92020-92029 were built with Franco-Crosti boilers. An additional boiler ran under the main boiler, taking the firebox gases back from the smokebox to a chimney on the right side of the locomotive forward of the cab. The standard chimney at the front was only used during lighting up. The additional boiler acted as a pre-heater, using low-grade heat that would otherwise be wasted.
However, the experiment did not deliver the hoped-for benefits. Efficiency was not increased sufficiently to justify the cost and complexity. The locomotives were unpleasant on the footplate in a cross-wind and the low temperature of the gases in the secondary smokebox caused serious corrosion due to the formation of sulphuric acid. The engines were rebuilt without the Franco-Crosti boiler, although they retained some of their distinctive looks.
