Paul, One way to test this would be to shut off the auto fill to the boiler and see if pressure increases in the system. If the pressure increases with the autofill shut off then it is more likely thermal expansion and not a PRV issue. If the pressure stays stable then that may indicate you have debris in the PRV causing it to over pressurize.
Paul, One way to test this would be to shut off the auto fill to the boiler and see if pressure increases in the system. If the pressure increases with the autofill shut off then it is more likely thermal expansion and not a PRV issue. If the pressure stays stable then that may indicate you have debris in the PRV causing it to over pressurize.