Over Heating Boiler Troubleshooting

When The Boiler Fails to Meet System Load
Possible cause is that the boiler thermostat is set incorrectly, this could be corrected by simply checking the thermostat
settings.
Another possible cause is poor combustion. And for that checking flue, flame, and assessing combution quality would resolve the issue.

When The Boiler and System Overheat
Possible cause is that system controls are faulty. Checking voltage, temperature settings, and safety switches, would fix this.
It could also be caused by the control thermostat failing and high-limit thermostat activated, or both controls may have failed. In that case check temperatures — if they are higher than the high-limit set point, then replace the high-limit thermostat.