getting to hot. At this point I don’t see it being needed.



Above is a closer view of the current fire tunnel and feed area of the system. This is obviously were I’ve been experimenting and playing with different ideas. Generally the stove is easy to start with little or no smoke. I usually start a fire just inside the fire tunnel. As it is getting started I’ll blow a little to encourage the heat up the riser and that’s usually enough to get things started. The lower area is a clean out area. My thought was to use a stainless steel grid or grate that spans from the end of the fire tunnel over this ash clean-out area. As wood burned it would fall through the grate and could be easily cleaned out there.

The Main Issue…
I think the main problem I’m having is not enough draw to start with. If there is much breeze or wind outside, the draft is easily overcome and smoke begins filling the greenhouse. As is visible I’ve played with different feed tube designs but inevitably have had to cap them off to keep the smoke and eventually fire from coming out the feed tube. I’ve actually been using the capped feed tube as a place to hold sticks of wood that gravity feed down into the ash pit area where they burn there. Oxygen is supplied down at the door of the ash pit. I’m aware that I need to better seal and insulate this fire tunnel area but was wanting helpful input as to move me in the write direction or even point out some concerns elsewhere before I proceeded. I may even have to rebuild something (perish the thought).

Also at times when the stove is running well, I need to feed it about every 45 minutes if I’m to keep the rocket going. Any ideas on getting a halfway decent nights sleep in the wintertime? I have had some thoughts myself but am interested in yours out there. I was thinking the feed tunnel could be lengthened to allow longer sticks of wood to be used as fuel. That will not work however if the fire is just going to work it’s way right up the feed tube.

There is much more information I could give but think I’ll wait for questions and answer those.

Thanks again for all your insight!

Ed, if your chimney is as drawn, it's sub optimal. You need to insulate the vertical part, and the part going through the wall. As well as adding an outside insulated chimney, which would clear the nearest tallest building's top. May be in a 50ft radius.

Make sure the transition from barrel to flue has at least 1.5 CSA.

And if you want to heat more, and and store much more energy, look into batch rockets.

Yes, insulated chimney first, as that is causing many of your problems. Depending on the chimney height, that could also be a major factor.

If you want to get heat overnight, you need a large enough system to produce a cold day's worth of heat in half a day. The batch box is ideal for this, as it burns a given amount of wood faster than a J-tube for a given system size, with no tending for the hour-ish it is burning. An 8" batch box can produce a tremendous amount of heat.
https://permies.com/t/40007/rocket-stoves/Results-batch-box-thingy-Innovators
Then you need enough heat transfer surface to absorb it before it goes up the chimney. For this application, a split duct with parallel runs under the tank may be your best bet. Access to adjust the flow in each run may be needed to balance them for best effectiveness.

May I ask your clay:vermiculite (mass:insulation)ratio around your steel riser?

I had less than 40% perlite (insulation) around my steel riser and it was giving me issues. I removed the riser surround and it worked great after that. Too much mass:insualtion ratio). Not enough perlite insulation. Since then I have installed a stainless steel liner, and removing the mass from around the riser helped a lot.

Have you tested the core by itself for performace/issues. I'm sure the insulated chimney sure would help a lot as others mentioned also. A mass like that sure would take a long time.

Also curious, why the two 90 elbows and 3' horizontal exhaust, after the 9' vertical, but I guess you have to go thru a wall there, so you should have more vertical pipe after that?

Successful perlite-clay mixes I have done have been somewhere around 5-10 times as much perlite as clay by volume. I don't premeasure, but dust powdered clay on dampened perlite, mix, and repeat until all the perlite grains are covered with clay and it can form a solid snowball that pops with firm finger pressure. This mix becomes quite strong when fired.

great suggestions so far with insulate the chimney and increase the height of the chimney. if that were my set up i would upgrade the heat riser fire brick with a thick outer section packed with lose perlite instead of clay mixed perlite and that should substantially increase heat and draw for sure. i believe i almost doubled my draw alone by upgrading my heat riser and insulating my chimney that way.

John McDoodle wrote:May I ask your clay:vermiculite (mass:insulation)ratio around your steel riser?

It's actually been over a year since I made this stove so I don't remember for sure. I do remember however tearing down the first insulation around the heat riser because I felt as, you have pointed out, there was not enough air in the mixture. I've thought about increasing the air in the insulation even more.

John McDoodle wrote:Have you tested the core by itself for performace/issues. I'm sure the insulated chimney sure would help a lot as others mentioned also. A mass like that sure would take a long time.
Also curious, why the two 90 elbows and 3' horizontal exhaust, after the 9' vertical, but I guess you have to go thru a wall there, so you should have more vertical pipe after that?

If you are speaking of removing the barrel to see how well the foundational system draws I can not. This is in a green house. If you notice in the second picture of the entire system the ceiling is plastic. I was initially thinking to keep as much heat in the building as possible so I kept the rising chimney inside. Then I needed to get it outside but rather then go through the plastic ceiling and roof I took it though the wall.
Is the main reason for insulating the chimney an effort to keep any remaining heat in the chimney thereby adding in the flow upward? At this point I'm ashamed to say there is no more pipe after it gets to the other side of the wall. At the moment the exhaust is simply expelled under and overhang out there. Going through the roof would involve some other issues I've yet to address. But to start with what would I insulate the chimney with? Fiberglass?

Glenn Herbert wrote:Successful perlite-clay mixes I have done have been somewhere around 5-10 times as much perlite as clay by volume. I don't premeasure, but dust powdered clay on dampened perlite, mix, and repeat until all the perlite grains are covered with clay and it can form a solid snowball that pops with firm finger pressure. This mix becomes quite strong when fired.

Thanks Glenn for you perlite-clay mixes. As to a batch box I have seen this about a year ago but not the post you referenced above. THANKS again for that link also. The size of the green house is 36' x 40' with the RMH set to one side. I'm needing to set the heat riser to one side like I have it now for the purpose of directing the heat into the room? I don't think the design I found at that link you gave above would do that. The heat riser is only half the hight of the double barrel hight. Also I've got a plastic ceiling over this whole thing. I currently have a heat deflector hanging overhead but may need to beef that up a bit. I do like the overall idea of the Batch Box and am sure the extra space in the top of the barrel is needed.

Ed, another tube around your existing one, and fibergass batt or roxul is perfectly fine. And realy you will need to extend your chimney.

Satamax Antone wrote:Ed, another tube around your existing one, and fibergass batt or roxul is perfectly fine. And realy you will need to extend your chimney.

Am I raising the chimney higher then any "Buildings" within a 50' radius in order to get above any obstructions to a breeze or wind? I'm located in a rather tight bowl between hills all around me. Two of them are both within the 50' limit you speak of. The winds here can from time to time be quit influencing and for the most part unavoidable.

The 8" batch box thingy is specifically planned to heat a large drafty shop/auditorium space, 1500 square feet plus 500 square feet of adjacent rooms (in Montana). You wouldn't need to have the second barrel on top, just feed the outlet to your tank duct(s). The barrel could also be offset on the riser as you have done. The critical part is the proportions of the batch box and its connection to the riser, and you can find formulas for that at http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions .

"Is the main reason for insulating the chimney an effort to keep any remaining heat in the chimney thereby adding in the flow upward?"
Yes, that's it. As you will not be getting serious high temperatures in the chimney, you can do a simple second outer duct 2" larger in diameter with rockwool packed between the ducts. Just keep rain out of the insulation space. You do also need to raise the chimney top to several feet above any nearby structures; not only will this improve draft at all times, it will greatly reduce backdrafting.

The heat riser is only half the hight of the double barrel hight.

Ed if this is true it may seem to me that you have alot of heat exchange (and you may want the heat for your green house) from all the space in the two barrels and that much heat loss being that its in a non insulated cold greenhouse may take a bit of the power out of your draw since it is going under water and water will also such the heat out of your duct work reducing more draw and since you do not have insulation reducing more draw and no way to allow the steam that will most definitely condensate in your duct with no way to divert it reducing more draw. if i was a betting man i would say you may be running into the same issue i did before i insulated my chimney and that is condensation pooling in my pipes (reducing draw). this may be a bigger issue than you may think and could give you chronic problems without insulating and finding a way to allow the condensation to drip out instead of down to the bottom to pool. can you lift off your chimney and use a flash light to look down to the bottom of the elbow and see if there is condensation?

if you do indeed have condensation forming in your duct maybe there is a way to make a drip edge inside your chimney and allow it to drip out before collecting underground and pooling in your duct creating a blockage.

I didn't see your description of the surrounding hills before my last post. With such topography, it may be difficult to get perfect draft, but the higher you can make the chimney, the better.

Condensation would be a possible consideration, and I concur with FStyles' comments on it.

Looking at your system photos, I think you could build a batch box to replace your existing burn tunnel/feed area, connecting to the existing riser, without touching the rest of the system. You might need to lower the floor, or if not practical, cut a notch in the barrel and raise the top of the firebox from where it is now.

Something I have not seen is a mention of how you have insulated the bottom of the duct under the tank. If it is not well insulated, you will be losing a lot of heat to the ground and not benefiting yourself. If you build a batch box, you could monitor the exit temperatures and find out if you need to double up the mass duct to extract all of the heat. If you did that, it would be an ideal time to upgrade the bottom insulation.

i was thinking how to make a drip edge inside your chimney and that would be to put the smaller duct edge facing up about 1 foot above the ground and bend the edge ever so slightly inward so that when you slip the next vertical chimney duct over it, that will become your drip edge and crimp a very slight gap in the over lapping area so that the condensation can drip out. this will hep in the condensation pooling in the bottom horizontal. that with insulation will do wonders for your draw.

F Styles wrote:Can you lift off your chimney and use a flash light to look down to the bottom of the elbow and see if there is condensation?

I may be able to put a mirror down the 8x9 channel leading down to the flue to look in there. I don't know however that any substantial water would be able to collect down in the flue. Because this sump is dug two feet down I had a situation with rain water collecting. This greenhouse is up against a 90' hi hill. I used a tire wall to hold the dirt back. The wall is in the greenhouse and can be seen in the pictures. When I built the wall I dug a french drain under it and filled it with creek gravel. The wall sits on the gravel. When it rains this drain fills with water. I found that in a good rain the water could backup into the sump area and get under my liner filling the flue. This being a potential issue I installed a bilge pump a foot lower then the sump's bottom. The bilge pump draws it's water from a gravel channel (the beginnings of which can be seen in the first picture below) running the length of the sump. I also was concerned that should my liner develop a leak it would fill the flue and begin corrosion especially if this happened in the summer when I'm not using the heater. It would never dry out. So in the final stages of the sump's bottom I dug several channels that lead from the center cavity where the flue rests down to the bilge's gravel channel. They are located at the two connections of the 24' flue which are two lowest points of the flue. Any moisture would run to these seams and slowly drain out there. I know my "Bilge" is always dry as I'm regularly checking that.

BTW: When I was saying "the heat riser is only half as hi as the double barrels" I was referencing the 8" Batch Box that Glenn linked me to. My system currently uses a heat riser over 60" hi.

Glenn Herbert wrote:Looking at your system photos, I think you could build a batch box to replace your existing burn tunnel/feed area, connecting to the existing riser, without touching the rest of the system. You might need to lower the floor, or if not practical, cut a notch in the barrel and raise the top of the firebox from where it is now.

Thank you Glenn! I'm liking the sound of that. Let me show some pictures that may help you guys see what I'd need to fit this new design to. The more I can keep what I've got the better. But I'm thinking things are not going to be that simple.


Above is a view of how the fire tunnel joins the heat riser. The heat riser is so tall and heavy I was uncomfortable leaving it supported by only cob around a few fire bricks. So concrete was used to fill in all the way around the base of the heat riser. This picture was taken before the lower ash pit was dug out in front of the fire tunnel.


This picture shows a better view of the fire bricks surrounding the carbon steel heat riser. Concrete is now poured around those bricks to help support the heat riser.

Glenn Herbert wrote:Something I have not seen is a mention of how you have insulated the bottom of the duct under the tank.

There is at least 2" of foam under the liner. The foam stops a couple inches short of the flue where vermiculite takes over. The vermiculite fills and lines the channel where the flue is. When I first ran the heater a year ago I got a good idea of how it drafted and acted under certain conditions. But when I filled the sump with water the stove acted more sluggish. I found that the water weight was actually compressing the flue by about and inch or two. When I emptied the sump again the stove operated as it had before. My solution was to remove a good deal of the vermiculite at the bottom of the sump around the flue to let the liner press in closer to the bottom of the flue allowing the water pressure to support it's original shape. That seems to be working much better. Although I have been concerned that the foam may be compressing a bit thereby reshaping the flue into that oval shape again. But the stove does seem to be operating as it did before now.

Below is a picture of the finished sump before the liner was put down. This picture was taken before most of the vermiculite was removed from around the flue.

Glenn Herbert wrote:The critical part is the proportions of the batch box and its connection to the riser, and you can find formulas for that at http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions.

I've loaded his formula into my spread sheet and have already been playing with the Batch Box size.

Ed Kelly wrote:

Satamax Antone wrote:Ed, another tube around your existing one, and fibergass batt or roxul is perfectly fine. And realy you will need to extend your chimney.

Am I raising the chimney higher then any "Buildings" within a 50' radius in order to get above any obstructions to a breeze or wind? I'm located in a rather tight bowl between hills all around me. Two of them are both within the 50' limit you speak of. The winds here can from time to time be quit influencing and for the most part unavoidable.

Ed, the 50 feet radius is "theoretical" it's; iirc what i've read here; the best practice usualy adopted by chimney fitters.

I think, if you raise your chimney 6m/24ft above ground, you should be all right. I have played with shorter ones. And Never had much luck with those.


I don't know how far in your build you are. But two or three details that bugg me.

There's a lot of mass in your actual firebox. Which doesn't seem well insulated.

There's a metal heat riser. That's no good. It will burn away at some point. But even before, it will give trouble, like expanding and cracking the joints of the firebricks around. And then when it starts spalling, flakes can also obstruct the heat riser or firebox. https://permies.com/t/52544/rocket-stoves/metal-burn-tunnel-heater-riser



And there's the "cold plug" the part where you go down with the bricks, after the barrel. On Allerton abey, they've had a cold plug problem. https://permies.com/forums/posts/list/160/41202 and i have experienced one or two too. Going down, with a rocket. Except with a real good chimney, is not a good idea.

Hope this will be of some help.

Also, at eight inches, your top gap of 2 inches is too small.

And two barrels radiating will not leave much heat left for the mass.

Ed you stated:

They are located at the two connections of the 24' flue which are two lowest points of the flue. Any moisture would run to these seams and slowly drain out there. I know my "Bilge" is always dry as I'm regularly checking that.

if your system has ever burned inefficiently you may and will put creosote in your duct work and will work as a "sealer" on your duct seams and wont let the water drain out and you wont see it in your bilge area... i know, i had that same issue, i speak from experience. i had an uninsulated chimney and the steam condensed on the uninsulated chimney and ran back down to my horizontal section and the water pooled instead of slowly draining out of my seems because even a few inefficient burns can seal those seems and then you are left with water pooling and less draw.

Satamax Antone wrote:Hope this will be of some help.

Those are all good thoughts. Thanks for all your help. I'll bee looking at these links.
BTW: I've not commented on the concern for a new chimney but I will be doing that. I may simply continue it underground and bring it up outside. If I'm going to insulate it there is no need for it to remain in the greenhouse. Then I'll run it through the roof at that time. Thanks also for the thought on the more room about the heat riser. It looks like I'll be redoing that as well.

F Styles: I actually thought this morning about creosote sealing those seams. Rather then using a mirror to look down a 24' black hole I will take the flue apart to be sure. Thanks for your insight!