Method of manufacturing a set of heat exchange cells (1a-1d) having a thermal power falling within a predetermined range of minimum and maximum values, each heat exchange cell (1a-1d) comprising a heat exchanger mounted in a respective containment casing, wherein the method comprises the steps of: a) providing a single containment casing of a plurality of heat exchange cells (1a-1d) of the set, said casing having a constant axial extension as the thermal power of the cell (1a-1d) varies within said range of thermal power values and equal to the axial extension of the cell having the minimum thermal power within said range of thermal power values; b) providing a plurality of helically-shaped heat exchangers each having a thermal power falling within said range of minimum and maximum values and each comprising at least one tubular duct for the flow of a first heat transfer fluid coiled about a longitudinal axis (X-X) of the helix according to a plurality of coils; c) mounting within said single containment casing a helically-shaped heat exchanger having a preselected thermal power of said plurality of heat exchangers of the set; wherein said plurality of heat exchangers of the set has an axial extension and an internal diameter that are substantially constant as the thermal power of the heat exchangers varies within said range of thermal power values of the set so as to define, coaxially and internally with respect to the heat exchanger, a feeding zone of a second heat transfer fluid having substantially constant dimensions both along the axial direction and along the radial direction as the thermal power varies; and wherein said at least one tubular duct of said plurality of helically-shaped heat exchangers has a radial extension of the coils which, upon variation of the thermal power of the heat exchanger, varies in a corresponding manner so as to maintain substantially constant the axial extension and internal diameter of said plurality of helically-shaped heat exchangers as their thermal power varies within said range of thermal power values of the set and equal to the axial extension of the heat exchanger having the minimum thermal power within said range of thermal power values of the set. a) providing a single containment casing of a plurality of heat exchange cells (1a-1d) of the set, said casing having a constant axial extension as the thermal power of the cell (1a-1d) varies within said range of thermal power values and equal to the axial extension of the cell having the minimum thermal power within said range of thermal power values; b) providing a plurality of helically-shaped heat exchangers each having a thermal power falling within said range of minimum and maximum values and each comprising at least one tubular duct for the flow of a first heat transfer fluid coiled about a longitudinal axis (X-X) of the helix according to a plurality of coils; c) mounting within said single containment casing a helically-shaped heat exchanger having a preselected thermal power of said plurality of heat exchangers of the set; wherein said plurality of heat exchangers of the set has an axial extension and an internal diameter that are substantially constant as the thermal power of the heat exchangers varies within said range of thermal power values of the set so as to define, coaxially and internally with respect to the heat exchanger, a feeding zone of a second heat transfer fluid having substantially constant dimensions both along the axial direction and along the radial direction as the thermal power varies; and