To verify if the air ratio is properly set is possible to carry out measurements of temperature field across the combustion chamber combined with measurement of oxygen, carbon monoxide and hydrocarbons in the flue gas. Uniform temperature distribution by the height at low CO concentration indicates that the air ratio is correct. Any temperature imbalances indicate the opposite situation, e.g. too low temperature in the post-combustion zone together with too high oxygen content in the flue gas indicates that there is too much secondary air that cools down the combustion chamber.