Our heaters are natural draft with low NOx burners (15 % air excess ). Our heaters O2 analysers are on the top of radiante bank. We do not analysis CO/CO2 in the flue gás. Is there any “rule of thumb” for that ? What is the real O2 in our analysers considering all natural leakeage in the heaters, like sight view opened, etc ? Of course it depends on the heater maintenance, but in terms of regular state.