Your boiler is struggling to transfer heat to your hot water because a key internal component is likely blocked or damaged.

What causes this fault?

This fault occurs when the boiler's internal sensors detect that heat isn't being transferred effectively to your taps, usually due to a buildup of limescale or heating sludge inside the secondary heat exchanger. The most common reason is 'sludging' from the main heating circuit, which creates a physical blockage that prevents the cold water from heating up properly. In some cases, a failing diverter valve may also be preventing the hot water from flowing into the correct part of the system.