Woodstove Design Theory


All of woodburning products can be divided into four general categories. We will consider and discuss each separately.

Early Non-Air-Tight Concepts

Until about 1970, ALL wood burning fell into this category. It includes conventional open fireplaces, potbelly stoves, bonfires, campfires, and, until the 1970s, pretty much all other wood-fueled fires.

Advantages

Advantages of this mode is extremely clean burning (sometimes above 99% combustion efficiency). Combustion efficiency is the proportion of what heat is actually created in the fire compared to the amount of heat available in the fuel. That 1% or so that does NOT get changed into heat, winds up as ASHES, CREOSOTE and POLLUTION. Relatively speaking, 99% combustion efficiency is extremely good!

Disadvantages

The fire has somewhat uncontrolled characteristics. It burns as hot and fast as it naturally wants, making for very uneven heating. If thinly split pieces of wood are used, as is commonly the case, then it is common to have very short burning periods with too much heat being produced then. Since these thin pieces of wood burn so rapidly, they create a very rapid draft up the chimney, which tends to suck the heat being created up the chimney. The result is a lot of radiant heat being given off for a brief time, but virtually NO convective heat except what is drawn up the chimney, so the net efficiency is fairly low, and there are a lot of fluctuations in heat output. Potbelly stoves were about 15-25%. Open fireplaces were about 0-12%. Non-airtight Franklin stoves were slightly better, being around 25% on a regular basis.

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An additional disadvantage was related to the fact that nearly all the heat being produced was radiant heat. You needed to be near the fire to feel much heat, and only one side of you got heated at a time. Your back would get cold while your front was betting toasted!


For thousands of years, people put up with these characteristics of burning wood for heating. Even when Franklin stoves and Potbellies came into use about 200 years ago, the vagaries of burning wood were accepted as necessary consequences of the activity.

Then, beginning in the 1970s, technology entered the fray, and the three OTHER general design concepts were developed. Tgey are: Advanced Non-Airtight, Airtight, and Advanced Airtight. Several variations exist in each category. Nearly all the companies of the 1970s followed the examples of a couple of commercially popular products, which were airtight modifications of the previous non-airtight burners mentioned above. Those products' designs will be discussed below, followed by the later improved designs of airtight products. First, however, we will discuss the advanced designs based on non-airtight principles, which includes the JUCA design concept.


Advanced Non-Air-Tight Concepts

Several approaches were tried during the 70s and 80s to enhance the efficiency and the user friendliness of non-air-tight wood-burning. Only two were really effective: the labyrinthine fireplace, and the JUCA design.

Labyrinthine fireplaces

The labyrinthine fireplace recognized the extremely clean burning of a free-burning wood fire. Its approach is to make the smoke weave through a long path through many tons of brick and other masonry materials. The smoke passes so much masonry material on its way out of the house, that it winds up giving up most of its heat to all that masonry material. The idea is to have a brief, very hot, fire (using thinly split wood for the hottest fire) that only burns for an hour or so. Then, the many tons of hot masonry will gradually give off that heat (by radiation out to the house) over the next eight hours or so.

Advantages

This approach is generally a pretty good one. It retains the extreme cleanness of non-airtight burning, and only needs to be tended at about eight-hour intervals. This is true, as long as at least 15 tons of masonry materials is used in creating the labyrinthine smoke path.

Disadvantages

Extremely high cost is the greatest disadvantage of a labyrinthine fireplace. There have been some limited sized labyrinthine style fireplaces sold, that were sold for $5000 or so, but they did not have all the benefits of a true labyrinthine design. Generally, a mason who is experienced in building labyrinthine fireplaces must custom build the entire 15 ton (or more) structure. This involves a LOT of labor cost, and generally gets the installed cost up above $15,000.

The only other disadvantage of a labyrinthine fireplace is that there is a slow cycling of the temperature of the house. After eight hours beyond the brief burn, the house temperature is considerably cooler than right after the fire. Owners of labyrinthine fireplaces seem generally to consider a ten degree (F) variation of house temperature to be acceptable.

JUCA's Non-airtight Design

JUCA also chose to maintain the extremely clean burning of a non-airtight burning concept. But, the ways we accomplished evenness of heating and long burn times are quite different from the labyrinthine fireplace approach. We had long noted that, in campfire bonfires, the thickest logs were the only pieces still burning and giving off heat in the morning. During the whole night, those big logs had unlimited oxygen supply, but their thickness caused them to last all night. Burning can only occur at the surface of a piece of wood. Inner wood fibers could only start to burn after the outer pieces had burned away to expose them to the oxygen. A little research soon established that 8 inch thick hardwood logs tended to burn for 10-11 hours. A side benefit was noted, that during that long period, the heat produced was amazingly constant.

This explains the very large fireboxes on JUCA products, to allow being able to burn very large, thick logs.

A complication was that, even though those big logs gave off constant heat, the AMOUNT of that heat does not seem too spectacular. But, have you ever looked inside the conventional furnace that heats your house? The actual amount of flame is surprisingly small! That furnace is able to heat your whole house because there is a very efficient heat exchanger (heat capturer) above the fire, which transfers that heat into different air which can then be sent to the various rooms of your house through ducts. This told the JUCA designers that it was necessary to do the same.

This explains the sophisticated heat exchanger system that is the upper half of all JUCAs.

Advantages

Very clean burning is achieved, because the fire is allowed to burn with adequate oxygen for extremely complete combustion. It turns out that by burning very thick logs, the draft air moves fairly slowly (and consistently) and allows even higher combustion efficiency!

Burning very thick logs allows very long burning times, with surprisingly constant heat creation. The convenience and comfort are wonderful, generally rivaling the comfort of a conventional gas or oil furnace.

The heat exchanger design is so sophisticated that nearly any JUCA can entirely heat the whole house! Any climate. Any size house. (Nearly) any condition.

Prices of JUCA products are VERY reasonable! Many popular JUCAs are about $1400. And they're BUILT! Most of the popular JUCAs weigh 500 to 600 pounds.

Disadvantages

I suppose we're biased, but we are not aware of any major disadvantages of JUCA products. The products ARE much larger than competing woodstoves. And, if you use the capability of feeding into a duct system, installation is more involved. (And you DO have to cut and carry wood!)

Soapstone Stoves

Another interesting idea appeared which was an effort at trying to make the heat output last longer from non-airtight wooestoves. Someone had noticed that a natural material called steatite )commonly called soapstone) could store about twice the amount of heat as steel or cast-iron. So, large, thick slabs of soapstone were incorporated into the walls of a woodstove, to try to capture some heat to be radiated later, after the fire had died down.

Advantages

The theory is good, but the actual amount of soapstone necessary to have much effect is huge. If 200 pounds of soapstone was incorporated into a Potbelly stove, for example, then about an extra hour's worth of heat would be radiated to the room.

Disadvantages

Soapstone is rather expensive material, and a stove that included 200 pounds of soapstone would be thousands of dollars more than a similar stove without it. In addition, the benefit of extending the heat output disappears when ANY of the advanced methods mentioned here are used. For example, since the JUCA design NORMALLY is meant to continuously PRODUCE heat for 8-10 hours, an extra hour onto that has minimal conzequence.

Early Air-Tight Concepts

In the 1970s, there was another approach taken to improve on the previous early non-airtight products. To make the fire last longer, and to make it give out its heat more evenly, the idea was to restrict the amount of air (oxygen) available to the fire. The principle sounds like a good one. And it definitely achieved those two goals, which made airtight products immediately very popular. Compared to the previous early non-airtight products, the evenness of heat output and the much longer burn times were wonderful. It would be great if that were the end of the story. But it isn't. Unfortunately, when the oxygen supply for the fire was restricted, all of the chemical reactions involved in oxidizing the fuel molecules in the wood fibers could not always occur. In particular, the Carbon atoms in the wood must first combine with one oxygen atom, to form carbon monoxide, before then combining with another oxygen atom to form the desired carbon dioxide, which is the natural product of proper combustion.

When the air supply is restricted, often the first reaction could occur, but there was then not enough oxygen available for the second one to be able to happen. This had several bad consequences. First, of course, was the production of a substantial amount of carbon monoxide (a very poisonous gas) in the house, although most of it would go up the chimney. Second, it turns out that the second necessary reaction is very exothermic (that means it gives off a lot of heat). By not being able to have the carbon monoxide combine with the second oxygen atom, a LOT of energy was not released from the fuel. Third, since that energy was not released, it remains in the smoke, which gets carried up into the chimney. That means that it not only wastes a lot of heat (low combustion efficiency), but sends unburned chemicals up into the chimney, which meant pollution and creosote-associated materials.

The majority of carbon atoms (and hydrogen atoms) in a piece of wood are originally parts of very complicated hydrocarbon molecules in the wood. After all, that's what the tree had to create to become a tree in the first place! There are quite a variety of these complicated hydrocarbon molecules in a piece of wood. When a fire is allowed to burn naturally (with adequate oxygen) those molecules are in the actual flames long enough for the heat to break them down into smaller, simpler molecules, which are then able to combine with the oxygen, to achieve extremely complete combustion. All this happens before the molecules leave the tip of the flame! But in products that do not allow adequate oxygen there, many of these giant molecules do not get a chance to entirely break down into really small molecules that can combust, and, of course, there isn't enough oxygen present right there for the combustion to occur anyway. The consequence of this is that the smoke in airtight burning rises from the fire with a large assortment of medium sized hydrocarbon molecules in it. This material is what is actually referred to as creosote. There actually is no ONE chemical that is creosote. It is a generalized term to describe about 160 different chemicals, which have relatively similar characteristics, and which are all the products of partial combustion.

Well, this presentation is just an overview of the subjects. The full picture is far more complicated, and some details are not yet fully understood by modern science.

Advantages

Early airtight products achieved their two goals: evening out the great variations in heat output and holding a fire for a much longer time. Many products added various thermostatic draft controls, where when the unit got hotter, the draft control closed, to make for even better heat output consistency.

Disadvantages

Huge creosote production was the most immediate problem known. In 1979, a chimney manufacturing company was testing a popular airtight woodstove with their 8 inch diameter chimney, and they found that in just THREE WEEKS, the chimney had CLOSED ITSELF SHUT with creosote accumulation! (By the way, that test and others like it were the initial source of the instruction in ALL airtight products' installation instructions where an owner is told to check the chimney for creosote at WEEKLY INTERVALS!) Later, people realized that the products were limited to about 40% overall efficiency because so much heat was not ever being produced by the fire, because of the airtight operation. Independent tests showed that combustion efficiency could be as low as 67%, meaning that fully a third of the heat available in the wood was never being created, and was being carried up into the chimney as creosote and pollution.

These things in themselves would probably have never gotten many people's attention. But it turns out that creosote is capable of burning EXTREMELY intensely (possible above 5000°F). When accumulations in chimneys would catch fire, the intensity of the resultant CHIMNEY FIRE was so great that the chimney would sometimes fail, and a house fire resulted. This happened often enough that insurance companies started paying attention, and the government soon followed.

Advanced Air-Tight Concepts

In the middle 1980s, various branches of the government started setting new strict standards, as a result of those problems with the early airtight products. Chimney standards were increased. Pollution standards were initiated. Efficiency standards were begun. New rules regarding control of creosote dangers were imposed.

Nearly all these new laws and rules were written very specifically for airtight products, which represented the vast majority of all wood burning products on the market. Fortunately, the government rule-makers knew enough to exempt non-airtight products, because those products did not represent the safety and pollution hazards of airtight products. Also exempt were central furnace products. (Note: JUCA products are thereby exempt for two separate reasons, being non-airtight and being central furnaces.)

Airtight manufacturers tried (and sold) a variety of different ideas to correct the problems of their earlier airtight units. Two of the most popular were Secondary Combustion Air and a Catalytic Combustor. A third approach appeared later, and it will be discussed below.

The idea of secondary combustion air was based on the fact that the fire had not been allowed enough oxygen initially to complete all of the chemical reactions of the combustion process. In particular, the main second reaction, of carbon monoxide plus oxygen gives carbon dioxide, generally hadn't occurred. The only thing that had been withheld was the oxygen. Why not later introduce extra air above the fire. This should allow the completion of all the reactions, right? WRONG! It turns out that the second main reaction can ONLY occur at a temperature above 1211°F. In normal (non-airtight) combustion where there is adequate oxygen in the main fire, this second reaction occurs in the flame tips, where the temperature is about 2300°F, so it works fine. But, as smoke leaves the flame tips, its temperature drops off very rapidly, quickly dropping to near the critical 1211°F. But then, if you introduce (cold) secondary combustion air, it immediately cools the mixture to below that temperature, and no reaction can occur. Therefore, secondary combustion virtually never actually happened in all those products that were sold that had that provision.

The idea of the catalytic combustor was in response to the lack of success in secondary air systems. A properly functioning catalytic combustor, for very complicated reasons, reduces the required temperature from 1211°F down to about 550°F. This is a much more manageable situation, and allows a much better chance for a secondary combustion to actually occur. It still requires a very well designed source of secondary air to be available at the right place at the right time, but units with catalytic combustors DO work. They REALLY work well in laboratories, where the operators do everything precisely correctly for best operation. The CAN work well in the homes of normal people, but sometimes don't. If the product is operated such that the catalytic combustor is never heated to its necessary 550°F, it will not do anything, and it is even likely to clog up pretty quickly. As long as the catalytic combustor gets up to its proper temperature, it creates heat of its own, which tends to maintain its temperature and its functionality.

The third, and newest, approach in modern airtight products is the most fascinating to us! For a while, experts were suggesting to owners of airtight products to drill holes in their draft controls! They were advising effectively eliminating the airtight nature of their airtight products! This approach evolved into the currently most popular approach in selling airtight products. That is to buy a unit that is as small as is absolutely possible for a particular installation. The approach here is that the wood burner is so small that it will nearly always be operated at its maximum output, or, in other words, wide open. In other words, in a non-airtight type of operation! This generally works fine, and has been pretty successfully marketed by the manufacturers of airtight products, which are still the vast majority of all wood burning products sold today.

Advantages

Basically the same as earlier airtights, with evenness of heat output and extended burn times. In addition, advanced airtights have higher efficiencies and less creosote and pollution formation than their earlier forbears.

Disadvantages

Units with secondary air systems have disappeared from the market, probably because they seldom accomplished what they claimed to do.

Catalytic combustor units are generally rather expensive to buy. The catalytic element is susceptible to failure if anything other than wood is ever burned. Owners of catalytic units seem to expect to have to replace the $200 catalytic combustor about every two years or so. The newer advanced airtight woodstoves, generally called NON-CATs or non-catalytic devices, must necessarily have very small fireboxes. This is so they are normally used at their maximum output, so they can often be in a non-airtight mode of operation. These tiny fireboxes require splitting wood to very small sizes, and of not being able to put enough wood in them to actually do very significant heating.



Related Subjects

Pyrolysis

A number of modern articles about the physics of woodburning refer to several "stages" of burning. In general, they describe an initial burning of easily burnable fibers of the wood (or paper). The heat generated by this first stage causes "out-gassing" or pyrolysis of the wood. This is the release of volatile gases from the wood material, which can include carbon monoxide, hydrogen, methane and ethane gases. If these gases are then able to combine with oxygen (oxidizing them), they release additional heat energy, improving overall combustion efficiency. After the volatile gases are released, the remaining wood is effectively very similar to charcoal, which has high carbon content, moderate hydrogen content, and almost no volatile gases left. The first two stages tend to occur fairly quickly, while this third stage tends to be the long lasting part of a fire.

We at JUCA don't often emphasize these aspects of the chemistry and physics of fire. In non-airtight operation, the first two stages tend to occur very quickly, well before any gases or smoke leaves the flame tip of the fire. And since adequate oxygen is always present, nearly all the pyrolyzed gases are able to complete their combustion while still in the flames. This makes for VERY high combustion efficiency in a JUCA.

In airtight products, the subject becomes much more important. Since the lack of oxygen often does not allow the completion of the combustion of the pyrolyzed gases in the fire, they must be handled later (often in a catalytic combustor) to avoid poor combustion efficiency. In airtight products, the three stages of burning often must be dealt with as separate events, with suitable environments designed into those products to maximize overall combustion efficiency.

Another reason why JUCA is so adamant about non-airtight operation!

Firebox Size

Most competing woodburners have rather small fireboxes. JUCAs all have very large fireboxes. Why is that?

Competing (airtight) products MUST keep the firebox small to minimize the amount of time that actual airtight operation is necessary (see above). They also do it to keep their products physically small, because small products sell better, because people consider them less overwhelming in their room.

But there are some problems with small fireboxes. The most obvious is that you cannot put much wood in it. A 2 cubic foot firebox woodstove can only hold about 30 pounds of wood if it is packed completely full. Since each pound of wood has about 6000 Btu of chemical energy available in it, that means there are 30 x 6000 or 180,000 Btu of energy available. Modern airtight products are often 70% efficient, which means they could actually get 126,000 Btu of heat to the house from that load. Sounds like a lot, huh? Well, no! That heat could give 50,000 Btu/hr to the house for 2 and a half hours! Or 12,000 Btu/hr for 10 hours. Certainly enough to heat a good sized room for a night, but certainly NOT enough to heat a whole house for much beyond two and a half hours.

There is actually a more important problem of the small firebox. Since our example was packed full of wood, much of that wood is very close to the metal walls of the firebox. It's pretty complicated, but the 2300°F flame tips are near 500°F metal walls, and the walls seem COLD to the fire. The effect is that heat is drawn away from the fire that is needed to maintain proper temperatures of everything for complete combustion. An analogy is when you stand near a large single thickness glass window on a cold day in Winter. You can FEEL the heat being sucked out of you! It is very uncomfortable, and you soon move away, because your body cannot maintain the temperature it needs to operate properly. This is sort of like the effect of the (relatively) cold firebox walls on a fire. The solution is to keep the fire far enough from those walls so that they do not adversely affect the burning. At JUCA, we did a lot of research on this, and we found that an average distance of about 6 inches is a good distance from the actual flame to the wall surfaces. (We still tell people that they can build the fire near or against the back wall, though.)

In the same vein, our heat exchangers are considerably above the fire, for the same reasons. Relatively few airtight products have any heat exchangers (because they would have even worse creosote problems if they did) but often there is a metal surface just a few inches above the fire. This again, adversely affects the fire.






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