Short answer
The high-limit thermostat is an independent safety device that shuts the boiler down if water temperature exceeds a safe value. A trip almost always means the boiler genuinely overheated; occasionally the thermostat itself has failed.
Common causes
Loss of water flow (pump, valves, air).
Faulty control sensor allowing overheating.
Scale or sludge in the heat exchanger.
Failed high-limit device.
What the engineer checks
Investigate why the temperature rose before resetting.
Check continuity of the device when cold.
Repair and prevention
Fix the cause of overheating, then reset. Replace a failed thermostat with the correct rated part. Bypassing a high limit is dangerous and illegal practice.
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:
Need a commercial gas engineer in London?
Gas Safe registered commercial engineers, 24 hours a day. Call now or book online and we'll confirm a time.