Pressure keeps falling with Vokera system boiler

I have a Vokera 'Minute 20SE' boiler. After recently having a Megaflow mains pressure hot water tank fitted and some additional radiators, I have noticed that the pressure in the system needs topping up from the filling loop every 3 weeks or so. Before I had the Megaflow and rads fitted, I only topped up approx 2 or 3 times a year.
As well as the pressure falling, some of the upstairs radiators require bleeding every few weeks too. (I bleed them after topping up the pressure in the system.)
Please can anyone suggest why these things are happening, and what I can do to fix them? I don't want to keep injecting oxygenated water into the system, and the water must be going somewhere!
Is it significant that the plumbers fitted an 'automatic air valve' to the pipework beside the Megaflow? Could this be the cause? (How should the small finger screw be set on these valves, by the way?)
As a precaution, I have arranged for a Vokera engineer to take a look at the boiler next week. Could a safety valve could be leaking enough to cause these problems?
Any ideas very welcome.
David Green
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There could be a small leak on one of the new joints. You may not see it, as the water evaporates immediately on reaching the outside air. Go round with a brillo pad (green scouring pad if plastic rather than copper) and clean every joint well. After a few days/weeks, the leaking joint may show some runs or staining, even if it is dry.
Also, the system may not have had inhibitor inserted when it was refilled after the renovation. This may lead to corrosion, which produces hydrogen that need frequent venting. This can be fixed by flushing the system and refilling with inhibitor.
Christian.
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