Volkera Linea 24 - Starts up intermittently then stops?

Hi,

I have a Linea 24 boiler and it's behaving strangely. It's set to run for an hour or so for heating in the morning and evening, with HW on demand.

Every hour or two (that I can tell), it sounds like the fan is starting up like it's about to fire into life. There's no spark ignition or anything like that, just a fan type noise for 10 seconds or so and this is outside of the programmed periods of operation and no HW taps have been turned on.

Does anyone have an idea what it could be, it works fine apart from the HW flow is a bit poor.

Many thanks in advance.