Without air-conditioning the inside temperature of passenger coaches would in many cases exceed outside temperature due to the following heat sources: human beings, lights and other electrical equipment, sunshine through the windows.

For an outside temperature of 12° C and a coach with 80 persons (~ 7 kW) and sun shining through the windows (~ 2 kW), 20° C inside temperature are reached without any additional heating.

If these heat sources cause overheating (> 20° C) of coach and outside temperature is still < 20° C, the coach may be cooled by outside air. For this purpose the air intake may even be increased to values beyond the necessary intake for ventilation. Such an excessive operation of ventilation may save energy air-conditioning. For comfort reasons there is however an upper limit for interior air flow and thus for maximum ventilation (UIC 553, EN 13129).

Excessive ventilation is usually an option for outside temperatures between 10 and 22° C depending on sunshine and occupancy.