Cooking with wood gas
If you’ve always wanted a jet engine to cook marshmallows with, this baby might be for you.
What this is about?
It is about a “batch-loaded, inverted down-draft gassifier”. Also known as a wood gas stove, it works with one charge of fuel at a time to produce a hot, smokeless fire.
I designed this. It weighs 3.8 ounces (108 g), and can be drop-kicked. Not bad.
This is about the simplest wood-burning stove worth building.
Some use batteries and electric fans. Clever. If you like batteries. And fans.
A big step down is a can with holes punched in it. Simple. Crude. Smoky.
A wood gas stove falls in the middle, being simple yet sophisticated.
- Burning wood produces smoke.
- It is not the wood that burns but the smoke.
- The trick is to generate lots of smoke and then burn it before it gets away.
- You do this by burning the fuel from the top down.
- The fuel supply (dry wood) is infinite. No need to carry it.
- The stove is dead simple. Once you get it built, that’s it — nothing to adjust or maintain.
What’s this “inverted downdraft” stuff?
An “inverted downdraft” is an updraft. Commercial wood gas stoves are fancy, pull air in from the top and blow smoke back through the fire using fans. That’s the downdraft.
But it can be easier than that: use an updraft.
Our stove lets air flow from in at the bottom and out at the top, which is either an inverted downdraft or an updraft, depending on how fancy you want to sound. But the fuel still burns from the top down, unlike the action in other wood stoves.
And then what?
To use this stove, fill it about half full of fine, dry twigs, as thick as a pencil, or finer, and about an inch (25mm) long, laid horizontally.
Once the fuel is loaded, sprinkle a bit of flammable oil or alcohol onto the top of the wood — maybe a teaspoonful — as a primer. Let it soak in a few seconds, then light it at the top. The wood burns from the top down.
The heat of the flames at the top vaporizes the wood below the flame, and the open bottom of the stove allows fresh air to rush in and create a strong updraft. Additional air holes near the top of the stove inject more air. All these vents, combined with keeping the cooking pot well above the top of the stove, allows complete combustion. No smoke!
Yes. When loaded, lit and used properly, this stove will burn almost without smoke. You get an occasional wisp or two, but that’s about it. This is totally unlike the average wood fire or can stove, which have to be tended constantly, and which smoke before they get going, while they are burning, and after they burn down.
The wood gas stove catches, runs like a blowtorch, then subsides to a cool smolder once the volatile gases have burned off. At the end you get a warm charcoal glow, which eventually fades out, leaving a little clean ash behind. This takes about as long as lunch lasts.
If one charge of fuel isn’t enough, and you add more fuel at the top, you’ll get smoke. Don’t be surprised. But you shouldn’t often need to do that.
These instructions describe things adults might or might not do with fuel, flames, tools, and sharp pieces of metal. Anyone working with fuel, flames, tools, and sharp pieces of metal assumes full responsibility for their own actions. Anyone attempting any activities mentioned here is assumed to be intelligent, creative, responsible, and prudent. If you are not an adult, and are not also intelligent, creative, responsible and prudent, then do not act on anything you read here. Neither the author nor Sectionhiker.com is responsible for anything you might do. You are.
To make this stove you need some tools and you assemble some parts using bolts and nuts and so on, so it takes some work. But not much. And once it’s done, it’s done.
This stove is made using just about the smallest possible can, about three inches (75 mm) in diameter. Larger cans are be easier to work with but are heavier and bulkier. The can described here may too small, because you need to reach one hand inside it to get a nut threaded onto a bolt. If you can manage this with tools instead of your hand, OK, otherwise you’ll need a bigger can if you can’t get your hand in.
- Height (can only): 4.4″ (112 mm).
- Diameter (can only): 2.9″ (74 mm).
- Full finished dimensions (including top and bottom supports): Height: 6.6″ (168 mm); Diameter: 2.9″ (74 mm).
- Weight: 3.8 ounces (108 g).
- Composition: Steel can (Del Monte sliced peaches in the 15.25 ounce size), galvanized hardware cloth top and bottom (to serve as integral legs and pot stand), plus a few bolts, nuts and washers.
- One empty 15.25 ounce (451 ml) Del Monte sliced peaches can or equivalent.
- Marking pen
- Galvanized 1/2″ hardware cloth (a piece 4″ by 24″ (102 by 610 mm ought to be enough)
- Sandpaper (100 grit, or thereabouts)
- Electric or manual drill, with 1/16″ (or 3/32″), and 1/4″ bits
- Paper hole punch
- Sheet of paper
- Wire cutters
- Work gloves
- #6-32 x 1/2″ slot-head machine screws (or equivalent)
- #10-24 x 5/16″ Tee Nuts (or equivalent flat washers)
- Lock washers
Drill some holes in the bottom and sides of the can, then bolt a collar of hardware cloth to both the top and bottom.
- Open the can. Eat the contents. Remove the top, the label, and wash the can. Sand down any sharp edges inside the top of the can. Now try to stick your hand inside. If you can’t, then you need a bigger can. Look for a can about twice as tall as it is wide (think “smokestack”).
- Drill bottom vents. Mark the bottom, dimple the spots with a punch or a nail, and then drill them out. When done, the bottom will be chock full-o-holes, forming the fire grate.
- Drill side vents.
- Make a template from a full-sized sheet of paper (8.5″ by 11″) with a strip trimmed from the long edge. It should be 1.5″ (38 mm) to 2″ (50 mm) wide.
- Wrap this around the can, overlapping the ends. Mark the point where the paper overlaps itself, remove the paper from the can, and cut off the short end.
- Measure down from the top rim of the can about 1.4″ (36 mm) and see where you are. The side of the peach can is corrugated. You want to be in the trough (low point) of one of these corrugations. Find the nearest trough and record that distance. It might be 1.4″ or 1.5″ or 1.55″ or 1.6″, but it shouldn’t be less than 1.4″.
- Take the measurement and mark your strip of paper. You want to end up with a line running the length of this paper strip, and 1.4″ (36 mm) or more in from the original factory edge. Use your actual measurement. Fold the paper in half four times and crease it hard. Unfold it and use a paper punch to make a hole at each point where the horizontal line you drew intersects with a vertical crease. You will have 15 holes.
- Wrap the paper strip around the can again, with one side of it snug up against the top of the can, and tape it in place. No holes or the taped ends should fall over the thick seam in the side of the can.
- Mark the can at the center of each hole you punched in the paper strip, and put a sixteenth mark at the point where the two ends of the paper strip meet, in line with the other marks.
- Now drill a small hole at each mark, then go back and enlarge them out to full size (0.25″/6.4mm).
- Now you’ll have the bottom of the can full of holes, and you’ll have another 16 holes in the side of the can roughly 1.4″ (36 mm) from its top edge, and the can will have no top. Sand off any sharp edges.
- Drill holes for the pot support.
- Take another strip of paper and wrap it around the can, then cut it to length. It should be 0.6″ (16 mm) wide. It will turn out to be just about exactly nine inches (229 mm) long. “Just about exactly” is close enough.
- Measure in three inches (76 mm) from one end and draw a line across the strip. Repeat from the other end. Now you’ll have a strip 0.6″ wide by nine inches long (16 by 229 mm), with two vertical lines dividing it into thirds.
- Wrap this strip of paper around the top of the can. Arrange it so that none of the marks or the taped ends fall over the seam in the side of the can. Then use a small drill to bore a hole at each mark.
- You want three holes equally spaced around the can, and about 0.6″ (16 mm) down from the top edge of the can.
- Drill holes for the stove base.
- Do the same as in the last step, but use a strip of paper 0.5″ (13 mm) wide. Wrap this around the bottom of the can. Again, avoid the seam in the side of the can and drill your holes.
- OK, now you have a can with six evenly-spaced screw holes in its side, three near the top and another three near the bottom.
- Cut and attach stove base.
- To do this part, you have to stick your hand inside the can to get a lock washer and nut attached to each of the three machine screws that hold the stove base in place. Watch out for sharp edges, or you WILL get cut.
- The pot support and stove base are both made of half-inch hardware cloth.
- For both the stove base and pot support, you want to cut the hardware cloth, gently bend it into a cylinder, and then bolt it to the stove. Look out for the edges. There is a sharp nubbin of wire every half inch along each edge, and they all bite.
- Protect your hands with work gloves. Using a wire cutter, cut a piece of hardware cloth 1.5″ by 10.5″ (38 by 267 mm). Using the wire cutter again, trim back any stray pieces of wire as much as you can. Sand down sharp nubbins.
- Bend the strip of hardware cloth into a loose cylinder to match the shape of the can. There should be one square of overlap at the ends (1/2 inch) when you wrap this around the can.
- Slide a machine screw through one of the flat washers. (I used Tee nuts because I couldn’t find any big flat washers. Use what you can find, as long as it’s big enough in diameter to cover the 1/2″ by 1/2″ hole in the hardware cloth.
- Squeeze the hardware cloth together with one hand and fit it over the bottom end of the can. A heavy rubber band can hold it in place. With the other hand, slide the machine screw with the washer attached through the topmost square in the hardware cloth where the two ends overlap, and then through the hole in the top of the can.
- Reach inside the can and slip a lock washer over the machine screw, and then thread on a nut. Hold the nut in place first with your fingertips, and then with a pliers and tighten the screw from the outside, using a screwdriver. Don’t cinch it down yet. Just get it snug.
- Place the other two machine screws, make sure that everything is in place and level, then tighten down all three screws to the max.
- Now one horizontal “rung” of the hardware cloth should be resting on the bottom edge of each machine screw, and the flat washers on the outside will be holding the hardware cloth tight against the side of the can.
- Go back to the place where the two ends of the hardware cloth overlap and twist on a piece or two of wire to hold those two ends tight together. Use a pliers, then clip off the excess and tuck it in so it won’t snag on anything or cut you.
- When you’re done, you’ll have the bottom of the can sitting 1/2 inch (13 mm) off the ground, held up by the cylinder of hardware cloth firmly attached to the can. This will provide breathing space under the stove.
- As noted, there will be one “rung” of hardware cloth up against the BOTTOM of the machine screws, supporting them from below.
- Cut and attach the pot support.
- Use work gloves to protect your hands. With a wire cutter, cut a piece of hardware cloth 2.5″ by 10.5″ (64 by 267 mm). Trim back stray pieces of wire and sand down any remaining sharp nubbins.
- Repeat the process you used above to make the pot support
- The pot support will bear down on the TOP of the screws that hold it, while the stove base will push up on the BOTTOM of the screws there. This probably doesn’t matter too much since the screws and washers are going to be squeezing everything pretty tight, and the whole shebang isn’t really that heavy, but better to over engineer, eh?
- You’ll see how it works as you put it together.
- OK, done. The parts won’t all be exactly square with each other, but close enough so you can’t tell from a foot away. The stove should not be obviously leaning.
- One advantage of the hardware cloth is that it has all those little sharp, pointy nubbins, like kitty teeth, and while you have to be careful not to cut yourself or snag clothing on them, they help to hold your pot in place. The bottom ones grip too (even though you want to use this stove on a fireproof sheet of metal, to be safe).
- Burning in and testing.
- The stove needs to be fired before using it to cook with.
- Hardware cloth is galvanized, and the inside of many cans. Zinc is the galvanizing stuff, and zinc fumes are toxic. You don’t want to breathe them or get them into your food.
- Choose a safe, fireproof area such as a fire grate at a local park and charge the stove with dry twigs until it is about half full (don’t overfill, less fuel is better, leave the side vents uncovered). Light the stove and let it burn out. This will take around 20 minutes, because once the wood burns down you’ll be left with charcoal, which will slowly burn down without smoke. You will be surprised at how long this takes.
- Repeat this a couple of times. You should see very little wood smoke, and after the first burn, you should notice no smoke from the galvanized metal parts, or smell anything odd. The zinc smoke is very acrid. Don’t breathe it. Keep a pot of water nearby in case you need to quench any flames, or if the stove gets knocked over while burning.
- As always, watch for problems, take your time. Relax, and keep it fun. Try boiling a pot of water. Yowsa! This stove puts out a huge amount of heat compared to alcohol stoves.
Get a pot lifter.
You can’t use bare hands, or gloved hands either, considering the blowtorch effect. Keep your sleeves well clear of the stove, especially while wearing synthetics.
When using this stove, put down a sheet of aluminum foil (oven liners or cookie sheets work), charge the stove with fuel, set the stove on the foil, and light it.
Use fuel and primer as described in the intro. Do not under any circumstances use gasoline, turpentine, thinner, acetone, or anything else explosive to prime with. You can try paper, or wispy, dry bark. Experiment, but stay away from anything guaranteed to explode in your face and spray you with flaming liquid.
Once primed, the stove is ready. Light the fuel at the top. The wood will take about half a minute to fully catch, but before long you’ll have a flame that looks a lot like it’s coming out an afterburner turned up to 11, and there will be almost no smoke. Three things are critical:
- Good ventilation through the bottom of the stove, where most of the air enters
- Ventilation from the holes around the side of the stove, which inject extra air
- The height of the pot support at the top
The pot should be at least two inches (50 mm) from the top of the stove. A smaller gap results in a stove that smokes badly. If you have problems with smoke, it’s probably poor ventilation, though fuel loading matters too. Laying twigs in flat, in tight, dense layers, works best. Starting with a small can means that this stove is relatively unstable. One of those ultralight tradeoffs. A fatter can will give a more stable but larger stove. This is your choice, which is what the ultralight idea is all about anyway. Experiment.
About Dave Sailer
Dave Sailer, author of The Ultralighter Blog, is a longtime hiker and convert to the light side, has written a pretty darn complete guide to ultralight backpacking stoves that most every backpacker will want. He calls it Fire in Your Hand: Dave’s Little Guide to Ultralight Backpacking Stoves. And he has officially seen the light. Not only seen the light, but dropped his pack weight by 20 or 30 pounds. Strange but true, and a whole lot easier than you might expect. And a whole lot of fun. Who would think that you can make your own backpacking stove? I mean, how crazy is that? But hey, it’s really easy, and fun. This book has a whole bunch of good info, but mostly it’s just fun.
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