[O2,dry]: Dry oxygen content

[ϑL]: Combustion air: Temperature

[ϑA]: Exhaust gas: Temperature

[qK1]: Boiler: Fixed losses

[qK2]: Boiler: Variable losses

[A]: Constant according to Siegert

[B]: Constant according to Siegert

[qA]: Exhaust gas: Losses

[ηF]: Efficiency: Firing

[ηK]: Efficiency: Boiler

[En]: External release