My water pressure reducing valve will control pressure under flow and can be
adjusted to various settings. But when flow is shut off, pressure jumps to
about 125 lbs/sq inch almost instantly regardless of pressure when there is
flow. Thus this is not a water heater thermal expansion issue.

Does this mean that the pressure reducing valve needs to be replaced?
Present unit is about 15 years old, Watts 25 AUB.

Thanks.

John