A combustion cell for a heating system, the combustion cell comprising: - a fan unit configured to supply a flow of premixed air-gas required for the combustion; - a combustion chamber extending along a longitudinal axis from a first end to a second end; - a lid, for closing the first end of the combustion chamber; - a burner, having a convex profile and including a central region and a peripheral region, wherein the central region is perforated whilst the peripheral region is free of holes; - a distributor, having a convex profile and including a peripheral region provided with apertures, the distributor being located upstream of the burner to feed the premixed air-gas flow to the burner and being arranged so that the peripheral region of the distributor faces the peripheral region of the burner; - a flange, configured to connect the lid to the burner; - a heat exchanger coil, arranged around the longitudinal axis to surround the combustion chamber and provided with a plurality of spirals including a first spiral proximal to the burner and a last spiral distal to the burner, wherein the flange has a receiving surface, which receives the first spiral of the coil in direct contact with it, wherein a minimum absolute distance between the coil and the burner varies with respect to the angular position around the longitudinal axis, between a minimum value and a maximum value, wherein a minimum axial distance between the coil and the burner along the longitudinal axis varies between a minimum value and a maximum value, and wherein at least one of the following conditions is met: i) the minimum value of the minimum absolute distance is less than 15 mm; ii) the maximum value of the minimum absolute distance is less than 25 mm; iii) the minimum value of the minimum axial distance is less than 20 mm; iv) the maximum value of the minimum axial distance is less than 30 mm. - a fan unit configured to supply a flow of premixed air-gas required for the combustion; - a combustion chamber extending along a longitudinal axis from a first end to a second end; - a lid, for closing the first end of the combustion chamber; - a burner, having a convex profile and including a central region and a peripheral region, wherein the central region is perforated whilst the peripheral region is free of holes; - a distributor, having a convex profile and including a peripheral region provided with apertures, the distributor being located upstream of the burner to feed the premixed air-gas flow to the burner and being arranged so that the peripheral region of the distributor faces the peripheral region of the burner; - a flange, configured to connect the lid to the burner; - a heat exchanger coil, arranged around the longitudinal axis to surround the combustion chamber and provided with a plurality of spirals including a first spiral proximal to the burner and a last spiral distal to the burner, wherein the flange has a receiving surface, which receives the first spiral of the coil in direct contact with it, wherein a minimum absolute distance between the coil and the burner varies with respect to the angular position around the longitudinal axis, between a minimum value and a maximum value, wherein a minimum axial distance between the coil and the burner along the longitudinal axis varies between a minimum value and a maximum value, and wherein at least one of the following conditions is met: i) the minimum value of the minimum absolute distance is less than 15 mm; ii) the maximum value of the minimum absolute distance is less than 25 mm; iii) the minimum value of the minimum axial distance is less than 20 mm; iv) the maximum value of the minimum axial distance is less than 30 mm. i) the minimum value of the minimum absolute distance is less than 15 mm; ii) the maximum value of the minimum absolute distance is less than 25 mm; iii) the minimum value of the minimum axial distance is less than 20 mm; iv) the maximum value of the minimum axial distance is less than 30 mm.