Conduct a visual inspection of the intake to the heater, the warm air ducting and the air outlets. A restriction to the airflow can make the heater overheat.
Has the ducting been crushed, the air intake to the heater inhibited, or have any or all of the outlets been closed off.
Check that the airflow into and away from the heater through the ducting system is not restricted.
For water heating systems, check that there is adequate water flow around the piped system. Ensure that regulating valves are open and functioning correctly. Ensure that no air is trapped within the heater emitters, i.e. the radiators, fan matrices or fin radiators. Bleed the air from the system if necessary.
Important: when topping up the header tank, us a 50/50 mixture of antifreeze to water solution.
DO NOT TOP UP WITH NEAT TAP WATER.
Check the heater exhaust pipe and combustion air tube for any damage or possible restriction. Water ingress in either tube or any mechanical damage can influence the operation of the heater. Please check this out.