A laundry treating appliance comprising: - a cabinet; - a drum, rotatably housed within said cabinet, in which laundry can be loaded; - a heat pump system, using one or more flammable refrigerants, configured for exchanging heat with an operating fluid; - a circulating system configured for circulating said operating fluid through said drum; wherein said heat pump system comprises a heat exchanger (70a, 70b) comprising: - a plurality of metallic pipes wherein said flammable refrigerant flows, wherein the external diameter (de) of said metallic pipes is comprised between 6.5 and 8 mm; - a plurality of fins, stacked, spaced and parallel to one another, each provided with a plurality of through-holes, each through-hole housing one of said metallic pipes, said plurality of through-holes of said fins being distributed on the respective fin to define a plurality of mutually parallel rows of aligned through-holes, wherein the centers (Ce) of the through-holes of each row are aligned to each other along a respective straight line, wherein said metallic pipes comprise two or more straight pipes, parallel one another and perpendicular to said fins, each one of said two or more straight pipes being housed in one of said plurality of through-holes of each stacked fin, said two or more straight pipes being connected in twos, at one end, by a curved pipe, to define as a whole a single duct wherein said flammable refrigerant flows, characterized in that - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 16 and 26 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 24 and 50 mm, or - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 26 and 50 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 18 and 50 mm. - a cabinet; - a drum, rotatably housed within said cabinet, in which laundry can be loaded; - a heat pump system, using one or more flammable refrigerants, configured for exchanging heat with an operating fluid; - a circulating system configured for circulating said operating fluid through said drum; wherein said heat pump system comprises a heat exchanger (70a, 70b) comprising: - a plurality of metallic pipes wherein said flammable refrigerant flows, wherein the external diameter (de) of said metallic pipes is comprised between 6.5 and 8 mm; - a plurality of fins, stacked, spaced and parallel to one another, each provided with a plurality of through-holes, each through-hole housing one of said metallic pipes, said plurality of through-holes of said fins being distributed on the respective fin to define a plurality of mutually parallel rows of aligned through-holes, wherein the centers (Ce) of the through-holes of each row are aligned to each other along a respective straight line, wherein said metallic pipes comprise two or more straight pipes, parallel one another and perpendicular to said fins, each one of said two or more straight pipes being housed in one of said plurality of through-holes of each stacked fin, said two or more straight pipes being connected in twos, at one end, by a curved pipe, to define as a whole a single duct wherein said flammable refrigerant flows, - a plurality of metallic pipes wherein said flammable refrigerant flows, wherein the external diameter (de) of said metallic pipes is comprised between 6.5 and 8 mm; - a plurality of fins, stacked, spaced and parallel to one another, each provided with a plurality of through-holes, each through-hole housing one of said metallic pipes, said plurality of through-holes of said fins being distributed on the respective fin to define a plurality of mutually parallel rows of aligned through-holes, wherein the centers (Ce) of the through-holes of each row are aligned to each other along a respective straight line, wherein said metallic pipes comprise two or more straight pipes, parallel one another and perpendicular to said fins, each one of said two or more straight pipes being housed in one of said plurality of through-holes of each stacked fin, said two or more straight pipes being connected in twos, at one end, by a curved pipe, to define as a whole a single duct wherein said flammable refrigerant flows, characterized in that - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 16 and 26 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 24 and 50 mm, or - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 26 and 50 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 18 and 50 mm. - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 16 and 26 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 24 and 50 mm, or - the distance (Tp) between the centers (Ce) of two adjoining through-holes of a same row is comprised between 26 and 50 mm, and the distance (Rp) between the straight lines of two adjoining rows is comprised between 18 and 50 mm.