Your boiler has detected that the water is heating up far too quickly or there is a blockage preventing hot water from flowing out of the unit.

Maximum delta t

What causes this fault?

This fault occurs when the boiler's internal sensors detect that the temperature is rising too rapidly or is too high compared to what the system can handle. The most common reason is a lack of water circulation, often caused by a failing pump or a blockage in the pipework that prevents heat from being carried away from the boiler.