MORE NOTES TO EFFICIENCIES GRAPHS
The unburned gases are due to incomplete combustion which is primarily caused by lack of sufficient air (oxygen) in the right places. The other common cause is poor fuel wood which has not been considered in these graphs. Airtight stoves are the worst offenders in not supplying enough oxygen since they control the fire's intensity and burn time by restricting the air supply. Many introduce "secondary combustion air" to re-burn these gases, an admirable idea. Unfortunately, currently available airtight products have been shown by independent researchers to have ineffective secondary combustion systems.
The proportion of unburned gases to net output can be used as a guideline to determine how much creosote will be produced relative to the heating supplied.
The waste heat up the chimney is an indication to the relative effectiveness of heat exchange or heat removal. Large areas of heat exchanger and/or slow smoke velocities increase the effectiveness and reduce these losses.
Airtight stoves that create a lot of creosote cannot be made with very high heat exchange rates without causing a lot of the creosote to condense in the smokepipe and chimney. As long as they waste a good amount of the smoke's heat, then the smoke stays hot enough so little creosote would condense in the chimney. This is not necessary in products that tend to produce less creosote in the first place-very little exists to condense in the chimney even at very low chimney temperatures.
The house air up the chimney waste is due to having to replace the warm house air that went up the chimney, with cold outside air that must be heated. This obviously depends on outdoor temperature. Under some conditions, this loss is so great that masonry fireplaces can operate at NEGATIVE EFFICIENCIES!
THE NET HEAT OUTPUT TO THE HOUSE IS THE HEAT REMAINING AFTER THESE LOSSES. THIS IS THE EFFICIENCY FIGURE THAT IS THE MOST IMPORTANT. It relates the amount of wood you burn to the benefit received.
The house air loss must be subtracted from this to get the NET EFFICIENCY as shown in the bar chart.