How to Improve your Stove

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Short Description

The following concepts describe how you can improve your stove in terms of efficiency. There are different aspects which will be analyzed because one can do more than just insulate the combustion chamber. In the picture below an example of an efficient stove is shown.

Insulating your house

The most fuel efficient heating stove is one that is never used! Therefore it is most important to reduce uncontrolled air exchanges by filling cracks in the walls, around windows and doors and secondarily to insulate the house. A house that is relatively airtight and insulated like a thermos bottle or a box full of hay does not require constantly burning wood in a stove to maintain interior temperatures. The �haybox house� helps to reduce fuel consumption just like the heat exchangers that can be added to a heating stove. Capturing the heat more effectively diminishes the need for burning wood all the time to keep warm.

Clean burning

In the combustion chamber wood or other biomass is turned into heat. A good chamber is when this process does not create much smoke or creosote (condensed wood tars). Complete combustion of wood results in two byproducts: carbon dioxide and water vapor. In contrast, incomplete combustion creates unburned particles that cause pollution and creosote that fills chimneys and can cause chimney fires if it ignites. Nearly complete combustion in a wood burning stove can be achieved by doing the following things:

1) Metering the fuel.
Cutting wood up into smaller pieces and feeding them at a proper rate into the fire as they are consumed.
2) Making a hot fire.
Creating a combustion zone where fuel, flame and air are mixed by turbulence, at a high enough temperature, for a long enough time to completely combust. Combustion temperatures must be hot enough to assist burning all escaping gases released from the wood.
3) Insulating the combustion chamber.
Insulation helps to keep temperatures high.
4) Igniting escaping smoke.
Passing smoke, which is un- combusted fuel, through a flame.
5) Providing sufficient oxygen.
Starving the fire slows it, cools it down, and produces smoke.
6) Warming and increasing the velocity of the cold air entering the fire.
Air is warmed as it passes through a small opening into the combustion chamber. For systems without a fan, make enough small holes under the door into the combustion chamber so the holes have as much cross-sectional area as the chimney exiting the stove. Position the holes so that primary air is sucked into the coals and up into the combusting wood. Do not allow the user to block the holes reducing primary air. Blocking the necessary amount of air will create pollution. The rate of burn in a heating stove should be determined by the amount of fuel in the combustion chamber, not by shutting off air to the fire.
7) Forming a grate out of the firewood.
Sticks burning close together heat each other and keep the temperatures high. The pattern should be stick, air, stick, air, with even spaces between the sticks.
8) Creating sufficient draft.
Use a tall enough chimney or better yet a small fan. An insulated chimney creates a lot more draft than an un- insulated chimney. High velocity, low volume jets of hot air entering under the fire, up though the coals, create mixing which reduces emissions. Do not use a damper in the chimney. Design the stove to run efficiently with enough air entering and leaving the stove to burn fuel cleanly.

Complete combustion cannot occur when starting a stove because the combustion chamber is too cold. An insulated combustion chamber will heat up more quickly and then, when burning metered amounts of biomass, make less smoke. Throwing a big log on the fire, however, always makes smoke. The log cools the fire and releases more pyrolysis gasses that overwhelm the available air and are too cold to be combusted. Without enough air, wood cannot burn cleanly. The size of the air inlets into the fire should add up to be about as large as the chimney exiting the stove. The power level of the stove should be set by the wood loading rate, not the air flow. When users try to control the power of the stove by shutting off air to the fire they can send horrible plumes of smoke out of the chimney. A stove must have enough air to function efficiently.
Metering fuel makes clean burning easy. In a regular wood burning stove, the same thing can be accomplished by burning small pieces of dry wood and watching to make sure that a fierce flame is present. A little observation teaches the operator quickly how to maintain clean burning. Unfortunately, adding fuel at regular intervals is much more demanding and time consuming than just throwing a log onto the fire and then ignoring the smoke polluting the environment.

Insulating the combustion chamber

Insulative ceramics used in stoves undergo repeated heating and cooling (thermal cycling), which may eventually produce tiny cracks that cause the material to crumble or break. All of these recipes seem to hold up well to thermal cycling. The only true test, however, is to install them in a stove and use them for a long period of time under actual cooking conditions. If you can afford it, buying commercial fire brick is certainly an easy and workable option.
Insulative ceramics need to be lightweight (low-density) to provide insulation and low thermal mass. At the same time, they need to be physically durable to resist breakage and abrasion due to wood being forced into the back of the stove. These two requirements are in opposition; adding more filler to the mix will make the brick lighter and more insulative, but will also make it weaker. Adding clay will usually increase strength but makes the brick heavier. A good compromise is achieved in a brick having a density between 0.8 gm/cc and 0.4 gm / cc.
These recipes described below are a starting point for making insulative ceramics. Variations in locally available clays and fillers will probably require adjusting these proportions to obtain the most desirable results.

In this formulation, fine sawdust was obtained by running coarse sawdust (from a construction site) through a #8 (2.36-mm) screen. Clay was added to the water and mixed by hand to form thick mud. Sawdust was then added, and the resulting material was pressed into rectangular molds. Excellent insulative ceramics can be made using sawdust or other fine organic materials such as ground coconut husks or horse manure. The problem with this method is obtaining large volumes of suitable material for a commercial operation. Crop residues can be very difficult to break down into particles small enough to use in brick making. This method would be a good approach in locations where there are sawmills or woodworking shops that produce large amounts of waste sawdust.

In this formulation, raw charcoal (not briquettes) was reduced to a fine powder using a hammer and grinder. The resulting powder was passed through a #8 screen. Clay was hand-mixed into water and the charcoal was added last. A rather runny slurry was poured into molds and allowed to dry. It was necessary to wait several days before the material dried enough that the mold could be removed. Dried bricks were fired at 1050� Celsius. Charcoal can be found virtually everywhere, and can be used when and where other filler materials are not available. Charcoal is much easier to reduce in size than other organic materials. Most of the charcoal will burn out of the matrix of the brick. Any charcoal that remains is both lightweight and insulative. Charcoal/clay bricks tend to shrink more than other materials during both drying and firing. The final product seems to be lightweight and fairly durable, although full tests have not yet been run on this material.

In this formulation, commercial vermiculite (a soil additive), which can pass easily through a #8 (2.36 mm) screen, is mixed directly with water and clay and pressed into molds. Material is dried and fired at 1050� C. Vermiculite is a lightweight, cheap, fireproof material produced from natural mineral deposits in many parts of the world. It can be made into strong, lightweight insulative ceramics with very little effort. The flat, plate-like structure of vermiculite particles makes them both strong and very resistant to heat. Vermiculite appears to be one of the best possible choices for making insulative ceramics.

Perlite Mix/Clay:
For best results, perlite must be made into a graded mix before it can be combined with clay to form a brick. To prepare this mix, first separate the raw perlite into three component sizes: 3/8� to #4 (9.5 mm to 4.75 mm), #4 to #8 (4.75 mm to 2.36 mm), and #8 (2.36 mm) and finer. Recombine (by volume) two parts of the largest size, one part of the midsize, and seven parts of the smallest size to form the perlite mix. This mix can now be combined with clay and water and formed into a brick, which is dried and fired. Perlite is the mineral obsidian, which has been heated up until it expands and becomes light. It is used as a soil additive and insulating material. Perlite mineral deposits occur in many countries of the world, but the expanded product is only available in countries that have commercial �expanding� plants. Where it is available, it is both inexpensive and plentiful. Perlite/clay bricks are some of the lightest usable ceramic materials.

Pumice Mix/Clay:
Pumice, like perlite, produces the best results when it is made into a graded mix. Care should be taken to obtain the lightest possible pumice for the mix. Naturally occurring volcanic sand, which is often found with pumice, may be quite heavy and unsuitable for use in insulative ceramics. It may be necessary to crush down larger pieces of pumice to obtain the necessary small sizes. The mix is prepared by separating pumice into three sizes: 1?2� to #4 (12.5 mm to 4.75 mm), #4 to #8 (4.75 mm to 2.36 mm), and #8 (2.36 mm) and smaller. In this case, the components are recombined (by volume) in the proportion of two parts of the largest size, one part of the midsize, and four parts of the smallest size. Clay is added to water and mixed to form thin mud. The pumice mix is then added and the material is pressed into molds. Considerable tamping or pressing may be necessary to work out the air and form a solid brick. The mold can be removed immediately and the brick allowed to dry for several days before firing. Pumice is widely available in many parts of the world and is cheap and abundant. Close attention to quality control is required, and this could be a problem in many locations. It is very easy to turn a lightweight insulative brick into a heavy non-insulating one through inattention to detail. Pumice (and perlite as well) is sensitive to high heat (above 1100� C). Over-firing will cause the pumice particles to shrink and turn red, resulting in an inferior product. Despite these concerns, pumice provides a great opportunity to supply large numbers of very inexpensive insulative ceramics in many areas of the world.

Heat exchangers

There are three different common types of heat exchangers described below: Mass heat exchangers, air to air and air to water heat exchangers. They have all advantages and disadvantages:

Massive Heat Exchanger
The great thing about air-to-mass heat exchangers and air-to-water systems is that the stove can be fired very hot for a long time without overheating the room. The heat goes into the cool stone or water, instead of immediately into the room air. Big fires are very hot and for that reason can produce less harmful emissions. The harmful gasses burn up in the hot fire.
1. The mass stores heat that can keep the house warm overnight.
2. Gentle radiant heat feels good.
3. Burning time can be reduced.
4. The fire can be huge and hot resulting in clean burning. Since the heat is stored at a lower temperature to be released more slowly, the room doesn�t tend to overheat.
1. Stored heat is there if you need it or not. If the day suddenly gets warmer, the room can overheat.
2. The mass takes up room. To store sufficient heat, the heat exchanger must weigh thousands of pounds.
3. The cold mass will take a long time to heat up and warm the room. Coming home and lighting the stove for warmth will not work with a high-mass, slow-response heating stove. The stove needs to be kept warm.
4. Creating the ductwork in stone, brick, or adobe frequently requires experience.

Air to Air Heat Exchanger
A stove using a high-mass heat exchanger can get away with short hot burns. An air-to-air heating stove has a harder task to accomplish: to create an equally hot, smaller fire that matches the heating demand of the space. The air-to-air type of stove is more dependent on doing things right to reduce emissions since it�s not creating one huge hot fire. The factor favoring air-to-air solutions is that they can be built inexpensively and quickly.
1. It is inexpensive and easy to make.
2. It doesn�t weigh very much.
3. It takes up less space.
4. It heats the room quickly.
5. If the weather suddenly warms, the heat can be adjusted.
1. It doesn�t retain heat and is cold after the fire goes out.
2. It discourages big, hot, clean-burning fires (which overheat the room) and can encourage small fires that pollute.
3. It is better suited to less drafty houses.

Air to Water Heat Exchanger
Air-to-water heat exchangers for house heating seem so full of potential problems. Imagine trying to repair lots of leaking pipes buried in your floor. So far, potential difficulties and cost have steered back to simpler solutions. Heating water is theoretically a great idea but can be complicated.
1. Can provide very efficient heat transfer
2. Retains more heat than other thermal mass
3. Allows for control of the amount of heat by opening or closing radiators
1. Usually requires a thermostat
2. Needs safety release valves
3. Water can leak
4. Minerals in water can reduce internal pipe diameter, leading to reduced water flow, greater temperature rise and increased pressure loss in the system.

Even in an optimized design, a heat exchanger requires a lot of surface area. Just piling mass near a stove will result in poor heat transfer to the mass. Only a small percentage of the heat will end up in the mass. Hot flue gasses need to be forced to scrape against surfaces over long distances for efficient heat transfer to occur.

Chimney design

The chimney pipe is designed for longevity, not for heat transfer. To optimize heat transfer it�s better to make a chimney with a different shape, not cylindrical, but with the same cross sectional area. The shape should be wide, shallow, and rectangular. Even though the same amount of hot gases pass through the inside, a great deal more surface area on the outside is in contact with the substance you want to heat. Temperatures in the combustion chamber are above 2,000� F when yellow flame is present. We want exit temperatures to be about 250� F. In an optimized design, approximately 40 square feet of surface area is needed to transmit this much heat into the room. As a rule of thumb, it takes about five square feet of optimized surface area, in a heat exchanger, to lower exit temperatures from the stove about 250� F.


The push created by hot rising air is very gentle. Even flame itself doesn�t travel at much more than three miles per hour. Natural convection produces a lazy draft that cannot be asked to do too much. The draft produced by a hot fire Fan can easily be defeated by friction inside of a chimney pipe if there are many twists or downturns. Many amateur designers hope that natural draft will overcome impressive obstacles such as long runs with little rise. Unfortunately, it just isn�t so!

Two Heating Designs Using Fans
Fans are great because primary air (the air entering the fire) can be preheated, which greatly improves combustion (see the very first picture on this article). Forced air helps the coals to burn down completely, leaving only a bit of ash. The rush of low- volume high- velocity jets of hot air do a great job mixing fuel, air, and fire which clean up combustion. A fan can also push air through such a long length of heat exchangers that close to 100% of the heat stays in the room. Doubling heat transfer efficiency can double fuel efficiency. Fans make everything easy.
The fan is pushing air through a one-inch-in-diameter pipe that is in contact with the very hot outer surface of the combustion chamber. Insulation around the combustion chamber and pipe keep both very hot! The air enters the combustion chamber at temperatures of around 800 � F, depending on the heat of the fire. It is amazing to see the effect of a fan on a fire, especially with preheated air. The logs burn very brightly. The fire is easy to light and combustion is more complete. The combustion chamber is usually glowing red hot. (Only combustion chambers made from refractory cement or firebrick can withstand this kind of heat.) Due to the draft created by the fan, the heat is driven through lengths of heat exchangers that would obviously stall a stove dependent on natural draft. It�s possible to add heat exchanger surface area until exit temperatures are equal to room temperature air. Adding a fan to a stove makes it easy to achieve clean combustion and very good heat transfer to the room. Air is pushed through pipes in contact with the fire until the swirling air entering the combustion chamber is very hot. The fan then pushes the hot flue gases through a big enough heat exchanger so that most all of the heat stays in the room.

Why aren�t fans used more often in wood burning stoves?
One reason is that if air is preheated and blown into the combustion chamber temperatures can rise to the point where steel begins to melt. Blowing preheated air into a big fire creates a blast furnace. Also, being dependent on a fan means that stoves may not work correctly when most needed, like during a winter storm when the electricity fails. Some people dislike the whirring of fans, preferring the silence of natural draft. However, when fuel efficiency is the highest priority, the amount of electricity used by the fan is very small when compared to the benefit received. If preheated air is used, the combustion chamber needs to be made from stone or high-temperature ceramic, refractory bricks, or refractory cement.

References and Further Reading

  • Designing Improved Wood Burning Heating Stoves, Dr. Mark Bryden, Dean Still, Damon Ogle, Nordica MacCarty, Aprovecho Research Center,

Related Articles

- How to Build a Winiarski Rocket Stove
- How to Make a Cooking Box (Hay Box / Hay bag)


--HTP Meret 21:53, 27 February 2010 (UTC)

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