The lowdown on my rocket stove performance

by ziggy on December 9, 2009 -- 30 comments -- Follow

The past few weeks have been the first real test for my rocket stove with outdoor temperatures dipping below freezing at nights, and daytime temperatures in the 30s-40s. (This week has below freezing daytimes, with wind chills in the negatives – but I am with family in NJ and April is at home tending the stove, so I keep hoping that it works well for her…) I have been firing the stove fairly regularly, and I’ve learned a good deal about its performance – or unfortunate lack of, at crucial times!

First and foremost, the rocket stove does not draw 100% of the time. When the air is still, or when the wind comes from the south (sometimes), the stove hardly draws at all. As you may or may not know, the flue pipe exits at ground level on the south side of the house, but I have added a vertical stack (6″ pipe, not insulated) to get the stack above the eaves (but not higher than the very peak of the roof). The stove is very sensitive to the wind. At times, the draw is so weak that the stove smokes into the house, and we cannot use it. The draw is strongest when there is a wind from the north or northwest. It is obviously not ideal to have to depend on the wind to be able to use the stove, because the house takes a lot of energy to get warm, which leads me to…

The stove takes hours to get warm. I am fairly certain that this should not be the case – that it should be warmer with less time (and wood). After several hours of firing the stove, the barrel just doesn’t get that hot! I am convinced that the heat riser, which is built out of firebrick, is leeching some of the initial heat before it even hits the barrel… this is my guess. I am not certain, though. With the barrel itself taking hours to get really hot, I have little hope that we can get the mass of the cob bed warm. It just doesn’t seem possible with the amount of time and energy it takes to get the whole thing really cranking. It is very difficult to raise the air temperature in the house – it’s been hovering in the low 50s, and after four hours of firing the stove, we can usually only get the air temperature up 3-4 degrees. It’s pretty cold in the house. We have gotten the house to 63 degrees inside, but that was after two days of pretty constant stove firing during the daytime (with a strong north wind).

I am not sure what to do to improve the draw. Rocket stoves are very finicky, and there are many variables to consider. Is it simply because of the two 180 degree turns in the flue pipe in the mass of the bed? Is it because the stack exits horizontally, and is only vertical outside of the house? Is the outdoor stack not tall enough? Is it something in the guts of the barrel, a tight gap somewhere where the gases slow down? Clearly the stove is capable of drawing well (such as on days with a northerly wind). But why does it not draw so well when the air is still?

And for getting it hotter faster, switching out the firebrick heat riser is only my theory. It could be because I am using a 6″ system, and the stove has a small firebox, too. There is only so much wood we can burn at a time. (By the way, it is good firewood – kiln dried oak scrap wood from a furniture place, mostly. Very small dimensional pieces.) But that barrel should get way hotter way faster!

Rocket stoves present a huge learning experience. Clearly this stove is not perfect, and it would be naive to think it would work without a hitch in its first iteration (and I have never built one of these things before, either.) But I do hope some solution can be achieved, because it could use some improvements! Brrr….

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  • My understanding of stoves is that a taller chimney is better for draw but harder to start the draw. A hot fire is best to start the draw but with smaller pieces of kiln dried oak you should be covered on that. It might be that with the long flat part through the bench a shorter chimney would work better. When the wind is blowing from the north is the house in front, behind or next to the chimney?

    As for efficiency, is there a fresh air intake for the burn box? I looked at the pictures in your build blog entries but didn’t see one. Without one cold air is being sucked through the cracks around doors and windows and warmed air is being sucked up the chimney.

  • Hey Ryan: The outdoor stack is on the south side of the building, so the house would be in front with a north wind. There is no fresh air intake. I’ve wondered if it would help with things, yea…. I would have to bust a hole in the north walll (no small task) and reconfigure the stove a bit for that, too…

  • Bryan

    Hey if you can drill a hole about 1inch So you can get
    A flex tube in it then on the inside of the house get the flex
    Tube really close to the burn box opening the input I
    Did a 1 waterhose and I put screen on the out side to keep
    Bugs out on the inside you don’t even see it it’s covered
    With cob .. Also you may want to conside a small fire place
    Not only for heating and cooking nut for looks I know two
    Burn places to look after is hard but we burn the rocketstove all day and the
    Fireplace at lunch and dinner and keep it going all night with a few big logs
    B

  • When I younger and a starving artist (potter) I helped build and fire several wood fired kilns. Some of the Japanese style kilns were multi-chambered or very very long which would make them very hard to get lit.

    The solution is simple… build a small firebox just under the stack… light this fire first to warm the flue up, then light the main box and close up the small firebox. Voila… you have draft.

    Does that help?

  • Marke

    I hate to hear you are having troubles with the rocket stove! Especially at a time of year when having it functional is critical! If it turned out that the 180 degree bends were the source of the problem, would you go so far as to tear the bed apart and reconfigure the pipe?

  • Bryan: Your fresh air intake… where is it in relation to the firebox? The side, front? Do you have a picture of it? I considered something like this…

    Michael: I can open up one of the cleanouts to light a small fire to start a draft, but the problem is more that the intensity/direction of the wind can easily alter how strong the draft is…

  • Heya Ziggy! This is Bucket from Twin Oaks.

    I have used your plans quite a bit to help construct the rocket stove I have just completed at Acorn Community.

    So far it has done very well to warm up the radiating barrel and put some heat into the thermal mass tubes, and it did all this while it was wet, it is still wet actually.

    There are a few things I have done differently. For one, I put the exit tube out horizontally rather than through the roof. However, it rises in the building up 7 feed inside before exiting out the side.

    I also am using a stainless steel triple wall flue pipe surrounded by perlite as the heat riser.

    I am also only running about 15 feet of indoor flue.

    I bet if you created a heat riser indoor before exiting the building out the side it would fix your draw problems.

    I have also seen chimney caps that are shaped in a certain way that cause any wind that blows through it to create a vacuum out.

    I don’t understand why you are not getting much radiant heat from your barrel. Perhaps it is too high or too low and resetting it might help? I have heard that 6″ systems are small and not very powerful, but we definitely get radiant heat quickly from our 6″ fire chamber. Perhaps you are right about the bricks, but everyone else seems to use them.

    I am very interested in hearing more on how this turns out.

    -bucket

  • Roofus

    Ziggy, you thought about putting one of them spinning things on top of the stovepipe? You’d have to put it up a bit higher than I think you’ve described it, but that way it would be able to pick up the least wind from any direction and create a stronger draft.

    I know “them spinning things” isn’t the most helpful term but its a start, sorry I don’t know the technical term.

  • Hey Bucket!

    Cool. Yea, I could well imagine that if I had a bit of indoor stack, it would help. It sounds like you have a super insulated heat riser, which is very ideal, too. And few indoor turns… It’s a wonder that my stove draws at all with the 180 degree turns, 25 feet of pipe, and horizontal exit from the house!

    Just to clarify… you are using a 6″ flue throughout your system? What are the dimensions of your firebox? What size barrel are you using? Do you remember the size of the gap between your insulated heat rise and the interior of the barrel?

    I’m glad to hear you are having success!

    Roofus: I don’t believe I’ve seen those fans before. I’ve seen people come up with other things to prevent wind from entering the pipe (like a weird makeshift t-joint thingy)… That is something I need to think more about, too. Thanks for the tip.

  • Roofus: Is it something like this?

    http://www.chimneyfansource.com/emailnote.htm

  • heya Ziggy,

    We are using a 6″ system. The combustion is no bigger then whatever the square inches 6″ ends up being, as that is what the book said to do. They stressed that no part of the system should be smaller then the combustion chamber.

    inside the house, we have 2 180 degree turns, followed 2 90 degree turns before the smoke exits the building.

    We are using a standard size barrel. I went for 1 1/4″ raise over the heat riser edge with the barrel, but i think it might have sagged some. The first time I had the barrel on it was several inches too high and did not get the barrel very warm. (the system was also very wet) I re-seated the barrel much lower and got better results.

    I wonder if you can not just dig a new hole in your cob wall higher up and put a stack inside your building.

    Here is a link to the special chimney caps:

    http://www.google.com/products?hl=en&client=firefox-a&rls=org.mozilla:en-US:official&hs=4bc&resnum=0&q=chimney+top+that+creates+a+vacuum+in+wind&um=1&ie=UTF-8&ei=HdUnS6SBKMjT8Aapt6WmDQ&sa=X&oi=product_result_group&ct=title&resnum=3&ved=0CCUQrQQwAg

    http://www.chimneycaps.com/ICP_pages/products1.html

  • Roofus

    Similar to, but the ones I’m thinking about are basically ball-shaped with fan blades. They don’t require any electric power, that’s what made me think of them.

    If it helps at all, when you look at them from the side they look like this : ((((()

    FOUNTAIN of useful info right here, sorry Ziggy.

  • Eric (Knobbly)

    Sorry to hear about your troubles. I am sure you have already, but did you clean the ash out of the burn tunnel? Your problems sound like what Ianto described in the rocket stove book. I don’t have my copy with me, but I think he mentioned using a ladle to get the ash out of the burn tunnel-interior chimney elbow, which caused poor draft and backdrafting sometimes.

    I think a Rumsford might help you out. Won’t keep the bed warm, but will dramatically heat up your interior.

    It doesn’t look like you can take apart your bed and simplify things either 🙁 (note to self- make a frame of cob and infill with sand)

    Well, good luck Ziggy!
    I sure love reading your blogs!

  • Stone Guy

    Tear it out and build yourself a Masonry Heater that includes an oven.

    Environmentaly friendly, very efficient, easy to operate and you can cook with the same energy that heats your home.

    I’m sure you’re familiar with these, but in case you are not, check out the masonry heater association. here you’ll find pretty much anything you’ll need to know about these systems.

    Tthere are many variations, but I personally am a big fan of the Swedish 5-run system.

  • Since I hope to build a rocket stove someday I did some research on this, in other words this is all theory.

    It seems that getting the draw started is the problem. The fan that you posted a link to would definitely assist with getting the draw started. An old computer fan and a 9v battery would be a cheap way to test the fan theory, however if it works it will eventually get to hot to leave the fan in the flue. With out electrical assistance heat is used to create the draw. As the smoke travels along the flue the heat is slowly sucked out through the mass of the cob bench but I think the exterior flue pipe is where heat is getting sucked out the most. Perhaps when the house is blocking the wind that is why the flue works better? Insulating the outside flue should help some. Perhaps disconnecting the outside flue and making sure that the stove can create a draw through the bench would be a good test. The idea of starting a secondary fire also sounds good but from your pictures it would take some serious work.

    The shape of the flue cap or height of the flue could also be be effecting the draw when the wind is blowing. It could also be that when the wind blows from the north the house is creating turbulence that hits the flue cap just right to create suction.

    Good luck!

  • Bryan

    Ryan I dig the small fan thing I thing he could do this really easy it’s just the battery
    Thing and he would have to put it far away so it would not melt
    …. And my fresh air intake is right next to the fire box all I find a camera soon my last on grew legs and ran away ……

  • Kevin

    Ziggy,
    Sorry don’t know much about rocket stoves, but I’ve been heating with wood in Minnesota for years. A couple of things. 1 fire brick is great once it gets to temp. It takes a while to get to temp, typically you need a hot fire with a good draft going for a while. Once it’s up to temp it will happily keep it going at a high rate. Typical stove damp down the air at this point, but a rocket stove should probably be left running full speed to warm up the mass and keep it warm. Rather then try a down stream draft fan though I would try a blower fan on the front end. You may be able to modify your fuel/burn can with a side hole and put in either a duct fan or a mount a computer case fan to the side. Make sure that is the only intake and run it once you get the fire lit. You will probably need to run the fan until the riser gets up to temp and starts a good draw on it’s own.
    Also as others have said a fresh air intake for the fire is a must if you want to keep your warm air inside the house. Dryer flexible vent hose makes a pretty good intake hose that’s not too expensive.

    Best of luck to you.

  • Kevin

    Ooops sorry I didn’t realize you were trying to be completely electric free. Instead of a fan you can use a bellows. They used to make hand crank blowers, but I haven’t seen them since the late 70’s. As an alternative maybe a dual action inflator pump for filling pool toys might do the trick.
    Running a side pipe in like the tuyere inlet in a blast furnace would allow you to blow air creating a hot zone. The advantage of all the fire brick you used is that you probably can over heat the fire box. In heavy steel or cast iron it is possible to get the fire so hot it causes material failure.

    Again good luck

  • I think Roofus is referring to attic vents: http://yourhoustonhomeinspector.com/Photos/HousePhotos/Attic%20Vent.JPG

    These are cheap and should increase the draw at least some.

  • Ash should not be a problem. I can easily clean it out (and do so often) so it shouldn’t be blocking up the system.

    (Actually, the front two firebricks of the firebox are loose, so I take them both out completely to slide in a ash shovel.)

    Starting the fire is also not a problem when the air is just right. It really is all about the wind, the intensity and direction of the wind. When it’s right, the fire takes off. When it’s wrong, it’s a struggle the whole time.

    Here’s my current standing. I think the fresh air intake from the outside into the firebox is probably going to be really beneficial. That attic vent might help with the draft, too, and would be the easiest thing to do without any major material adjustments.

    I’d like to start with trying a couple of smaller adjustments before I go ahead and decide to tear out the stove, which might actually have to happen if things just don’t improve. If I can’t get that bed warm, it defeats the whole purpose of having the stove as it is, and I might have to go with a long bench (with no 180s). That would be pretty damn difficult and tedious… but the house needs effective heat, and stored heat!

    I appreciate all the ideas and would love to have them keep coming…

  • Craig

    The only problem with the attic vents is they only draw when the wind causes them to.

  • Roofus

    Owen : THAT’S THE THING! Thanks man!

  • I never saw a rocket stove used in this manner. I thought you would rather go with a European style fireplace that creates a thermal mass by bricks, then having the smoke go through the mass in a S pattern. This fireplace uses more wood than a rocket stove, but it is more effective for warming a house. In my rocket stove most of the heat is being used in the first chamber for cooking, and not much coming out the flue.

  • What do folks make of this? http://www.amazon.com/Vacu-Stack-Wind-Resistant-Chimney-Cap/dp/B0013KXZMQ

    Does anyone know of anything similar that is… less expensive?

    I know you can home-rig something similar, but I kinda want something that has the best chance of actually helping…

  • After searching the cap you posted is the cheapest I saw. Before shelling out the cash it would be easy enough to make the chimney cap with some scrap sheet metal and pop rivets, maybe even soup cans. If it helps some then you will feel better about paying for the store model. Who knows, your version might even work great and save you some money.

    I was also thinking about the fresh air intake. An air intake works best when the fire box is sealed against the inside of the room. The following link seems to say that a 100mm diameter (4″) intake is good enough for most fireplaces. Your walls are pretty thick but a masonry bit would probably go through cob without much effort.

    http://www.woodheat.org/outdoorair/outdoorcmhc.htm

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  • Dave

    My 2c never built one and just came across these stoves recently so have done quite a bit of reading. I wondering if the gap between your barrel and heat riser is too big so the gases are not getting mixed correctly I read somewhere significant turbulence was required and your not getting a secondary burn. My understanding is that these stoves dont rely on draw to move the air but rather push. Hope that helps.

    cheers

  • anne

    i have heard of people using some kind of rotating cap with a weathervane on top of the chimney with a vent on the leeward side so that it turns with the wind, so the vent is always facing away from the wind. i’m not sure how to construct such a contraption but there would always be draw no matter which way the wind is blowing.

    good luck and may the force of wool blankets be with you!

  • jeb781

    Zig,

    I didn’t see this one mentioned. I think a directional cap is what you need (and a taller stack for those who go horizontal). This one will even increase the draw as it orientates itself with the wind (along the lines of what bucket had mentioned). I don’t think you will have any broblems starting it up in windy weather either as there will be a vacuum created at the top with any wind.
    http://www.rutlandguttersupply.com/Chimney%20Caps/ChimneyCap_CC015.asp

    There is also this one: http://www.hvacsolutionsdirect.com/product.php?productid=726&cat=0&page=1

    Hope this helps.

  • Richie

    Hey i think that i would try what roofus suggested. They are called whirly bird attic vents. In my opinion they would be your cheepest option to increase draw for your stove especially since you have problems when the wind is blowing from certin directions. They dont cost to much either. And because you have such a long flu run i wouldent think that you would have to worry about the heat wrecking the bearings.
    http://www.bizrate.com/building-supplies/turbine-ventilators/

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