Assuming that the fuel parameters are known, only the oxygen or carbon dioxide concentration, ambient temperature and flue gas temperature have to be determined. Since that does not give a 100% certain reading (see chart sections 8 & 9), it is really necessary to measure CO as well.
Combustion efficiency is a simple (and occasionally misleading) measure of the heating efficiency of a boiler. It is equal to 100 percent minus the percentage of heat lost up the vent (called "flue loss" or "stack loss"). The formal methods for measuring flue loss vary by furnace or boiler type, but, in essence, the combustion efficiency measures the total heat energy that is spread through the heating system or escapes from the boiler jacket itself (jacket loss) instead of going up the stack or flue to warm up the environment. Certain factors may place a limit on the combustion efficiency that can be reached. In particular, the need to keep the gases above the dewpoint for sulfur dioxide may be a major factor. Such considerations must be borne in mind at all times when adjusting a burner system.
Combustion efficiency is based on the flue gas temperature and inlet air temperature. These must be measured in an appropriate fashion. The inlet air at the air inlet, avoiding any wind chill factor that the air movement might produce. This is a relatively simple, but important factor. The flue gas temperature must be measured in the hottest part of the gas stream. Generally this will be in the centre of the flue, but eddy effects may cause it to be slightly displaced. It is vital to have the correct fuel programmed into the flue gas analyser before measuring combustion efficiency. The fuel parameters have a noticeable effect on the value of combustion efficiency, and failure to follow this step will result in erroneous data.
Combustion efficiency measures the so-called "steady-state" efficiency of a boiler, which may or may not be appropriate in specific cases, especially where a burner is used as back-up and only ever operates for short periods of time to cover peak usage. many burners operate on a short cycle which is intrinsically less efficient than continuous use at full power.
As stated above, combustion efficiency may be limited in certain cases and with the condensing boilers it is a definite problem. Here the quantity of condensate produced in a set time must be measured for any calculation of efficiency. The high specific heat of evaporation for water makes this a major factor in any true calculation of efficiency. Nevertheless, it is a useful factor for seeing changes made by adjustments. Generally this data is also required when annual checks are made on equipment.