Plumbing Questions? Ask a Plumber and Get Answers ASAP.
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It sounds like you have 2 problems. First your plumber seems to be relying on the air eliminators/vents on the system to get rid of the air in the system. This is not how it's done. The system should be actively purged of air. The air vents are only there to get rid of the small amount of residual air in the system. Also there should be a feed valve (see attached picture for typical) on the water line that supplies the system. This device regulates the water pressure let into the system and will automatically add water to the system should the pressure drop too low. Do you have a feed valve on the system? To be perfectly candid it sounds like you need a new plumber.