An apparatus for heating vessels, the vessels having enclosed spaces therein and controlling air ingress into the enclosed spaces through gaps. The method includes providing a lid structure for the vessel having the enclosed space, the lid structure having a burner assembly mounted therein. The burner is configured to provide a predetermined flame diameter. The vessel and lid structure are mated such that the gap is formed between the vessel and the lid structure. Fuel and oxidant are discharged from the burner assembly under conditions to provide the predetermined flame diameter and impart a flame velocity sufficiently large to create an outward gas flow from the enclosed space through the gap and control air ingress.