A bypass system comprising: a source of a fluid; a bypass valve mounted on a line to selectively bypass the fluid prior to reaching a downstream component, said bypass valve including a valve poppet a valve seat, a piston, a valve housing and a wax portion, said piston is adapted to move along an axis allowing movement of said valve poppet toward and away from said valve seat, said piston is adaped to move within said valve housing, said wax portion, in operation, expands as a temperature increases, and causes said valve poppet to move against said valve seat, said wax portion, in operation, cools if the temperature lowers such that said valve poppet moves away from said valve seat; and there being a sealing surface on said valve poppet in sealing contact with said valve seat and said sealing surface being part-circular in a plane in which said axis lies; wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet, and wherein said recess is formed in a frusto-conical surface of said valve poppet; the system being characterised in that: said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet. a source of a fluid; a bypass valve mounted on a line to selectively bypass the fluid prior to reaching a downstream component, said bypass valve including a valve poppet a valve seat, a piston, a valve housing and a wax portion, said piston is adapted to move along an axis allowing movement of said valve poppet toward and away from said valve seat, said piston is adaped to move within said valve housing, said wax portion, in operation, expands as a temperature increases, and causes said valve poppet to move against said valve seat, said wax portion, in operation, cools if the temperature lowers such that said valve poppet moves away from said valve seat; and there being a sealing surface on said valve poppet in sealing contact with said valve seat and said sealing surface being part-circular in a plane in which said axis lies; wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet, and wherein said recess is formed in a frusto-conical surface of said valve poppet; the system being characterised in that: said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet. wherein said valve poppet is part-spherical about said axis, and wherein said valve seat has a sharp break point selectively in contact with said valve poppet to provide said seal, or wherein said bypass system further comprises a cylindrical seal and wherein the cylindrical seal is received in a recess of said valve poppet such that said seal provides said part-circular portion of said valve poppet, and wherein said recess is formed in a frusto-conical surface of said valve poppet; the system being characterised in that: said valve poppet is threadably received on threads on said valve housing to facilitate replacement of said valve poppet.