Single Wall versus Double Wall Woodburner

First Dan, I commend your hard work and attention to detail. I would say that if efficiency was the only purpose of using a double wall, your experiments have shown it's not worth the trouble.

There is another concern however. I've seen hobo stoves turn red hot which means anything that inadvertently comes into contact with the stove can ignite. My insulated stove can get too hot to touch, but it never gets hot enough to start a fire.

I also see that you didn't use a windscreen/chimney to keep the hot gasses close to the pot after they exit the firebox. I'm guessing it would help a lot because I my typical boil times for 2 cups are about 4 minutes. Of course there are a lot of variables here, like size of firebox (mine's about 3 in. in diameter) species of wood, etc., and of course the starting temperature of the water.

Again, there is also a safety aspect. There is no exposed flame with a chimney surrounding the pot, and the inherent baffling make it difficult for embers or sparks to escape the firebox.

If I have time and low winds tomorrow, I'll try my stove without the chimney and see how it affects power to the pot. Regardless of the outcome, the chimney is handy because I store the sooty pot in it, which keeps the rest of my gear soot free.

One other thing. I put a double thickness piece of fiberglass cloth in the ash pan (below the grill) of my stove. I put 10 to 15 drops of kerosene on it, light it and then drop my wood on the grill. It gets the wood burning a lot more reliably than the same quantity of alcohol, especially if the wood is damp, which is common in the woods.

Thanks for your comments Hermin. I kept the testing down to the very basics. I should also mention the water temps were the same through out the testing. A two gallon bucket of water was acclimatized to the outdoor temperature at the time of testing. I had to move the last tests outside due to the high temperatures in the greenhouse where I did the first testing. There was a slight breeze during the last 5 tests that I had to contend with. A wind block will be noticed to the left of the stove in those videos.

I will do more testing when the weather gets cooler. The tests will be for further addition to this thread. Clothes pins will be used for their consistant size and weight. American made will be used if they can be found new.

Thanks for the info on what can be done to increase the retention of heat when using woodburners. Low odor mineral spirits works well or charcoal lighter.

Thanks again for your input.

I tested with and without chimney and got approximately equal results in no wind conditions. When the winds came up, I tried it again. Without the chimney, I didn't get the water to boil. With the chimney, it took about 20% longer to boil the water.

Hermans comments:

I invite all to add to this discussion.

Despite what the "scientific" literature says, the windscreen didn't appear to make a difference in efficiency in still air which surprised me. It made a HUGE difference in a light breeze which didn't surprise me.

I'm going to keep using it because with it, there is no exposed flame which I think is good.

zelph, good (hard) work. Thanks for sharing it over here at BPL.

I have spent some time some while back trying to understand how wood stove can be made to work more efficiently and your results support my conclusions. I think a single wall stove with large windscreen which also covers the pot would make the most efficient setup.

Hi Dan,
I would also agree you have put a lot of effort in to try and make these tests fair, well done. That said either I dont understand the results or I cant see how they show any result. The final temperatures of each set of results are surprisingly different. If you tried to show statsistical significance with such variation and such a small sample you will almost certainly not acheive it. This does not mean there is no difference just that the experiment does not show it. At this point I would say we are all quite capable of believing whatever our prejudices lead us to.
The only practical way to get significance would be to somehow get rid of the variation within a set of results. The task of proving there is no small difference is even more difficult than the task of proving there is a difference.
Anyway its not just efficiency, for me reduction in smoke is very important; my aluminium outer jacket weighs very little.

Herman, what we read in scientific literature needs to be taken with a grain of salt as you have experienced.. You have seen for yourself that actual testing is what is needed for us as DIY stove makers.

Huzefa, you are welcome. I'm glad you were able to use the information and videos. I'm glad to be of help to all DIY'ers.

Derek, what you have seen here is the progression of tests. You've been able to see how the stoves perform under test. There is very little significant difference between the two if we look at water temperature. Like you said, you can't see how they show any result. There is alot of difference in the amount of smoke in the double wall stove.

I'm preparing to do an additional set of six burns. I'll use the same stove, same fuel and tinder, same amount of fuel, all tests done same day. Water temperature will be taken at 8 min. Starting water temperature will be the same. The only difference in the test will be the stacking of fuel in the stove. All pieces will be placed in an upright position, narrow end of clothespin facing downward to allow easy passage of incoming air. Three tests of the double wall and three of the single wall. The tests will be started at break of day under calm conditions inside my greenhouse.

I'll do my best to get rid of the variation within a set of results. Videos will be made of each test burn.

Darek, do you feel that more than 3 tests per stove should be made? 4? 5? or 6?

Anybody have any suggestions?

Hi Dan,
I looked closer at your videos because your remark that double wall smoke was different did not say it was less.
I am still not sure because I do not know how the vidoes go. Have lumps been edited out? does the hour hand of the clock not matter? did you reset the minute hand to zero each time and start the second hand on ignition.
From what I see it could be that double wall stove provides more smoke not less. I cannot see a mechanism for this. In the interests of fairness you have lit the whole batch at once, it therefore takes a long time for the ingition at the bottom to burn through and ingnite the smoke. This is not how you or I would use a woodstove. From the videos it looks like the hottest water tests were those where the smoke happened to light earlier.
The only difference I can see that affects double/single is that from memory when you ignite the smoke from a double wall it more often sets light to all smoke and the flames dont lick up the pot quite so much. The single wall some times does not fully ignite the smoke or only round the edges, leaving a central column of smoke. When the single's do ignite fully the flames are quite long.
I would theorize that smoke, and flames higher than the water level are pretty useless.
My stove has a potstand that more fully encloses the flame column. You can see sometimes the wind blowing the flames around which means that design needs a bit more help from a windscreen, nothing to do with double/single though.
I will post saome more in a bit a customer has come

Hi Derek,
The double wall stove produces smoke for a longer period of time. In one of the tests I put a butane lighter to the column of smoke to show how it ignites with and outside source.

My camer allows me to make videos in 20 second increments. It then stores the image and then goes back into standby mode which is "OK to do another segment" There are 13 videos, one for each test burn. Each one is made up of 20 second segments. Each video varies in the amount of segments. The clock is set so that the hour hand is set to correspond to the test taking place(in the last 5 tests) The min. hand is set to 59.5 min. befor the hour. That gives me 1/2 min to get ready to light the stove and have the camera ready. The alcohol is poured into the shallow tray, the stove placed over the tray. My butane lighter is ready and when the second hand reaches the 12 the lighter ignites the alcohol fumes that are gathered inside the stove at it's base. The flame of the lighter is focused through one of the base holes. It ignites with a "poofff". I then hold my hand over the top of the stove to be sure heat is coming up through the wood. I've tried to capture the smoke igniting as the first segment of video. It's a hit and miss situation. In some cases there are alot of sements showing smoke and then all of a sudden you see fire. Well, during the time the camera is "loading" the smoke/gasses have ignited and I was not there for that "Kodak" moment.

As you have noted that there was a breeze that was pushing the flames away from the pot and I was slow to provide a wind screen for the stove. At the end of the testing I had it in my mind to do additional testing to try and eliminate more variables. I have 400 new pieces of clothes pins being acclimated in my greenhouse to be used this coming weekend for 6 more tests. I'm hoping to use a digital temperature recorder to be more accurate in the temp recording.

The tests are to see how the stoves perform once they are fired up and nothing else. Our methods of preparing are going to be different in the real world. Choice of kindling and fuel will vary, the use of a windscreen should be a "must"

As you notice in one of the videos where I was trying to ignite the smoke/gasses as they were coming out they would ignite and then go out and then I would wait a few seconds and retry to light and then it was successful in staying lit. I speculate that is similar to what we see and hear of in tests of building canister stoves where the term "Flame Liftoff" occurs. Too much velocity and not enough heat in the case of the wood stove. Only when temperature(heat), oxygen and fuel are in the right proportion will the gasses ignite when an ignition source is introduced.

Maybe it's just my good luck, but I'd swear the smoke from my stove is keeping the skeeters at bay.

My stove puts out a lot of smoke until it gets going and even when it's going straight up, we're not bothered by bugs.

One other useful design feature: I put a bail handle on my latest model so it can be carried short distances while it's cooking. This comes in handy for wind shifts. The other campers at the table are a whole lot happier when the stove is down wind of them.

But I think your experiment suffers from inaccurate assumptions. Coming from a scientific background, I can attest that real world experiments are extremely sensitive to initial conditions and you must be very careful in how you set up the experiment.

Double wall woodstoves are based on the "reverse downdraft woodgas" stove principle, which is a modification of a forced air downdraft stove. (obviously, reverse downdraft means updraft or natural convection) That means that the fire starts at the top and migrates downward through the fuel. In this mode the flame front stays small and is close to the woodgas being produced, thereby creating more heat but less upflow of air. (the woodgas is actually being produced from the radiant heat of the flame in the fuel BELOW the flame, which then passes up THROUGH the flame to be ignited) The insulation of a double wall stove helps hold the smaller amount of heat in, and preheats the secondary air right at the point of combustion.

When you light the fire from the bottom, you completely change the physics of the burn. The bottom fire heats up all the fuel above at one time, creating the huge cloud of unburned smoke, yet the fire itself is inches away (at the bottom) from the woodgas produced. Therefore, you waste a huge amount of the woodgas before it ever ignites. Once the flames propogate up enough to ignite the woodgas, you get a huge burst of flame and burn through your fuel very quickly.

Since there's this big fireball, air is sucked through the stove very rapidly, which pretty much negates any insulation quality of the stove itself. The stove just becomes a container for an open fire, much like an HVAC vent just moves hot air from one place to another without picking up much of the heat.

The key to an efficient and hot woodgas stove is to keep the burning woodgas inside the stove (and under the pot) for the maximum length of time to allow all the gases to burn. When your flames lick up around the outside of the pot, you're just heating the outside air, not the pot itself.

What you want is a slow, tightly contained flame that burns smoke free UNDER the pot. A double wall stove will help in that regard by preheating the secondary air. (The primary air supplies the flame front that propogates down, the secondary air mixes with the woodgas above the fire to insure complete combustion of the woodgas)

I would suggest running your experiment again, but ignite the fire from the TOP. If you use fluid to start the fire, make sure it doesn't drip down into the wood at the bottom and end up starting a fire there. Best way to do that is with a cotton ball soaked with a few drops of kerosene placed on top of the main fuel with a little kindling. (wood chips)

If you enclose the area between the stove and the pot with an enclosed standoff, you will also increase the secondary draft while heating up the woodgas and air mixture for more efficient burning. If your flames are licking way up the sides of your pot, you've got too much primary air or too much fuel being wasted.

I think under these circumstances, you'll find the stove to be more efficient and the double wall and insulated modes to heat water faster, with less smoke.

Good luck!

Hi Jay,
Thank you for your input. It’s good to have someone with a scientific background to join in on the subject of wood burning stoves. Most of the stoves we see being made are of the “Bushbuddy” type rather than the “Garlington” type.
The “Garlington” type being a TLUD, (Top Lit Up Draft)

As you pointed out the original was a forced air downdraft stove that was deemed not to be very easily made available to 3rd world countries that had no electricity or readily available batteries to power them. So what the maker did was use a ceramic “riser” as the container for the stove body. It was a thick walled container, not a double wall container. Primary air was supplied through it’s base and air flow was controlled via a valve. Very small amounts of air was used for initial ignition of fuel that was ignited from the top of the fuel pile. As the fire progressed downward the amount of primary air was increased.

If we watch how the Bushbuddy is used we see the fire is started from the bottom and works it’s way to the top. A chemical firestarter is used to ignite the fuel just as is the case in my tests. The Bushbuddy being solid, mine being liquid..

For now I’ll stay with continued tests with the fuel being ignited from the bottom. At some time in the future I’ll continue with some additional tests with the “Garlington” stove thread that I have started here somewhere. I’ll incorporate the suggestions that you have given here for a Top Lit Up Draft stove.

I appreciate your scientific background and your input.

I’ll see if I can round-up a photo of the stove made with a ceramic “Riser”

I found it…….

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Hello again Dan,
continuing from where I left off I wonder whether it might be more realistic to load half the load at say 3 minutes when the first part are on average burning merrily. I know you would not be able to arrange them in a standard way but it would be more like real life. I think it might reduce the variation in the sets of results. I suppose mostly I hate watching that smoke.

Dan,

As I'm sure you know, the bushbuddy was designed to feed fuel into the combustion chamber very slowly, after starting a tinder fire at the bottom. In fact, the bushbuddy is designed for constant stoking, hence the large opening in the standoff. Preloading all the fuel at once creates a huge amount of smoke, which is unburned fuel, and results in a overly large fire once it finally ignites. That also results in a far larger draft than that experienced in the bushbuddy.

As mentioned before, that large draft pretty much defeats the purpose of insulation since 90% of the air is rushing up through the primary vents.

So, if you want to test the bushbuddy design, I suggest feeding your fuel into the stove gradually (slow enough to prevent smoke) I think under those circumstances, you'll see a bigger effect of the insulation.

Hello Derek,

I hated to see all that smoke also =) that's why I interviened with the butane lighter. I should have let the tests go at will. No intervention!!!!

The tests were informative. If you view the videos of the single wall stove you sill see the flame pattern that looks like a jet of burning gasses coming from the top holes that so many say is wood gas coming up the wall of their stoves and igniting when entering the inner portion of the stove. The video shows that it's only air pushing it's way towards the flames. The air is following the natural draft of the rising hot air.

The tests also show that the rising smoke/gasses can be ignited. So when we are out in the real world and our campfires are putting out a lot of smoke, put a match to it. It may poof right into ignition.

I agree that it would be more realistic if we would load only half the amount in the stove at first and then add as needed.

As we see time and time again, stove makers are shaving off every un-needed gram from their stoves. They make them out of every conceivable material. Should we have a double wall wood burning stove?????? Can we ditch the extra wall and just use the outside wall????? The ideal size can to use is one that will hold enough wood for one batch, light it and walk away. It's recommended that the BushBuddy use fuel tabs to get it going. Put the fuel tab under the stove that's loaded and away we go.

My tests are to show the capabilities of the stove and only the stove. Double wall versus Single wall.

There are so many variations of holes that can be put into this can set-up that would make our heads spin. I used 1/4 inch holes. That's a size that most DIY'ers can handle. They don't have to go out and buy a punch that will do 1/2 inch holes.

One day I was walking through the isles at the local K-Mart and saw a stainless steel flour sifter and said: It's a Wood Stove. Nice convienient size, will last forever, no rust, looks fabulous after a couple of burns. High temps make the stainless turn pretty colors. I bought it and turned it into a stove. I named it Martha Stewart and then later changed it to Martha Stixx. (it burns sticks/stixx)

More tests coming up this weekend. No intervention ;=)

Cheers Derek, let's make some fire!!!!!

> flame pattern that looks like a jet of burning gasses coming from the top holes that so many say is wood gas coming up the wall of their stoves and igniting when entering the inner portion of the stove. The video shows that it's only air pushing it's way towards the flames.

Yep, kinda funny when compared with an alky stove. Sort of 'inverted' flames: air going into the fuel gas.

Cheers

I can see no intervention allows you to claim that the experimenter is not biasing the results by using their skill in fuel placing to get the result they want, but at the moment the all in one fuel batch looks too artificial, that is not like the real world, where we would feed either single or double wall stoves more continuously.

(rcaffin)>Yep, kinda funny when compared with an alky stove. Sort of 'inverted' flames: air going into the fuel gas.

Hi Roger, I just had the thought of using a small alcohol burner inside this wood burner. I would use HEET in the Red bottle so that it would burn orange/yellow/red flames and we would see the same flame patterns. Air entering through the holes into the flames making it look like wood gasses burning(as so many are led to believe)when in reality it's only air entering through the circular hole saying " make way there flames" I'm coming through here on my way upward. ;)

(Derek Goffin)>but at the moment the all in one fuel batch looks too artificial, that is not like the real world, where we would feed either single or double wall stoves more continuously.

I agree with you on that thought. My thoughts are befor we go out into the real world let's decide what we want to make and how we make it before we take it into the real world. Do we really need the double wall?

A week ago I did more tests but not at the same time of day and the results varied. I used a different name brand clothes pins made in China(U>S>A> made not available). The 1st set of 3 tests(morning) were with the double wall stove. 35 clothes pins acheived a boil in 6.5 min. in 2 of the 3 tests.

2nd set of three(evening) only one test acheived a boil. A storm front moved in, barometric pressure rose, ambient air temp was 12 degrees higher than the morning temp. Under the existing conditions the single wall burned too fast.

Big difference in results due to fuel quality and placement of fuel so that it was all in a verticle position.

I've just returned from a family campout and maybe I can get a video of the tests posted by tomorrow.

I burned some GooGone cleaner in the single wall stove to have some simulated wood burning type of flame color. It’s to show how the air going through the holes near the top push it’s way into the flames and give it the look of the so called “woodgas” on fire in a double wall stove. There is no such thing as wood gass coming in the outer wall of a double wall wood burning stove. So many times we hear of the downdraft gassifier wood stove burning woodgas coming up the wall of the double burner. It’s not true, “Urban Legend”

What you see in the photos is a single wall stove with plain ole air going through the holes into the path of the upcoming flames.

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Based on your tests, I built a new stove which is un-insulated. I used the space that was previously taken up by insulation to expand the capacity of the firebox. I also replaced the screw in feet with retractable feet for greater stability. I kept the intake on the bottom of the stove where it is wind direction neutral, and I retained the chimney with bail from the previous stove so it can be moved with a fire burning in the stove.

I think the overall performance is improved. The weight is now 10oz. for a two person stove. What I like most of all is the increased fuel capacity allows for 10 minutes between refueling vs. 2 minutes in the old model. Now I can do something besides feed the stove when dinner is cooking.

Stove is steel and stainless steel. Someday, when I can get some titanium and a spot welder, I'll make it lighter.

Photo's below:

Packed Stove

View of firebox stowed in pot.

Assembled stove.

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bottom of stove, leg locking detail. "S" bend prevents rotation, hook over edge prevents retraction, and notch in stove prevents swinging side to side.

Herman, that is quite an improvement, great modifications.
Not having to attend to the stove makes a world of difference for sure. 10 min between refueling compared to 2 min as in the original is a huge difference.

I realy like the design of the retractable legs. All out great improvements!!!! Well Done!!!

Thanks for sharing your findings and confirmations.

Dan's "GooGone" post made me think about something. His photos clearly show the air basically blowing through the top holes, creating that gassifier look we're accustomed to. I'm not sure if the photos prove the non-existence of woodgas in the double wall stoves. But at the very least air is drawn up between the inner and outer walls. It would have to get heated to some degree. So feeding hotter air into the flames… not woodgas, but it might help?

Hi Brad, I agree that the air is heated to some extent. Take another look at the single wall stove burning "goo gone" Does it look like it needs some assistance with pre-heated air to burn efficiently?

I feel that the small increase in air temperature is insignificant in the operation of the stove and is not worth the added weight.

All things considered, the single wall stove is my choice of design.