Short answer
A commercial boiler overheats when heat is being put into the water faster than the system takes it away. The control normally limits this, so a high-temperature trip almost always means water is not flowing properly through the heat exchanger, or a control or sensor is giving false readings.
Common causes
Failed, slow or air-locked primary pump.
Closed valves, blocked strainers or dirt-fouled heat exchanger restricting flow.
Low system pressure or air in the system.
Faulty flow sensor reading low, so the boiler keeps firing.
Low-loss header or hydraulic separation problems causing short-circuiting.
What the engineer checks
Measure flow and return temperatures and the temperature difference across the boiler.
Check pump operation, valve positions, strainers and system pressure.
Compare sensor readings with a contact thermometer.
Repair and prevention
Restore flow (pump, valves, strainers), clean or flush the system, replace faulty sensors and reset the high limit only once the cause is fixed. Fit dirt and magnetite separation to prevent recurrence.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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