A flow-control switch device (10) for gas-fired wall boilers. The flow-control device (10) comprises a casing (11) of plastic material, defining a longitudinal duct (16) having an inner surface which extends between an inlet threaded pipe fitting (12) and an outlet threaded pipe fitting (13), and a magnetic float (17) for operating a reed-type switch (26). The float (17) consists of a cylindrical body of magnetic material, having an axial hole (16) for the fluid, a fore cylindrical portion (17A) and a rear cone-shaped portion (17B) facing towards the inlet threaded pipe fitting (12); the rear cone-shaped portion (17B),and the inner surface of the longitudinal duct (16), define an annular thrust chamber (21) to entrain the float (17) by the fluid. Side ribs (25) enable the reed switch (26) to be removably positioned on side faces of the casing (11) of the flow-control device (10).