Sustainability Hacks – External Wood Burner

Sometimes sustainability is about making do with what you have. This is the wood burner that I use for my personal workshop. In my area, it is mandated by law that we have to clear brush on a yearly basis. I live on a pretty large lot so we have plenty of brush to be cleared every year. Add to that the occasional tree that falls down and all of the scraps from my shop and you have all the wood that you need to heat a shop like mine. With the exception of the work that I put into gathering firewood, my heat is free and is carbon neutral. This is somewhat of a double hack because as you have probably noticed, the wood burner is sitting outside of my workshop. My workshop is fairly large but I have it so packed full of tools that I just didn’t have room inside so I came up with this solution.

More details after the break.

Because I didn’t have room inside of my workshop, the only solution was to heat it from outside somehow. What I came up with was to take two barrels of different sizes and fit them together like Russian dolls. The inner barrel is the burn chamber and the outer barrel allows air to flow between the two barrels. On the inside of my workshop I have a blower fan that pushes air into the wood burner allowing it to circulate around and pick up heat before it exits through another pipe which goes back into the workshop. At first I just had the barrels by themselves but I found that the air wasn’t as hot as I expected so I pulled it apart again and welded on a bunch of small steel tabs that give the burn chamber a greater surface area. Once I did that, heat transfer was greatly improved.

Here you can see the original setup without the inner barrel attached.

The whole thing is held together by the original band clamp from the larger barrel. The burn chamber door and the feet were ordered as a kit for about $50. The first picture of this post is from a few days ago. You might notice that I made a few changes since these original pictures were taken. I increased the diameter of the outgoing pipe to allow for the expansion of the air as it heats up. I also added a second outgoing pipe that runs to my wife’s office, which is better insulated than my workshop and can actually get uncomfortably warm even on the coldest of days if the damper isn’t used.

50 thoughts on “Sustainability Hacks – External Wood Burner

  1. Really, burning wood is carbon neutral? Interesting definition! Don’t get me wrong, I think all this carbon neutral stuff is a pile of crap and a scam anyway; this just proves it!

    1. The “carbon neutral” stuff is something I’ve noticed creeping in a lot but people not understanding what it means.

      As far as I can tell, it really just relates to energy expenditure for gathering resources/doing tasks, oddly “measured” in carbon output.

      So in a sense, the article is right. Gathering the resources is done in a sustainable way and has not cost anything (in terms of fuel, travel etc) so it’s carbon neutral.

      Obviously that only applies to gathering resources. Once you do something with them, like burning them, it goes out of the window.

    2. Burning wood is carbon neutral in the sense that the carbon you are releasing is the same amount as was taken from the atmosphere as the tree grow. Net is no ‘new’ carbon added to the atmosphere.
      Fossil fuels on the other hand are adding carbon to the atmosphere that has been locked up for thousands of years, hence not carbon neutral.

      Something else most people don’t realize about wood is that the carbon released while it burns is exactly the same amount of carbon as would be released if it were to fall on the ground and rot. So again, carbon neutral.

    3. Nope. Actually, wood and biomass are carbon neutral fuels. The reason is that when they were alive, the plants absorbed all the carbon around them, making a theoretical carbon deficiency. When they are burned, they release the same amount of carbon that they previously absorbed, adding no new carbon to the environment.

      It’s a bit tricky, but this article has it right. Great job on the heating system!

      1. well then oil must be carbon neutral too … it was organic matter that has decayed and now we are burning it! Whilst it was alive it absorbed an amount of carbon causing a carbon deficit of sorts way back then and now we are releasing it.

        Somehow your argument just doesn’t sit right …

        People. Carbon doesn’t just spontaneously happen.

    4. The “carbon neutral” argument here is probably that all the fuel came from wood that fell naturally or had to be cleared anyways. Thus, they are not contributing any “extra” carbon, compared to letting the wood decompose (which would also release all the wood’s contained carbon) and buying fuel elsewhere. However, they are releasing that carbon from the wood far more quickly than would happen naturally.

    5. He says he uses brush, fallen trees and scraps. THAT’s what makes it carbon neutral. Brush and fallen trees especially are going to decompose and finally end up back in the atmosphere anyway. Burning does that, only faster.

      If he were chopping down trees, or using mined coal for his heater, it wouldn’t be neutral.

      1. chopping down trees is co2 neutral as well. those trees would eventually die of natural causes, decompose, and release their lifetime intake of co2.

        if he plants ad many trees as he cuts down, I believe it counts as co2 neutral

      2. Actually, a great deal of the dead trees and plants end up as topsoil which is slowly buried under new topsoil, so burning trees instead of leaving them be is not carbon neutral, since the real neutral state is that plants remove carbon from the atmosphere on average at approximately the same rate as volcanic activity releases it back.

        Even the tree that sits rotting is still sequestering carbon in its body. It’s like a race track. If you put a 5 mph speed limit somewhere, that’s where you’ll find most of the cars at any given time, despite the fact that they’re constantly going around in circles. Remove that speed limit, and the cars spread more evenly to the rest of the track – or in this case – burning the trees as soon as they’re grown spreads the CO2 back in the air a lot sooner, which increases the amount of atmospheric CO2 from the natural state.

        So, by burning stuff you stop a natural process that locks away carbon, and you shift the balance of CO2 from plants to the atmosphere.

        So carbon neutrality is a scam.

    6. What might be a useful addition to Thatchers comment: things that are not carbon-neutral usually involve fossil fuels. Since the carbon that went into creating these fossil fuels was absorbed from our atmosphere before our current climatic state, burning fossil fuels effectively adds new carbon to the environment. And that is why it is not carbon-neutral to burn fossil fuels and why burning wood is. At least as far as I know :-).

      That said, the actual post reminds me of the wood burning my parents used to do with wooden pallets from local shops. Nicely built and interesting to see that the metal tabs made so much of a difference.

      1. The carbon cycle is thus:

        Volcanic activity releases CO2. CO2 dissolves into water in the air, forms carbonic acid. The water rains down and dissolves rock, which forms minerals that trap the carbon and sediment it. The plants “eat” CO2 and eventually die, forming topsoil which is then washed into rivers and sedimented, or covered by new topsoil and buried that way. After a while the tectonic plates move and new volcanic activity releases the trapped carbon again.

        Now: by burning plant matter we effectively disable half of that mechanism by not allowing the plant matter to get buried. That alone will make the CO2 levels rise until the weather picks up and erosion is accelerated by warmer temperatures and more rain.

      2. @ Dax,

        I understand your train of logic, what with bypassing a major part of the carbon chain. The fact remains, though, that he is doing just that – bypassing it, not adding to it. If nature were to take its due course, that same amount of carbon would end up spewing out of a volcano somewhere, just at a (much) later point in time.

      3. I think the “later point in time” part is a key element – it may be carbon neutral in the sense that the same amount of carbon would have ended up in the environment anyway, but by burning it, the rate at which it is introduced into the atmosphere is much greater and thus the overall quantity of carbon in the atmosphere is greater at that point in time. If we all burned wood, we would put all that carbon back into the environment too quickly and it would take a long time to sort itself back out.

    7. What he is doing IS carbon neutral, or at least mostly so. He is clearing brush from a yard, not cutting trees from a natural forest. If you ever actually look at a brushy, untended area as the dead stuff piles up there is less room for new growth. It wouldn’t go so far as to choke off all new growth of course.. but because he clears it away each year there is probably quite a bit more new growth than there otherwise would be. Thus.. more carbon removed from the atmosphere to make up for what he releases in burning it.

      Also… what are ashes made of? Doesn’t a lot of the carbon remain in them? I suppose they either get dumped out to become part of the topsoil or sent to a landfill where they are even more removed from the atmosphere.

      1. It doesn’t matter how much new growth there is, if he keeps burning it and releasing the CO2 back.

        The point is that you should leave nature be, because that way the carbon stays in the plants and eventually gets buried, because that’s what largely regulates the amount of CO2 in the atmosphere.

        If plants are not allowed to die and get buried under new growth, then it doesn’t matter how much new plants you’re growing because none of the carbon ends up sequestered under the ground. You’ll just burn them in a few months, and the carbon is back up in the air anyways.

        And when you close that carbon sink, the level of CO2 will increase because of the natural CO2 emissions from volcanic activity.

      1. Precisely the point.

        How is new oil and coal to be made if not by having plant matter buried?

        The carbon locked away in fossil fuels is coming up on its own anyways, so it has to be put back underground by the living nature to keep the CO2 level stable over long periods of time. As the CO2 levels increase, plants grow more and more carbon gets buried. It’s a feedback mechanism that keeps most of the carbon locked away underground, because it takes such a long time to come out, and such a short time to get buried by the living nature.

        Now, if we hinder the process of burying the carbon by burning massive amounts of biomass, it means that the CO2 levels will rise because nature can’t put it under there as fast as it comes back out.

        The feedback mechanism becomes ineffective because any increase in growth just means we have more stuff to burn, so we do, so the new equilibrium settles to a point where the earth is much warmer than today.

  2. Nice! Good Idea using the barrels like that and the fins to spread the heat out to the heat exchanger.
    I don’t think there is much I could do to improve on that except to create a weather proofing cover for the whole thing, like a tin roof and shack.
    With or with out that it looks really well thought out.{the only reason i brought up weather proofing is because of the fact it is gonna negate any savings in cost in the long run if it rusts up}

  3. Barrel stove? Nice implementation. Since you’re burning wood just to get rid of the surplus, totally appropriate. If you were actually trying to heat your home or shop, a rocket stove would be much more effective for the fuel (small sticks and scraps).

    Keep on Hackin’!

  4. I don’t know what climate this guy lives in, or how full his workshop is, but surely tidying his workshop and installing it inside would have been easier and far more efficient (both in terms of effort and energy) than having it outside. Plus it comes with the bonus of having a tidy workshop where you can actually do stuff with the space. Piping the heat to the house is a great idea though.

    1. Well remember there is an added bonus of having the stove out doors. The smoke that leaks from the stove during filling does not end up filling the shop and making it hard to do useful work.
      Plus if tidying up was the problem he would have already have put it into the shop. Having space between tools like band saws and circular saw tables is kinda paramount over a tiny amount of efficiency.

      1. Sorry, I misread: I presumed that the workshop was a mess rather than just full. That assumption is probably a reflection of my own tidiness! There shouldn’t be a problem with smoke though – air should be going into the burner when it is hot (chimney effect).

  5. Seems as if you disregard the smoke removal, there is still the issue of heat loss of the pipes and chamber. I agree with the building of an enclosure and some insulation of the stovepipe. I don’t know what the fire code is there, but the fire department here would tell me to move it further away from the building. Good use of the brush though.

  6. I don’t know how hot the outer barrel gets to the touch, but if there wasn’t a risk of melting/fire you might consider insulating the outer barrel and the intake and output piping to bring up the efficiency a little. I’m thinking regular insulation or maybe reflectix. Either that or just use asbestos…

  7. Replies to several comments:

    Having it outside the building may make it easier to insure the building. A fireplace or woodstove can really increase insurance rates, so the inefficiencies of heat lost outside the structure may be well compensated by lower insurance rates.
    But now add in how much interior heat will be lost each time a door is opened to go out and come back in to replenish the fire, and how much of the interior heat will be used to “re-heat” the person once they are back in the shop.

    Making a small (insulated?) shed for the burner could reduce heat loss to the outside, allowing more time for heat to transfer to the shop.

    How about a spiral of copper tubing around the barrel and pump/recirculate heated water/anti-freeze mix into the building as well? A salvaged car radiator and electric fan to transfer the heat to the air in the shop? (With the obligatory AVR thermostat pump controller B^)

  8. Cool stuff, I have been planning to implement something similar. I am looking at building a rocket mass heater for my garage and/or shed. Shed is 160 sq/ft so it has some size to it. Rocket mass heaters are said to be somewhere close to 90% efficient when it comes to burning because you are burning the wood gas / carbon off.

  9. Actually, things considered, wouldn’t this actually be a carbon POSITIVE hack? Should certainly be a point of consideration in the “neutral/not neutral” debate…

    “In my area, it is mandated by law that we have to clear brush on a yearly basis.”

    Now, aside from buying a chipper to turn it into mulch, the only real way to “clear brush” is by burning. So this fuel would have to be destroyed anyway, the hack is harvesting the energy that would otherwise have been wasted under conventional methods of disposal. Additionally, the “carbon-saving” aspect increases due to not relying on something like an electric heater (likely powered by a fossil fueled power plant)…

    Point is that all things considered… This hack is certainly not carbon-deficient.

  10. I can suggest a change or two; instead of adding metal tabs to increase the surface area, he could add ducting, long metal strips open at opposite ends so the air flowing around the heat chamber has to travel a really long way, as well as a greatly increased surface area.
    In regards to the carbon neutrality aspect, if he decreased the oxygen flow to the combustion chamber or put the larger pieces inside yet another tin with only a couple of exit holes in it, he could make charcoal he could bury in his garden, which would make it carbon-negative, he’d be actively removing carbon from the carbon cycle as charcoal isn’t broken down in the soil.

    1. “long metal strips open at opposite ends so the air flowing around the heat chamber has to travel a really long way, as well as a greatly increased surface area.”

      Propbably a long strip of metal bent in to the V shape and cut in such way that it can bend inwards. Such “V” strip could be wrapped around the combustion chamber to form an spiral guide for the air. Other variant is using copper pipe to do exactly the same job on guiding the air and heating up the water. Good tip for carbon sequestration.

  11. Hmmm… How to improve it… How about combining a few different tech paths to make an all-in-one? Let’s see… Generate biochar in an oven over the combustion chamber of a rocket stove mass water heater, where the hot water is pumped under the floor of the shop to heat it. Alternatively, just install the rocket stove mass heating system under the shop floor.

    Something that you might want to consider is using both refractory bricks and firebricks

  12. Perhaps 3 or 4 spiral steel fins with say a 100mm pitch would greatly improve the heat transfer and thus the efficiency of the heat exchange.

    Nice hack although those barrels don’t survive long with fire inside especially in the wet were they rot out in no time. Id be very interested to see how long this lasts. Just to prove me wrong!

  13. I know this has probably been mentioned by other astute observers, but as a safety professional, I have to say: keep a CO monitor in your workshop. One leak an you’ll be down and out before you realize what’s going on.

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