Short answer
A heat exchanger can block on the water side (sludge, magnetite, limescale) or the flue-gas side (combustion deposits). Water-side blockage causes high temperature differentials, kettling and overheat lockouts. Flue-side blockage causes poor combustion, high CO and fan or air pressure faults.
Common causes
Magnetite and sludge from untreated or poorly maintained systems.
Limescale from hard water and frequent fresh water top-ups.
Combustion deposits from poor gas/air ratio or lack of servicing.
Debris from new or modified pipework not flushed.
What the engineer checks
Measure differential temperature and pressure across the heat exchanger.
Inspect the combustion side during the service.
Test system water for inhibitor level, pH and debris.
Repair and prevention
Clean the combustion side to the manufacturer's procedure; chemically clean or replace the heat exchanger if water-side blockage is severe. Flush and treat the system, fit a dirt separator and monitor water quality.
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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