You have already admitted 12 years old, no service, no inhibitor, get someone in to service your system and sort it out, it may need more than inhibitor being added.
iron grabs the Oxygen from the water, makes Iron Oxide (= rust) and you're left with hydrogen - which is why you possibly keep having to vent your sealed system. If you put a corrosion inhibitor in your system it will stop the reaction, and you won't have to bleed your rads any more.
£10 to £15 fom any DIY store.
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