Hi Ivan, is this the info you were looking for? Usually O
2 and CO concentration along with ambient and flue gas temperatures are measured. The other values including CO
2 concentration and efficiency are then calculated. Most combustion analysers allow selection of the fuel type to account for the stoichiometry so all the fuel related factors are lumped together in the software. As has been stated by others it's location, location, location of the probe when measuring. It will only give the efficiency of the burner and as has been said previously any ingress of air post combustion will give incorrect readings. You could use the CO and O
2 readings at various points and do your own calculations. Depends on what you are trying to achieve

Handheld analysers tend to use electrochemical cells which usually give reliable readings but do need replacing every few years and can be poisoned in weird and whacky applications. There can be interferences but again this doesn't happen in normal eg gas/heating oil boilers and the analysers can correct for it.