A Franco Crosti boiler is a conventional locomotive boiler with an additional second boiler. The latter heats boiler water with the assistance of gasses passing around it, which have been emitted by the locomotive's smoke. This second boiler is therefore called an exhaust gas pre-heater. The Italian designers Franco and Crosti built their first test locomotives with this technology as early as the 1930s. The savings in coal were approximately 20% with the improved efficiency. The German Federal Railroad took advantage of this idea for this reason and contracted Henschel to build two class 42.90 locomotives. The exhaust gas pre-heater is situated below the regular boiler in these locomotives. The flat smokestacks for operation project from both sides of the boiler, which results in a very striking appearance. The standard smokestack is no longer used to discharge the exhaust gases during operation; it is only required for firing up the locomotive. Despite the increased efficiency, the operating costs were rather high, pre-heater boilers were rather subject to corrosion.