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No best answer has yet been selected by wellright. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.wellright, If the problem is only recently starting to show on a 20 year old system, I would assume that you can discount a "design problem ". The most probable cause is that hydrogen is being created in the system by corrosion. To overcome this, you would need to add a type of sludge remover to the system ( available from plumbers merchants, large diy stores, etc. ), ensuring that you adhere strictly to the manufacturers instructions. If the corrosion / sludge is of a major proportion, your system may even be in need of a " powerflush ".
Another possibility is that you have a small leak/leaks on the system. This may seem strange, but as a minute amount of water is displaced from the system, air finds its way in !
As pointed out earlier, I would discount a design problem ( or possible pumping - over ), as your problem is only begining to show after 20years. Unless you are very competent re plumbing, I would suggest calling in a heating engineer.