Short answer
Without water flow a boiler overheats within seconds, so a pump fault usually shows as overheat, high ΔT or flow faults. Pumps fail mechanically, electrically, or because the control signal telling them to run is missing.
Common causes
Seized impeller after summer shutdown.
Air lock.
Failed motor or electronics on variable-speed pumps.
No run signal from the boiler, BMS or controller.
Twin-pump changeover failure.
What the engineer checks
Confirm the run signal and supply.
Measure differential pressure across the pump.
Listen and check temperature on the pump body.
Repair and prevention
Free or replace the pump, vent air, check twin-pump changeover and set speed for the design flow.
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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