Heating system for heating a liquid, in particular a liquid as water, using the heat generated in a natural way by the decomposing material in a compost heap, comprising: a plurality of hollow bodies (2;3;4), which are arranged inside the compost heap in direct contact with the decomposing material thereof and are crossed several times by the liquid to be heated; a supporting body, which is arranged inside the compost heap in direct contact with the decomposing material thereof, the hollow bodies (2;3;4) being supported by means of said supporting body; and a tank, the only openings of it being the openings for letting water in it and the openings for letting water out of it; characterised in that it further comprises: a feeding pipe, which is jointed with one of its ends to the tank and with its other end to a first closing/opening valve, to which is being fed the liquid to be heated coming from a source that is extraneous to the heating system; an outlet pipe, which is jointed with one of its ends to the tank and with its other end to a second closing/opening valve, from which comes out the liquid that was heated in the heating system; pipes (12,13,14;15,16), by means of which the hollow bodies (2;3;4) are jointed to the tank, the hollow bodies (2;3;4) forming together with the tank and with the pipes (12,13,14;15,16) at least one heating circuit within which the liquid circulates during its heating. a plurality of hollow bodies (2;3;4), which are arranged inside the compost heap in direct contact with the decomposing material thereof and are crossed several times by the liquid to be heated; a supporting body, which is arranged inside the compost heap in direct contact with the decomposing material thereof, the hollow bodies (2;3;4) being supported by means of said supporting body; and a tank, the only openings of it being the openings for letting water in it and the openings for letting water out of it; characterised in that it further comprises: a feeding pipe, which is jointed with one of its ends to the tank and with its other end to a first closing/opening valve, to which is being fed the liquid to be heated coming from a source that is extraneous to the heating system; an outlet pipe, which is jointed with one of its ends to the tank and with its other end to a second closing/opening valve, from which comes out the liquid that was heated in the heating system; pipes (12,13,14;15,16), by means of which the hollow bodies (2;3;4) are jointed to the tank, the hollow bodies (2;3;4) forming together with the tank and with the pipes (12,13,14;15,16) at least one heating circuit within which the liquid circulates during its heating. a feeding pipe, which is jointed with one of its ends to the tank and with its other end to a first closing/opening valve, to which is being fed the liquid to be heated coming from a source that is extraneous to the heating system; an outlet pipe, which is jointed with one of its ends to the tank and with its other end to a second closing/opening valve, from which comes out the liquid that was heated in the heating system; pipes (12,13,14;15,16), by means of which the hollow bodies (2;3;4) are jointed to the tank, the hollow bodies (2;3;4) forming together with the tank and with the pipes (12,13,14;15,16) at least one heating circuit within which the liquid circulates during its heating.