Jilly, this doesn't sound like a Ferolli problem (although they are a little prone to faults - it's what happens at the more economical end of the market...).
The "two black knobs" are your "fill-up valves" and are usually part of a "loop" of pipework which control the flow of mains water into your boiler on the occasional times when the boiler system has had a leak, or requires light flushing, and the pressure (as you should see very quickly on the gauge) requires "topping-up".
When the boiler is back in normal operation, these valves should usually be shut (the handles at right angles to the pipe direction) but that may depend on exactly how the system has been configured.
Beware of opening and closing these valves if you are not certain of what you are doing - your boiler's system pressure setting (find in the Maker's Instructions) will be between 1.1 and 1.5 bar approx whilst your mains pressure can vary, according to area, from 1.5 to perhaps 6 or even 8 bar - this kind of pressure your boiler does not want to see !!
At least, not unless a PRV (Pressure Reducing Valve) has been fitted. This is used to regulate the mains water pressure. Set it to 2.5 - 3.5 bar and your boiler and everything else in your house (taps, cisterns, showers etc) will run just that little bit quieter - and last about six times longer !!
Your best bet just now is to get hold of the former owner and get him to show you how the plumbing works before you start spending money in an endless murky pool!
Hope it's something simple
