For oil systems, the smoke number is also determined as part of the flue gas loss measurement (so-called smoke number measurement), and oil derivatives (harmful components indicated by yellowish discoloration of the filter) are identified. A CO measurement is also required according to the 1st BImSchV (to better assess combustion quality). The limit value is 1,300 mg/kWh. Here too, the chimney draft (differential pressure measurement) must be determined.