Dirt magnetic separator for a thermal plant, comprising: - a first hollow body defining internally a separating chamber for a fluid circulating in said thermal plant, said first hollow body being bounded laterally by a side wall that extends around a longitudinal axis and, above, by an outlet end wall, transverse to said longitudinal axis, - a filtering cartridge housed in said separating chamber and arranged for keeping and separating ferrous and non-ferrous particles (7a, 7b) from said fluid and causing the decantation thereof downwards; - a second hollow body, defining internally a quieting and collecting chamber bounded below by a bottom wall on which an inner collecting surface is obtained that is suitable for receiving and storing said ferrous and non-ferrous particles (7a, 7b) coming from said filtering cartridge, and a discharge opening for periodical evacuation of said ferrous and non-ferrous particles (7a, 7b), - an inlet connection for the fluid extending transversely to the said longitudinal axis and obtained on said side wall near an upper zone further from said quieting and collecting chamber, such that the advancement and turbulent movement of the main flow of the fluid substantially affect an upper portion of said separating chamber, far from said quieting and collecting chamber, - an outlet connection for the fluid obtained on said outlet end wall and extending in a direction parallel to said longitudinal axis, - said first and second hollow bodies being sealingly coupled together and being shaped in such a manner that said quieting and collecting chamber is arranged below said separating chamber, and said inner collecting surface is at a distance (D x) from said inlet connection that is such as not to be affected by fluid turbulence and dragging actions, - magnet means engaging removably in seat means obtained in said bottom wall for attracting and retaining the ferrous particles conveyed by said fluid, said magnet means being arranged in a position outside and below said quieting and collecting chamber to attract said ferrous particles to said inner collecting surface in a zone further from said inlet connection and far from turbulence and dragging actions of the flow of said fluid, - in said first hollow body a guide bulkhead being obtained that extends, from a zone near said inlet connection, transversely to said longitudinal axis to close above a part of said separating chamber and bound above said filtering cartridge, - said guide bulkhead being spaced away from said outlet end wall to define a gap for evacuating the fluid to said outlet connection, between said guide bulkhead and said side wall a passage opening being defined, diametrically opposite said inlet connection and shaped to enable the fluid to pass from said separating chamber to said evacuating gap. - a first hollow body defining internally a separating chamber for a fluid circulating in said thermal plant, said first hollow body being bounded laterally by a side wall that extends around a longitudinal axis and, above, by an outlet end wall, transverse to said longitudinal axis, - a filtering cartridge housed in said separating chamber and arranged for keeping and separating ferrous and non-ferrous particles (7a, 7b) from said fluid and causing the decantation thereof downwards; - a second hollow body, defining internally a quieting and collecting chamber bounded below by a bottom wall on which an inner collecting surface is obtained that is suitable for receiving and storing said ferrous and non-ferrous particles (7a, 7b) coming from said filtering cartridge, and a discharge opening for periodical evacuation of said ferrous and non-ferrous particles (7a, 7b), - an inlet connection for the fluid extending transversely to the said longitudinal axis and obtained on said side wall near an upper zone further from said quieting and collecting chamber, such that the advancement and turbulent movement of the main flow of the fluid substantially affect an upper portion of said separating chamber, far from said quieting and collecting chamber, - an outlet connection for the fluid obtained on said outlet end wall and extending in a direction parallel to said longitudinal axis, - said first and second hollow bodies being sealingly coupled together and being shaped in such a manner that said quieting and collecting chamber is arranged below said separating chamber, and said inner collecting surface is at a distance (D x) from said inlet connection that is such as not to be affected by fluid turbulence and dragging actions, - magnet means engaging removably in seat means obtained in said bottom wall for attracting and retaining the ferrous particles conveyed by said fluid, said magnet means being arranged in a position outside and below said quieting and collecting chamber to attract said ferrous particles to said inner collecting surface in a zone further from said inlet connection and far from turbulence and dragging actions of the flow of said fluid, - in said first hollow body a guide bulkhead being obtained that extends, from a zone near said inlet connection, transversely to said longitudinal axis to close above a part of said separating chamber and bound above said filtering cartridge, - said guide bulkhead being spaced away from said outlet end wall to define a gap for evacuating the fluid to said outlet connection, between said guide bulkhead and said side wall a passage opening being defined, diametrically opposite said inlet connection and shaped to enable the fluid to pass from said separating chamber to said evacuating gap.