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I'm a little confused ...... an open vented system won't absorb nor release air, nor will the water within the actual heating circuit evaporate. There will be a little at the header tank as they tend to be open topped but that's replenished by the valve. The actual system water, if there are no leaks (less likely long term due to it's low pressure), will require very little new water.
The water absorbs air in an open vent system and releases the air when the water heats, unfortunately often into a towel rail or a radiator on upper circuits in many systems, and repeats the cycle. Sealed system is possible to not have that problem. Got to be careful of the problem of auto vents leaking though and I avoid using them if possible.
The f&e tank will allow some evaporation, especially if it has no lid on it. But it admittedly would take many months to be of any significant amount. Still a water loss though that would be impossible in a sealed system.
I have some sealed systems, (installed with all compression joints fully sound and valves high quality) that hold water pressure without requiring top up in several years, which I think is unbeatable compared with open vent.