B

May be caused by strong winds or flue gas recirculation. The burner starts automatically after purging.
▶ Check the flue integrity.
▶ Check the flue terminal position.
▶ Check the CO2 settings and adjust if necessary.
▶ Check the burner and the heat exchanger gaskets and the inner flue gaskets.
▶ Check the gas pressure during burning at full load (working pressure, dropping low).