Abstract: A miniature heater system has electric immersion heaters in a heating enclosure which forms a compact monoblock composite unit resistant to high pressures. In this unit several horizontal tubes are connected at or near their ends by two vertical distributing tubes, one of which is upwardly extended by a circulating pump and the other by a water-mixing turbulator. The unit, with its control, regulating and safety devices is enclosed in a preferably metallic covering.