The Science of Efficient Heating with Wood

Did you know that burning wood in some fireplaces can actually make your house colder? All wood-heating appliances are not alike. Manufactured wood stoves built before 1988, when the EPA began controlling emissions from wood stoves, can be quite inefficient. New and clever ideas for wood heaters may have serious downsides. If you understand the chemistry and physics of wood burning, draft, and radiant heat you will be able to make better decisions when choosing a wood heating appliance. Here is some of the science behind creating an efficient wood heater.

It takes three components to produce fire: combustible material (like wood); oxygen; and sufficient heat to ignite it. Our atmosphere is about 15% oxygen. That doesn’t seem like much but it is enough for us to breathe and to sustain fire. Wood can ignite at temperatures lower than 500 degrees Fahrenheit, if dry enough and exposed for long enough, but the easiest way to provide the ignition requirement is to apply a flame via a match or lighter.

percentages vary with type of wood and variable moisture

Wood is mostly made of cellulose, hemi-cellulose (a collection of complex sugars related to cellulose), lignin (a complex polymer of aromatic alcohols), and a small percentage of extractives (fatty acids, resins, and terpenes). All these complex hydrocarbons are combustible. Wood also contains noncombustible components in the form of water and mineral compounds. The amount of water in the wood affects how well it burns. If your firewood has 20% to 25% water in it, then it should burn just right. A moisture content higher than that results in poor burning. Lower moisture can cause the wood to burn too fast. The minerals in the wood consist of calcium and potassium oxides and carbonates, and trace amounts of other elements. These do not burn and make up most of the ash you shovel out of your wood stove or fireplace.

Different species of trees produce wood with different compositions and, therefore, different burning characteristics. For example, oak contains more heat per pound than pine. Here is a link to a chart listing the characteristics of some common wood varieties.

Less than half of the potential heat available from wood comes from the wood, itself, burning. Most of the heat potential is in combustible gases that the wood gives off while burning. These gases consist mostly of carbon monoxide (CO), Hydrogen, (H2), methane (CH4), and various aromatic hydrocarbons. The combination of these combustible gases is called wood gas or producer gas. Wood gas can be used to run internal combustion engines but that is another story. The problem with inefficient wood burning appliances like open fireplaces and badly designed wood stoves is that the combustible gases go up and out of the chimney before being burned, thereby wasting much of the potential heat available.

The key to burning wood gases before they escape is summarized by the three T’s – time, temperature, and turbulence. The longer you can keep the gases in the firebox the better chance of it burning. Increasing turbulence mixes the air with the combustible gases within the firebox so they can ignite before going up the chimney. These gases need to be exposed to high enough heat in order to ignite. Keeping them inside the firebox longer and burning a hot fire helps.

Some stove manufacturers use a catalytic combustor to lower the burn temperature of the gases. This involves putting a honeycomb-like ceramic element coated with a catalyst just below the flue connection. The gases go through this device and catch fire. I don’t like this method. The catalytic combustor is not something you can make. They are expensive. They wear out. It makes the stove harder to clean. It’s better to just keep the gases in the firebox longer and burn hot fires.

Two ways to keep the gases in the fire box longer are to provide a longer path for the smoke to get to the chimney and to control the flow of air into the firebox. The simplest way to make the smoke path longer is by putting a thick steel shelf between the fire and the flue opening positioned so that the smoke has to go around it to get to the chimney. 

Turbulence is created by providing multiple entry points for air to get into the stove. Primary air is provided via openings low in the firebox, like at the front and back. Secondary air is provided via openings higher in the firebox. The secondary air puts air directly into the upper part of the fire box where the unburned gases have just risen from the burning wood. The air mixes with the gases, creating turbulence that keeps the gases in the firebox a little longer giving them an opportunity to burn before heading up the chimney. A common way to add secondary air to a wood stove is by putting two or three perforated steel tubes across the top of the firebox. The ends of these tubes are connected to air from outside the stove. Air gets sucked into the firebox through these tubes. The video below shows the gases burning just below one of the secondary air tubes in our stove and the great turbulence that is burning the gases.

 

Since burning wood requires oxygen, sufficient air must be supplied to the firebox. This is called “draft.” The simplest way is to create a natural draft with a vertical chimney. You can also create an artificial draft using a powered fan. A principle of physics is that hot air rises. With natural draft the hot flue gases naturally rises up and out. The rising air in the chimney draws fresh air into the firebox via the air inlets.. To get a good draft you should have a minimum of 6 feet of total rise in your chimney but more will be better. Stoves generally draft well when the rise is more like 10 to 12 feet. There is a lot more to efficient chimney design and that will be the subject of a future article.

Combustion air flow can be controlled using “dampers” at the air inlet or in the chimney. The best way is to control the air at the inlet. If you do that, you don’t need to put a damper in the chimney. It’s easier to clean the chimney without one. Using the dampers you can slow the flow of air which will keep the gases in the chimney longer.

It is a good idea to draw the air used for combustion from outside your building envelope. If you don’t provide for an exterior air source for your stove all the air that feeds the fire has to come from inside your house which will suck cold air from every possible leak in your building envelope – under doors, around windows, through recessed lighting cans in the ceiling, and even through electrical outlets. Open fireplaces with large chimney diameters can draw so much air from your house that the cold air replacing it can make your house colder even when the fire is going.

Wood stoves heat your home by radiant heat transfer. That is to say that the fire makes the stove hot and that heat radiates into the house. You get more even heat with increased mass. The fire heats the mass of the stove and the mass radiates the heat into your house. With sufficient mass, you can still be getting heat for hours after the fire has gone out.

Modern wood stoves are made out of either cast iron or 3/16” to ¼” thick steel plate lined with fire brick. The thick steel provides mass and is a good conductor of heat. The firebrick is a dense, fired ceramic that also holds heat. Firebrick is the standard way of doing it but there are other possibilities. Masonry, stone, rammed earth, cob, etc. can all be used as mass to absorb and re-radiate heat into your home. The important thing is that the material is dense and non-combustible.

our pile of split walnut drying in the sun

Wood is a dirty fuel, as compared to propane or natural gas. The non-combustible portion of the wood (minerals, dirt, etc.), as well as unburned hydrocarbons, produces ash that will fill your firebox and coat the inside of your chimney over time. It should be easy to clean every part of the stove and chimney. This is very important. Complex chimney paths, obstructions, or tight bends within the stove will make it very difficult to keep clean. A dirty stove will provide less heat and, once the systems starts to clog, will get worse quickly.

Burning wood for heat has been done for millennia. A lot has been learned about efficiency since those first campfires.  Now that you know the basics you can get the most heat out of the wood you burn. To learn more about burning wood the EPA has a great PDF online with more details than I have presented here.

 The next article about wood burning will cover how to install a wood stove and chimney safely.