Combustion efficiency is a measurement of how well the fuel being burned is being utilized in the combustion process. This is different from the efficiency number produced on the analyzer, which is reflective of the total amount of heat available from the fuel minus the losses from the gasses going up the stack.
Stack loss is a measure of the heat carried away by dry flue gases and the moisture loss. It is a good indicator of thermal efficiency. The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the appliance, and reflects the energy that did not transfer from the fuel to the heat exchanger. The lower the stack temperature, the more effective the heat exchanger design or heat transfer and the higher the fuel-to-air/water/steam efficiency is. The combustion efficiency calculation considers both the stack temperature and the net heat and moisture losses. This would include losses from dry gas plus losses from the moisture and losses from the production of CO.
Combustion converts the carbon and hydrogen in the fuel to CO2 and H2O. For each type of fuel there is a maximum CO2 that can be converted. When you select the fuel in the analyzer, the CO2 is calculated from the fuel type by the percentage of O2 left in the flue gas. Typically for natural gas the ultimate CO2 is 11.7%. This would be achieved when the O2 in the flue gasses was at 0% Some analyzers also allow for the max CO2 to be input by the user if the heat content of the fuel is known.
Again, the ultimate CO2 would be derived during stoichiometric combustion in which there is no excess air and no excess fuel present during the combustion process. In reality, in the HVAC industry we are striving not for stoichiometric combustion, but complete combustion in which all hydrogen and carbon in the fuel are oxidized to H2O and CO2.
For complete combustion to occur, we have to have excess air, or air supplied in excess of what is needed typically because of poor mixing of the fuel and air during the combustion process. If excess air is not provided we will not have the complete conversion of carbon to CO2, and will end up with the formation of partially oxidized compounds, such as carbon monoxide and aldehydes. While ideal operating range for burners is not as efficient as stoichiometric combustion, it does provide us with an additional factor of safety.
Each type of fuel has specific measurable heat content. The maximum amount of heat that can be derived from a fuel is based on using pure oxygen as the oxidizer in the chemical reaction and maximizing the fuel gas mixture. In field practice, the oxygen is derived from the air which is 20.9% oxygen, 78% nitrogen and 1% other gasses. Because the oxygen is not separated from the air prior to combustion, there is a negative effect on the chemical reaction. Air is primarily nitrogen. While nitrogen is inert, and plays no role in the combustion process, it cools the chemical reaction (burning temperature) and lowers the maximum heat content deliverable by the fuel. Therefore, it is impossible to achieve combustion efficiencies above 95% for most fuels, including natural gas, when air is used as the oxidizer in the combustion process.
The combustion efficiency or maximum heat content of the fuel is then based upon the quality of the mixture of fuel and air, and the amount of air supplied to the burner in excess of what is required to produce complete combustion. The efficiency calculated by the combustion analyzer is a modified equation that considers combustion efficiency and stack losses. It is a part thermal, part combustion efficiency calculation. The equation is a reasonable estimation of the steady state thermal efficiency of the appliance. This is true of all analyzers currently manufactured. It should be noted however that high efficiency appliances will have thermal efficiencies and AFUE numbers that very closely match if they are not identical. To achieve a high AFUE, the stand by losses have to be minimal and the maximum heat content of the fuel needs to be obtained including latent heat. So in short, your new appliance will look great on the analyzer and the old maybe not so bad because the standby losses are not considered.
A draft hood equipped appliance with intermittent pilot and a flue damper could be as high as 78% AFUE efficient with a thermal efficiency of 82% Take that same appliance, remove the flue damper and convert to standing pilot and it could drop to the low 70% AFUE range and still have an 82% thermal efficiency.
The ultimate thermal efficiency of the appliance is determined by dividing the heat output rate of the appliance by the rate of fuel input. During the combustion process, all furnaces that operate with the same combustion efficiency will produce the same amount of heat with the same fuel input. The combustion efficiency has no bearing on how well the appliance utilizes the heat produced after the combustion process has taken place. Heat exchanger design and its ability to transfer the sensible and possibly the latent heat to the room air determine how well the heat produced by the combustion process is utilized.
The entire system (furnace/boiler, ducting, and piping) must be evaluated to determine the true efficiency of the system. Combustion efficiency is a valuable part of the system evaluation, but it is only one part of the evaluation process and cannot be used as the sole reason or justification for keeping or replacing existing equipment.