For some time I’ve been considering the best way to deal with a pile of scrap wood that has been growing next to my home, the result of many woodworking and carpentry projects I’ve been involved in over the years. A few options I have considered are:
- taking it to the landfill
- cutting it into chips and using it as mulch
- burning it
From a climate change standpoint, the latter of these is surprisingly the least harmful in the long run. Mulching or burying do postpone carbon release to the atmosphere, but the carbon will be released eventually regardless. What’s worse, mulching or burying the wood will result in some anaerobic decomposition (that is decomposition in an oxygen deprived environment) which will result in the production of methane, a far more harmful greenhouse gas than carbon dioxide.
Burning also has a side benefit. It releases energy which may be captured and put to some use. Scrap wood and yard trimmings are burned in backyards across the country each year without any attempt to capture that useful energy. Rather than simply “disappearing” my pile of scrap wood, I wanted to extract as much value as possible by heating my home with it. To do so most efficiently, I built an ultra-efficient wood burning stove, more commonly referred to as a “rocket stove’. Rocket stove designs are most often used for small cook stoves but larger stoves for home heating are not unheard of. They are often referred to as rocket mass heaters.
Warning
Fire is dangerous. Building and operating your own wood stove of any design will almost certainly void any fire insurance you may have on your home and may also pose a serious risk to you and your family. As far as stoves go, a rocket stove is probably one of the safest designs since the combustion chamber is tiny, the exhaust volume is low, and the bulk of the exterior of the stove does not reach very high temperatures. However, I hope it goes without saying that you should build at your own risk, and keep a watchful eye on your stove whenever it’s burning. With a reasonably load of wood my stove burns vigorously for about 30 minutes before requiring more fuel. I consider this short burn time to be not a burden but a safety feature, and I don’t mind it at all since there’s a certain pleasure that comes from putting another log on the fire.
How is a rocket stove different from a regular wood stove?
The goal of a rocket stove is to burn a relatively small amount of wood at as high a temperature as possible, resulting in more complete combustion, and to extract as much heat as possible from the exhaust gases. To generate high combustion temperatures, rocket stoves separate the combustion, heat extraction and exhaust functions. They have insulated internal chimneys to generate a strong draft for vigorous combustion. My design uses a down draft combustion chamber. Scrap wood is loaded directly on top of the existing burning wood inside the combustion chamber. The flame is drawn downward by the strong draft rather than rising out of the chamber as one might expect. The result is that ALL combustion products pass through the hottest part of the fire resulting in very complete combustion, producing the greatest amount of heat and reducing products of incomplete combustion such as carbon monoxide and smoke. To capture as much of the heat as possible and radiate it into the room, the exhaust gases are passed through a secondary chamber (much larger than the combustion chamber) that absorbs and radiates the heat. Finally the relatively cool exhaust gases are expelled through an exhaust tube.
The following illustration shows the basic design.
Insulating the chimney ensures a large temperature difference between the exhaust gases inside the chimney and those outside it. This temperature difference causes a density imbalance resulting in a strong draft. The hot exhaust gases in the chimney rise, while the cooler exhaust gases outside the chimney fall, and the whole process draws fresh air into the combustion chamber, supporting vigorous combustion. In my design, the radiating chamber is about 18″ in diameter by about 36″ high, while the combustion chamber is only about 4″ by 4″ by 10″. Don’t let the overall size of the stove fool you. It only burns a couple handfuls of wood at a time. The large size is required to absorb and radiate the heat, not to contain the fuel.
Construction
Rocket stove mass heaters are often built from steel drums. These are convenient since they have a flat top that can also be used for cooking. I did not have one handy though. What I did have was my parents’ old electric hot water tank that they had just replaced since it was corroded and leaking. In addition to this I used some 3″ diameter steel pipe, some 4″x4″ square tubing, and some flat steel plate, all about 1/8″ wall thickness (though that is probably thicker than necessary). I also used some flexible aluminum tubing (dryer ducting) to feed the exhaust from the rocket stove into my existing fireplace.
Above is a picture of the internal parts of the stove (combustion chamber and chimney) set up for initial testing to make sure it would generate enough draft for vigorous combustion. The aluminum flex tubing is connected to the top of the chimney for testing only. In the finished product it will be connected to the side of the stove. The chimney is insulated with about 1″ of Roxul (a rock wool insulation that will tolerate higher temperatures than fiberglass insulation) and wrapped with adhesive aluminum furnace tape to hold the insulation in place.
The above image shows the hot water tank cut to accept the combustion chamber and chimney.
The above image shows the combustion chamber and chimney being welded into the side of the hot water tank. Note the block of wood between the end of the chimney and the inside surface of the tank to ensure an appropriate gap for the exhaust gases. This piece of wood will be removed after the chimney is welded in place.
Mad scientist at work…
… and after a bunch more welding and a paint job that I neglected to take any pictures of … viola! … the finished product. The silver band around the bottom isn’t a racing stripe. It covers the seam where I tack welded the bottom of the tank back on. I did not want to weld it on permanently since I may want to disassemble the stove later for inspection.
Notice that I removed a glass pane from the left door of my fire place and replaced it with cardboard. The aluminum flex tube passes snugly through a hole cut in the cardboard to expel the exhaust gases into the fireplace where, still warm, they rise and exit through the existing chimney. I know you’re probably thinking I must be crazy to use a combustible material like cardboard for this purpose, but the fact is that the exhaust volume from this stove is so low and the stove is so efficient at removing heat from the exhaust that this aluminum tube reaches a maximum temperature of only about 60 degrees C during operation. The top of the stove gets much hotter, of course.
I also made a simple metal screen that can be placed over the combustion chamber to prevent sparks from popping out into the room where they could ignite something (or more likely just leave burn marks as they smolder on the carpet). I have another cover, not shown, that is just a solid steel plate. It’s useful to quickly extinguish the fire and to prevent air infiltration when the stove is not in use.
Does it work?
Well… actually … it works too well. The difference between burning wood in the fireplace and burning wood in this stove is amazing. I’ve fed the fireplace for hours before with hardly a noticeable rise in room temperature but it only takes a few handfuls of wood in the rocket stove to raise the room temperature by several degrees. It may take a long time to get through all my scrap wood at this rate. Feeding the stove every evening after work, I have only managed to go through about one bin (perhaps 20kg) of wood per week. Part of the issue is the season. It just doesn’t take much heat to get the house to a comfortable temperature right now in early spring. I figure I’ll be able to burn a lot more wood in the stove next winter.
Savings
On average the heat energy available from burning wood is around 4.5 kWh per kg (assuming a 20% moisture content). Assuming roughly 80% efficiency of the stove (just a wild guess) the heat extracted will be about 3.5 kWh per kg. I heat my home with electricity that costs about $0.07 per kWh. Therefore, the heating value of my scrap wood is about $0.25 per kg and by burning about 20kg per week as I am now I save about $5 per week on heating costs or about 50% of my heating bill for this time of year. Savings would be much greater in winter when the stove can be operated more without the room becoming uncomfortably hot.
Why not just buy a fireplace insert?
I considered buying a fireplace insert but after learning about rocket stoves I quickly dismissed the idea because:
- an insert would almost certainly be less efficient
- it would require significantly more cost and effort to install
- it wouldn’t provide nearly the same conversation value
- I would not be able to repurpose existing scrap such as my parents hot water heater
- it would not be portable (I look forward to operating my rocket stove on our patio on cool summer evenings).
Improvements
My rocket stove is a portable design. When designing for permanent installation, performance could be significantly improved by including a large thermal mass to better extract and store the heat from the exhaust gases. Imagine replacing my short length of aluminum flex tube with 30 or 40 feet of duct buried inside a couple tons of concrete and you’ll have the right idea. Exhaust would come out the end of the tube at close to room temperature and the efficiency would be very high. The thermal mass would also even out the temperature distribution. When I operate my stove, the room heats up fast and when I stop operating my stove, the room cools down fast. By adding several tons of thermal mass, a fairly constant temperature could be maintained, even if the stove is operated infrequently. It’s common for rocket mass heaters to be built into adobe benches or similar permanent fixtures for this reason.
Further reading
There is a lot of good information on rocket stoves online. A Google search on “rocket stove” or “rocket mass heater” will yield good results but http://www.rocketstove.org is probably the best place to start. There is also an excellent book called Rocket Mass Heaters available for purchase online at http://www.rocketstoves.com in PDF format. There are also several videos online of rocket stoves in action. Search for them at http://www.youtube.com.
