The first step to understanding clean burning is to define it. At Dectra Corporation, we take an engieering approach to the definition:

Clean Burning is the act of turning the vast majority of any fuel into energy. This can be measured in "Combustion Efficiency".

Combustion Efficiency is the percentage of potential heat that is actually released during the combustion process.
For wood, the amount of potential energy stored is 8600 Btu's per pound. The actual weight of wood by volume varies from species to species and by moisture content (higher moisture content equates to less wood per pound because of the weight of the water). 100% Combustion Efficiency would be 8600 Btu's per pound of wood adjusted for moisture content.

Lower Combustion Efficiency leaves behind unburned fuel - smoke, ash, and creosote.

If you have ever seen a wood-burning device that smoked a lot, left ashes that you had to clean out, or caused creosote to build up in the chimney, this is primarily due to a low combustion efficiency.

The GARN® WHS units utilize a two-stage combustion process that leads to a 97% Combustion Efficiency.
Combustion of any sort requires three things: Fuel; Heat; and Oxygen.

Most wood burning appliances fail to create high combustion efficiency for two reasons: not enough heat to burn all of the fuel and cutting the air supply to try to control the rate of combustion. The last is intuitive for anyone who has ever seen the air cut off from a fire - the result is a lot of smoke and reduced heat.

GARN® WHS units use an induced draft fan to ensure the proper amount of oxygen and a two stage combustion process. In the primary combustion chamber the wood is burned quickly and efficiently (this is known as a "batch burn"). The ash and smoke from that lower heat combustion is driven into the secondary combustion chamber where it is "reburned" at temperatures approaching 2000 degrees. The final temperature needed to combust almost all the material in wood.

Why is Combustion Efficiency important?

First, it reduces the emmisions of the unit (please see the section on Emissions and Efficiency). Second, it reduces the amount of fuel needed to provide the heat required.

For example:
If you need 86,000 Btu's per hour of heat for your house, that amount of energy is naturally stored in 10 lbs of wood. If your combustion efficiency is 100%, then you gain all of that heat. If your combustion efficiency is only 50% (and most units that "dampen" the fire are lower than that) you have to use 20 lbs of wood to get the same amount of heat. In addition to the extra cost and work to use twice as much fuel, you also end up with 10 lbs of unburned fuel each hour in the form of smoke, creosote, and ash. That means disposing of the ash, breathing the smoke, and cleaning your chimney and firebox quite often.

GARN® WHS units with a 97% combustion efficiency produce very little smoke, very little ash, and no creosote buildup.