BoilerFaultCodesTechnical description
Heat exchanger blocked or pump failure (Flow/Return Delta T too high).
What causes this fault?
This fault happens because water is not moving through the boiler fast enough to carry heat away, causing the internal temperature to spike instantly. The most common reasons are a broken internal pump that can't push the water or a buildup of sludge and scale that has physically blocked the narrow channels inside the heat exchanger.
This issue often comes to light during the first cold spell of autumn when the heating is turned back on after being idle all summer, which can cause internal pump components to seize.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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