An insert tube is used for the repair or improvement of a pre-heating tube for air arranged in an exhaust gas pathway of a fuel burner. A pre-determined radial gap exists between the outer surface of the insert tube and the inner surface of the pre-heating tube for air such that the insert tube can be slid into the pre-heating tube for air. The insert tube is provided in the vicinity of each end with at least one groove that runs around its circumference. In the groove there is arranged at least one sealing ring of an elastic material. The sealing ring mounted in the groove has an external diameter that exceeds the internal diameter of the preheating tube for air. The sealing ring forms a seal against the inner surface of the pre-heating tube for air and prevents leakage through the sealing ring.