In this paper, thermal feed-back (TFB) as the way to reach lower and higher temperatures, is analysed. By definition, in TFB application one part of the fluid passing through the system is backed by feed-back connection from outlet to inlet, exchanging the heat flux with incoming fluid flow. The effect of TFB is analysed in two cases: (1) fluid heating and (2) air cooling by adiabatic humidification. In both cases, the TFB moves temperature limits of fluid at exit beyond the limits that are possible in standard installations. The global effect of TFB could be defined as some kind of transition "from quantity to quality".