In the classical use, air preheaters are connected behind high-pressure steam boilers or thermal oil boilers with a flue gas temperature up to 400 ° C. Their exhaust temperatures lend themselves to preheat the combustion air up to 300 ° C. In this way, an air preheater save up to 10% of fuel costs and thus enables payback periods of less than two years. Meanwhile, air preheater also be used in low temperature ranges in order to make the most efficient use of the fuels and counter the rising fuel prices.