Burning Sawdust in a Wood Stove
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You can burn sawdust for heat, but the stories in this thread make it clear that the practice is a risky one. February 15, 2015
Question
I've been trying this for a while now, and finally arrived at a pretty good system for making a nice shop fire with only scraps: Make a small fire with little kiln dried scraps. After it's going good, pile on the saw dust being careful to leave an edge of the fire uncovered. As it burns down, alternatively add a bit of scrap at the front, then scoops of sawdust. Dust out of the saw is perfect. Mix shavings and sanding dust for the right consistency. It burns fully, and puts out a good bit if heat. A nice fire and a clean shop: hard to beat.
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Forum Responses
(Business and Management Forum)
From Contributor S:
How will the stove burn on one fill up?
From the original questioner:
I tend it every couple of hours, but it would burn out in six-ish hours.
From Contributor O:
How does your insurance underwriter feel about the wood stove?
From contributor C:
Be careful with this. I had a friend who was doing some serious sanding in his small steel Craftsman shed/shop and when he opened his wood stove and tossed a handful of sawdust into it, the shed blew-up! He was found unconscious and only slightly injured about 50' from his shop! Yes, his situation was unique, but I learned a lesson about dust and fire that day. Oddly enough, the local grain elevator had a major fire within weeks of this mishap with far more expensive results.
From Contributor O:
Sawdust is particularly much more volatile - explosive - than wood. Many a dust collector has been launched by a mere spark mixed into the air and dust. The largest explosion on Mythbusters was sawdust in an open container. The reason sawdust is only used to heat in very tightly controlled situations is because of its explosive nature - its feed rate must be controlled and the flame spread has to be prevented from burning back into the storage or delivery system. It is your shop and you can do what you want, but you should realize that you are playing with fire, in my opinion.
From contributor W:
I use to burn sawdust the same way 25 years ago. One day I filled my shop with panels that I had just finished and stoked up the stove with sawdust just like you describe. Just as I started to walk out the door the stove door burst open and burning sawdust flew 40 feet to the other side of the shop. And that was the end of my sawdust burning. I have done some dumb things in my day and that one was right at the top of the list. Sawdust doesn't just burn it explodes.
From contributor G:
When I wanted to add a wood burning stove to my shop it wasn't my insurance company that said no it was the building division and permits. For in-shop finishing I would need a none combustible room made of concrete or steel. A wood stove would be nice but to use it safely it should outside with a fan blowing the warm air in.
From contributor C:
A wood stove that is sealed and used properly is no more dangerous than anything else in the shop that could cause a fire, like an electrical issue, nail going through a machine into a dust collector , spraying solvent based material without a spray booth, linseed oil rags, etc. Sawdust in the air needs the right sawdust to oxygen ratio to be explosive. If you are near that explosive ratio my guess would be that you don't have a dust collector. There are also machines that press sawdust into a brick that can be used for burning. Be smart with your stove and you won't ever have a problem.
From contributor T:
Interesting as it is, there does exist pellet stoves. They burn pellets of compressed saw dust scraps that are bought from the wood industry. So theoretically speaking you could make your own or trade your dust for pellets?
From contributor G:
I do concede that a wood burning stove could be used safely in a shop. My biggest concern is human error. For example an employee who dumps a five gallon bucket of sawdust on hot coals to restart the fire, heavy overspray that settles on top of the stove or someone who leans a piece of dried oak against the vent pipe.
From contributor C:
You are exactly right, wood stoves are an insurance issue whether they are in your shop or your house or wherever. The reason being is they can easily cause a fire if used improperly! Simple things like over filling, having combustibles too close, or embers falling out when filling can all cause a fire and they are all user errors!
From contributor Y:
Several years ago I would have totally agreed that a wood stove set up in the shop was fine (even had one in my home shop that was used occasionally) but in my real shop we had a 75HP torit collector with filter bags that was originally purchased when we were doing mostly panel processing (fine dust). Over the years as we started doing more millwork we put a moulder, rip saw, etc. on line and the filters that worked so well with the fine dust didn't work so well anymore and once a week or so we would have to actually climb into the machine to manually shake out the filters or the machine would not be as effective etc.
Well, it was a Friday afternoon and we had a large rush job of several thousand feet of trim we were going to run through the moulder on Saturday so I had a couple guys go in to clean the machine out so that we could be running on Saturday morning without any collector trouble. The machine was a little more packed then usual and as it was getting later the guys couldn't see that well and one of them decided to get one of those halogen light stands to put on the platform so they could see while they worked. I walked outside just in time to see the light fall over and break, the minor spark with the sawdust floating around was like a bomb going off. There were two guys on the platform that were blown off and fell approximately 20 feet to the ground - thank god there was nobody in the machine!
The fire department got there very quickly with several trucks and spent a couple hours to get the fire out. It’s important to remember there was not a large buildup of dust in the machine, it was more the floating particles that were most volatile. The firemen were shocked at how the fire reacted and how the little bit of floating dust was so dangerous. Luckily nobody was hurt and we ended up replacing the collector to better fit our needs and learned a hard lesson. So yeah, I would not have any kind of open flame anywhere near the shop. The story above where the guy was doing a bunch of sanding and had the fireball, no doubt that could happen and happen very easily.
From Contributor O:
I ran an 8,000 square foot shop in the 80's that had two wood burning/gas furnaces. Duel fuel so they could burn wood and then change over to gas when the wood burned out. They worked well, but had lots of warnings about not ever using sawdust for fuel. It was only years later that I found out the insurance carrier refused to cover the entire building. They covered the rest of the buildings, but nothing with a wood stove in it. They would only cover it if we put in a full sprinkler system. We grew that shop and built two more, and had quite a bit of wood waste, so I looked into burning it on a larger scale. This required quite an elaborate system in order to pass fire laws, ecological restrictions, local ordinances and more. It ended up as a co-generation plant, where the wood is burned in very controlled situation, heats a water boiler, and that turns a generator to make electricity used on site or fed back into the grid, but the system could not handle sawdust or fine shavings. It started at about $1 million and took over a year to put into service. We backed away.
Anyone that wants to play with fire can do so. Just remember that this internet thing has no control over it and that burning sawdust is a very risky thing to do inside a wood shop, or any indoor situation. Yes, it can be done, with or without malarkey, but that does not mean it should be done. I have produced hundreds or thousands of cubic yards of shavings in my career, and almost all of it has gone to stables for horse bedding, then spread onto farm fields. The last several years, it all gets composted on site and is spread under the hundreds of trees I have planted.
From Contributor B:
I will have to admit that I do burn a lot of sawdust and chips when I have time otherwise they go the stable route or dumped on my farm and burned. I completely agree that the insurance issues in many locations would knock this option off the table quickly for most, and for anyone with employees tending a conventional stove it would be lunacy to have one.
That said, for clarity, the instance mentioned by Contributor W can happen with any wood fuel (chips, split wood, sawdust, and so on). Anyone who has burned wood in some form of an air tight or semi-air tight stove for a long period in their homes or elsewhere has likely had the exact thing happen at least once. You have a low smoldering fire with coals or even a bit of a fire and you load the stove and shut the door (or worse leave it ever so slightly ajar). There is not enough fire to get the loaded material burning right away so the hot coal bed just heats and heats and heats giving off combustion gasses the entire time. The stove fills to saturation with combustion gasses but there isn’t the heat/air mix to ignite. The stove stalls for a second, takes a gulp of air, and BOOM!
I have been sitting next to my wood stove at both my homes and had this very thing happen and it was never with sawdust. Often times it will blow a flame out any open air intake a foot or more like a torch. This is of course followed by your house being filled with smoke and hopefully you screwed your flue pipe together well because if not it may be dislodged from the chimney and your scrambling for buckets while trying to call 911.
All this aside the entire event is operator error. No different than the halogen stand light tipping over, breaking, in a combustible environment, operator error. They make explosion proof lighting for this very reason. Of course everyone makes mistakes and accidents happen but operator error is operator error. It’s like blaming a gun for a murder. The halogen light didn’t blow the place up. A scary sawdust situation I once saw was a guy sanding hardwood floors and walked out to a fire burning and proceeded to empty the bag. A ball of fire basically wrapped around him instantly - again, operator error.
I agree that chips and sawdust are actually pretty hard to burn in any conventional stoves. I have often burned large piles of sawdust when the stables don’t want them and it’s a real chore. You just get a slow surface burn and the bulk of the pile never even tries to burn. The same thing happens in a stove. The bad stuff happens upon loading the stove. The only good part is, at most, you’re talking about a shovel full of chips, and unless some idiot (operator error) tosses the shovel full into the stove, loading basically puts the fire out. I have a scoop that fits the door of my stove and a pusher. I basically slide the scoop load of chips into the stove, and use the pusher and just pull the scoop out from under the load of chips. It works quite well. I get a lot of free heat, and don’t have to handle the chips but I’m a small shop. When it’s busy though I just give the money to the gas company as I don’t have time to fill the stove.
From Carl Hagstrom, Systems Administrator at WOODWEB
I followed this thread with interest and I happened to pass the link along to a stair builder (and good friend) I know. He sent me a note that included: "I heated my shop for years with a homemade stove that burned nothing but sawdust. It was made from a plan in popular mechanics. It had a 50 gallon drum with the top that's held on with a metal clamp. Inside was a topless 30 gallon drum with a 4" diameter hole in the center of the bottom. This drum was held about 4" off the bottom of the big drum. I took off the top, put an tapered (4-6") mandrel in the hole, and rammed sawdust into the 30 drum, then pulled the mandrel out - leaving a hole in the packed sawdust. I dropped a piece of burning newspaper down the hole and put the lid back on. It burned clean and steady for about six hours, a big sawdust donut with a fire in the middle. It eventually rusted out, and the old machinist guy who made it got too old to make stuff. I don't think it was any more dangerous than any other wood stove."
And also: "Not saying sawdust won’t explode. Sometime light a match and shake out the little bag from your palm sander over it...." Also included is a video of what appears to be a European stove designed for sawdust. From what I can gather, it's the dust to air ratio that can produce volatility on the order of a bomb.
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From Contributor H:
Personally I've always considered it a bad mix to put a wood stove in a wood shop. I know others that have done it without incident but for me it has always been just too risky. Even the best laid out setup and tending policies can go awry. However, many years ago I had a coal burning stove in my house basement. At night I would carefully add a couple shovels full of sawdust to the top of the freshly loaded stack of coal. This made for a long lasting fire that went through the night.
From contributor A:
There is certainly a lot of danger when it comes to wood dust (any combustible dust for that matter) and fire. Anything that disturbs that pile of saw dust in the stove can cause an explosion. A piece of solid wood that pops could do it. Then you can have that situation that Contributor B mentioned. Ever see the movie Backdraft? That is what happened there. The fire is starved for oxygen and there is probably a tiny air leak around the door seal or some other part of the stove. When the oxygen finally builds enough in the stove and makes its way to the ignition source, it goes boom. This is called back puffing in wood stove burning because it causes the stove to puff smoke out of all its openings from the explosion. I think the idea of a wood stove in your home shop isn't a terrible idea. It's much easier to control everything in the situation. In a professional shop where there are several people working, it is certainly much more difficult. All it takes is one person who doesn't take the threat seriously. Even by yourself the threat of a dust collector back coming off unexpectedly or dust from sanding is a real one.
From contributor K:
About 40 years ago I was sharing a shop with four other guys in Fayetteville AR, which we heated with an old wood stove. It had a heavy cast iron top for loading. One cold morning, a young fellow poured a big scoop of sawdust down on top of the hot bed of coals from the night before. All four of us were standing around that thing when the loud BOOM launched that big cast iron lid into the rafters of that old warehouse. It didn't happen instantly, but took a while for the gas to build up, and an ignition spark. I'm sure none of us have ever heaped dust onto coals since that day. We were all lucky that day. It is not something I will ever forget.
From contributor J:
I heated my shop with gas unit heaters (open flame) for 45 years. My home, upstairs, was heated with a wood stove. My favorite fuel was sawdust, planer chips to be precise. I'm not a dummy, I was careful and I knew I was taking a risk due to the randomness that's in the world. However, you need a precise mix of air/fine dust for an explosion and my lungs would bug out if I even got close to it. I started my stove each morning by loading it full of chips. It would never start from coals, but I would put a burning splint in the front and let the draft through the door vent make what is now called a jet stove. It doesn't take much draft to make a powerful and controlled fire. To damp it down, just open the door. When the breeze is less focused, the flames go out quickly and you are left with a very thin bed of glowing embers, which lets out very little heat.
Note that chips are combustible, but not explosive. I followed the advice in the Woodburner's Encyclopedia, which said that a slow, oxygen starved fire leads to clogged chimneys, but a hot oxygenated fire is great. Each year when I cleaned the chimney I'd get a couple cups of soot from burning the equivalent of a couple cords of kiln dried hardwood mostly in chip form. Sure, I'd sometimes throw in a handful of sanding fines to watch the puff of fire. Put in a shovel and you make a smelly smoky mess.
I never felt safe enough to put a stove in the shop, but half of that is because we didn't have a good place for it. I thought of building one like Carl mentioned. I know it would work well for chips, probably for ripsaw leavings or chips mixed with fines as well. In the article I read the author suggested having a couple inner barrels (35 gal, which sit in a 55 gal barrel) and loading one up as you were burning the other. Then let the first go out and the stove cool, pop the top, replace the 35 gal. inner drum with a full one and relight. Those who caution about random people using the stove, throwing things in, etc. have a point. I started very cautious and learned what the margins were and then stayed well within them.
Like most things, it is quite safe if you handle it well. Because the potential danger is high you do well to be very careful. If I was setting up a shop again I would burn chips in a heartbeat with an isolated stove that only a trusted few lit or fed. If someone out there is considering a setup like this my best advice is to play with sawdust in a woodstove and see what it does and how it burns. You'll quickly get a sense of how to handle it and what to do or not. Don't try it if you can't maintain a careful attitude.
