Heating system of a liquid comprising: - a vessel for receiving the liquid to be heated, said vessel comprising an upper portion, a lower portion and an intermediate portion between said lower and upper portions, with the liquid to be heated being present in said upper, lower and intermediate portions in fluidic communication, - a heat generating source with a compact immersed tube for heating the liquid in said vessel, - a first fluid loop in a closed circuit connected between a first liquid outlet and a first liquid inlet of the vessel and wherein circulates the fluid of the vessel, said first outlet of liquid being arranged to take liquid in the upper portion of the vessel and said first liquid inlet being arranged to bring the liquid in said lower portion or said intermediate portion of the vessel, - a recovery heat source arranged outside the vessel and able to recover heat in the environment of the vessel and to heat the liquid of the vessel circulating in a second fluid loop in a closed circuit connected between a second fluid outlet of the vessel and second and third fluid inlets (84, 85) of the vessel, said second liquid outlet being arranged to take liquid in the lower portion of the vessel and said second and third fluid inlets (84, 85) of the vessel being arranged to bring liquid to first and second levels of elevation in the vessel respectively, said first level of elevation being located in the lower portion or the intermediate portion of the vessel and said second level of elevation being located in the upper portion or the intermediate portion of the vessel, said second level of elevation being located above the first level of elevation, - a first solenoid valve mounted on said second fluid loop in order to place in fluidic communication said second liquid outlet with said second liquid inlet and a second solenoid valve mounted on said second fluid loop in order to place into fluidic communication said second liquid outlet with said third liquid inlet, and - a control circuit for controlling said first and second solenoid valves (54,55) according to the heat recovered by said second heat source characterised in that said compact immersed tube is fully arranged in the upper portion of the vessel. - a vessel for receiving the liquid to be heated, said vessel comprising an upper portion, a lower portion and an intermediate portion between said lower and upper portions, with the liquid to be heated being present in said upper, lower and intermediate portions in fluidic communication, - a heat generating source with a compact immersed tube for heating the liquid in said vessel, - a first fluid loop in a closed circuit connected between a first liquid outlet and a first liquid inlet of the vessel and wherein circulates the fluid of the vessel, said first outlet of liquid being arranged to take liquid in the upper portion of the vessel and said first liquid inlet being arranged to bring the liquid in said lower portion or said intermediate portion of the vessel, - a recovery heat source arranged outside the vessel and able to recover heat in the environment of the vessel and to heat the liquid of the vessel circulating in a second fluid loop in a closed circuit connected between a second fluid outlet of the vessel and second and third fluid inlets (84, 85) of the vessel, said second liquid outlet being arranged to take liquid in the lower portion of the vessel and said second and third fluid inlets (84, 85) of the vessel being arranged to bring liquid to first and second levels of elevation in the vessel respectively, said first level of elevation being located in the lower portion or the intermediate portion of the vessel and said second level of elevation being located in the upper portion or the intermediate portion of the vessel, said second level of elevation being located above the first level of elevation, - a first solenoid valve mounted on said second fluid loop in order to place in fluidic communication said second liquid outlet with said second liquid inlet and a second solenoid valve mounted on said second fluid loop in order to place into fluidic communication said second liquid outlet with said third liquid inlet, and - a control circuit for controlling said first and second solenoid valves (54,55) according to the heat recovered by said second heat source