Short answer
Condensing boilers produce large volumes of acidic condensate — a large commercial boiler can produce many litres an hour. If the trap, pipe or pump blocks or freezes, condensate backs up into the combustion chamber, triggering a condensate sensor, flame failure or fan fault.
Common causes
Blocked trap from debris or sludge.
Frozen external condensate pipe.
Failed condensate pump.
Pipe without adequate fall or too small a diameter.
What the engineer checks
Inspect and clean the trap.
Check pipe fall, diameter and external routing.
Test the condensate pump and its high-level alarm.
Repair and prevention
Clean traps at every service, insulate or re-route external pipes, fit neutralisers where required and size pumps for the plant output.
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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