Meaning
Siphon fill mode. The boiler runs at a low burn for about 15 minutes to fill the condensate trap. This happens after the boiler has been turned off, if it has not been used for a day or so, or if the control knob has been turned down and back up.
This is a normal safety feature. After about 15 minutes the boiler should return to normal. If it continues for more than half an hour, call a heating engineer or Worcester on 0330 123 9559.
Gas work stays with a Gas Safe registered engineer. These steps are guidance from the manufacturer sequence, not a substitute for the handbook.