A method of operating a plant having a furnace including at least two vertical shafts connected by an overflow duct, wherein at least one burner is arranged above the overflow duct in each case such that the burner gases therefrom flow downward in burning operation of the respective shaft. A cooling gas supply is provided beneath the overflow duct in each case such that, in combination with the operation of a burner in the burner-operated shaft, the burner gas flowing downward is deflected in the direction of the overflow duct by the cooling gas ascending in the burner-operated shaft, and a supply of cooling gas is adjusted such that the temperature of the burner charge through which the burner gas flows at least in the burner-operated shaft is kept above the deacidification temperature thereof.