Abstract: System for regulating temperature of hot water in a wall-hung instantaneous mixed gas heating unit, in which the value of the temperature detected by a sensor inserted on the output of the secondary circuit of the hot water exchanger is compared with a set value, and their difference or error value is sent to the input of an amplifier of proportional-integrational-derivative type (P.I.D.), whose output is connected to one input of a selector of minimum value or power, to the other input of which is sent the difference or error value between the temperature of the heating system water and the corresponding maximum tolerable set temperature. The output of the selector then commands the gas modulating valves through a power amplifier, as well as the gas on/off operating valve through comparison with a signal proportional to the minimum thermal power that can be supplied by the heating unit.