Abstract: The invention relates to air preheaters and discloses a preheater in which the air is heated in two stages of heat exchange, the heat source being flue gases. Air to be heated is passed along a path having two portions. Air in the second portion is in heat exchange relation with the flue gases and air in the first portion is in heat exchange relation with air in the second portion. By this means, direct heat exchange between flue gases and cold air is avoided, with the consequent reduction in the risk of condensation of harmful fluids from the flue gases. The respective portions of the air flow path are normally one within the other, enabling the conduit defining the inner portion to be removed without interfering with the flue gas ducting.