First let me say that, when it comes to burn efficiency, we have 3 main elements; time, temperature and turbulence. I think turbulence also means mixing with oxygen properly or at right stages along the burn path. I would like to add two more elements to this list, myself, from Rumford’s studies; insulation and thermal mass (a recurring theme in the green scene). Though insulation and thermal mass are elements that affect time and temperature. There is plenty that I do not say in this article about stove installation and connections of pipes and types of pipes, wall connections, etc. I don’t talk about modern stoves with catalytic converters in the smoke stacks, which bring the cost up in the $1,000’s. There are certified installers for stoves, as well. I also don’t talk about creosote build up due to premature cooling of smoke gasses. Yes, there is quite a bit more material covered by books and websites about all this. I, like most people, like the looks and pleasing feel of the open fire in or out of a home. I personally have no bias against highly inefficient open fires places, unless I need to conserve wood as fuel because of a shortage or because I want to spend less time gathering, cutting, splitting and stacking or moving wood. You may get a metallic reflector to place in the back of your fireplace to add efficiency. Yet, fireplaces with stove inserts, of course, are more efficient than open fireplaces. Inserts are not as efficient as stand alone stoves, however. Count Rumford was a guy who studied thermodynamics in the late 1700’s. He wrote a set of books about his research. He determined and recorded the BTU ratings for many types of wood around Europe and the world. His book is called “Complete Works of Count Rumford.” You can download a free scanned copy of the original rare book from a publisher’s website (GeneralBooksClub.com). You can also preview excerpts of the book there. I have seen vol 2 and vol 5. I don’t know how many volumes there are. You will find downloads of the PDF with web searches. Here is a firewood facts page with some BTU for species info.
This image above shows the forward slant on traditional yet not on the Rumford. I’m pretty sure Rumford has a slight forward slant on top of the fire box, though not as extreme as the conventional one shown in the image. Of the open fireplaces, Rumford’s design is most efficient. This design is not deep from front to back. The back wall slopes forward for about half the height up to the fireplace flue opening. It would have a lot of thermal mass around it. It has a smoke shelf and chamber above the top of the fireplace so that hot smoke can circulate around and warm the thermal mass, instead of simply shooting straight up and out the chimney. There are Rumford design stoves and cooking ranges/ovens, as well. A brand of Rumford design wood cook ovens was AGA Brand. I don’t know that much about his stove/range/oven design yet and I can’t really find drawings of it. As for Count Rumford’s wood cook ovens and ranges, he used deep fire wells with good use of thermal mass and insulation. He used insulated lids and doors. The idea was to only produce the amount of heat needed and keep it where needed. His first ovens were so efficient that they had to install a small open fireplace to keep the cooks warm on winter days. When cooking on wood ranges, pots and pans must be moved around from cooler burners to warmer burners. To get extra heat, a burner plate could be removed to set the pot or pan directly on the heat or flames. Turning of the pots and pans is frequently necessary because each burner will have a warmer and cooler side. To cool an oven that is too hot, place a pan of cool water inside the oven. Some ovens vent smoke around a warmer box above the stove for keeping food warm until served. Some stoves also have hot water wells for on demand hot water.
Rocket Stove is made with a pipe that drops nearly to the bottom of the stove, so that air is sucked downward and horizontally into the pipe. Mass Rocket heater is a similar concept, except that it uses a J shaped flue with firebox, which circulates the smoke/gasses back around the flue and down into heater ducts. These ducts flow through thermal mass of some kind and should be 6″ from the outside surface of this thermal mass. The flue is only about 3 feet tall and is insulated with high heat tolerant insulation, such as vermiculite beads (activated mica), perlite (volcanic glassy material), pumice (volcanic ash), kaowool (sometimes called kawool, I think), or, heaven forbid, asbestos. Actually, there is a type of asbestos that does not get caught in the lungs or cause cancer or lung disease. Even though geologist have proven its safety, it is still not allowed. Wools like kaowool may be called “High Temperature Insulation Wool” or HTIW.
Rocket mass heaters draw air from the top of the fire hole (pit) downward and sideways into the flue. They have their drawbacks. For example, they need a lot more attention than the normal stoves or open fireplaces when it comes to feeding them. However, they absolutely make the most efficient use of heat, heat storage and thermal mass. And they burn the most cleanly and efficiently possible. The exhaust from a rocket mass heater is almost clear, has mostly CO2, little CO maybe and water vapor. The heat coming out of a rock mass heater flue would only be around 200F degrees or a bit more instead of 600 to 900 degrees. This, in itself, demonstrates why they are efficient. This shows heat retained by the thermal mass and inside the structure. A rocket stove design at alt-nrg.org
There are many uses for rocket mass heaters and one interesting one I heard of was used for a wood drying kiln. The kiln was 8 feet high, 20 feet long and 10 feet wide. A rocket stove would pipe its exhaust through a box filled with sand the length of the kiln. I think I’ll try this some day, because you can use all the waste wood and possibly saw dust for fuel in drying the good wood. Other uses mentioned was in the heating of a hot tub or pool water. Rocket Mass stoves for cooking are not out of the question either. Unlike the open fire or stoves, using heated thermal mass means taking advantage of all the forms of heat transfer radiation, convection and conduction. If done properly you can place mattresses and/or lay or sit directly on the thermal mass for the transfer of heat into your body. This is the most efficient way to warm the body. Normal warming comes from radiation (stoves or open fires) or convection (central heating) mostly. Cob (Native American Horno (bee hive oven) or brick ovens for baking pizza and bread seem like a great idea to me, too. I’m a big proponent in utilizing thermal mass for this stuff. The cob oven I saw in the Cob House book resembled a mud dome. It had a door that was made of wood actually. You build the fire inside the oven to warm it. After you warm it, you remove the coals and ash and wait a small bit of time for the temp to drop to the baking temp. Then bake. This oven can be made more efficient borrowing from the rocket stove design. It would need a small chimney or flue at the top. It would also need an intake flue that would drop down so that intake air is drawn down to the floor level. I’d say a pipe with some slots or holes drilled in the bottom would work fine. This intake flue would be removed just prior to baking. An insulated plug might be needed for the intake flue hole. Chimney might need to have a damper or plug. The thermal mass of the 1 foot thick mud oven dome keeps the oven warm long enough to bake pizza or bread or maybe even a casserole. It’s a matter of designing it so that it keeps a given temperature range for a given time for a given recipe. It’s likely that what would work for one recipe will also work for a myriad of others. Russian fireplaces and stoves were made using thermal mass principles as well. Russian chimneys were often made so that the smoke circulated in a zig zag fashion around block or rock, so that more heat was absorbed by the thermal mass before the smoke escapes the chimney. They also had places to sit that had been warmed by the smoke and fire. Some Russian beds were actually made on top of stoves, where there was about a foot of thermal mass between the bed and stove. The bed would be high up in the room, near the ceiling, also, which would be the warmer part of the room because of convection. It gets quite cold in Siberia (-120F maybe).
One last note about chimneys. First settlers in America made chimneys and fire places from logs and cob. Cob being mud/straw mix packed between the logs and as thermal mass between flue and logs and fireplace and logs. The logs were stacked in a loose fit manner. The mud held the logs in place and the logs held the mud in place. Once fired a few times, the mud would become like brick. The logs overhung the mud on the outside enough to prevent erosion from rain. This is probably not a method I’d use unless I was in a pinch. If I wanted an el cheapo cabin somewhere as a camp site I might try this method as well. Lehman’s sells $8,000 wood cook ranges and ovens imported from Europe. A quick search on eBay will find used wood cooking stoves and ovens for much lower prices.
|Rocket Stove, Rocket Mass Heater|
|Building with Cob, shows how to build a simple cob oven.|
Some comments from the blog post with additional info. http://politowoodfireovens.com.au/ A cob oven maker in Australia
Masonry Heater Association Mother Earth news article about Masonry Heaters Also, for rocket mass heaters made from 55 gallon drums, you can make your own ovens by using metal bowls turned upside down or large metal cans. You can cook breads, pizza and even casseroles, or even just warm something up. You could insulate with ceramic wools and use aluminum foils and such. You could add oven thermometers or other high temp thermometers, also. There is a lot one can do with cooking on wood fired stove tops.