the internal bi-metal disc may have fatigued

As a veteran-grade fan pulley

you must use an ultraviolet-specific amplifier from the 7800 series

or failed inducer motor

Grundfos 97523134 Pump 318423-701 the internal bi-metal disc mayThe Grundfos 97523134, also identified as the UPS50 60F, is a high performance three speed circulator pump engineered for demanding hydronic heating, cooling, and district heating systems. This wet rotor circulator features a cast iron pump housing and a canned rotor design, where the motor and pump form an integral unit without a mechanical shaft seal. By utilizing the pumped liquid for bearing lubrication, this unit ensures quiet, maintenance free