Your boiler is temporarily reducing its power while heating your hot water cylinder because it is reaching its temperature limit too quickly.

Limitation of cylinder charging in output in kW

What causes this fault?

This occurs when the boiler is generating heat faster than your hot water cylinder can absorb it, causing the water inside the boiler to get too hot too quickly. To prevent overheating, the boiler automatically throttles its power back to a lower level to allow the heat transfer to catch up. The most common reasons are a buildup of limescale or sludge inside the heating coil, or a pump that isn't moving water fast enough through the system.