"Siphon fill mode. The boiler will run at a low burn for about 15 minutes to fill the condense trap. This happens after the boiler has been turned off, if it hasn't been used for a day or so, and if the control knob has been turned down and back up. This is perfectly normal - it is a safety feature of the boiler. It will last about 15 minutes, the boiler will then go back to normal."
I'm pretty sure that is the sign it is re-setting/ flushing the system or something similar after having had the power switched off, should go back to normal after 15 minutes or so. (I had a panic when ours did that - same make and model)
