Liquid heating system

A liquid heating system, comprising a high-pressure hydraulic pump for suctioning the liquid from a liquid reservoir and delivering it thereto through a liquid conduit into which said pump is coupled, a variable speed internal combustion engine of a motor vehicle or an operating apparatus provided for driving said pump, a pressure-reducing element comprising a non-adjustable restrictor coupled into said conduit downstream of said pump for reducing the pressure and elevating the temperature of the liquid, said element having a fixed flow passage, a heat exchanger coupled into said conduit downstream of said element, and means for controlling the discharge rate of flow of said pump as a function of the calorific heat required to be generated by the system and/or of the temperature of the liquid, said control means including means for adjusting said pump for maintaining a predetermined discharge rate of flow thereof upstream of said pressure-reducing element even at low idling speeds of said engine, said adjusting means comprising a transmission assembly between said engine and said pump, said transmission assembly being inversely coupled to said engine so as to effect a decreasing transmission ratio as engine speed increases.