I have a 30-year-old forced hot water heating system that runs on gas (via a conversion burner, it was originally oil). I have it tuned up in the fall but it always seems to get air in it fairly soon. In late December one zone got air-bound. I had both zones thoroughly purged of air, but within 2-3 days I could hear a little gurgling, and now, 6 weeks later, both zones are noisy when the heat comes on. The noise doesn't bother me that much, but I'm concerned about the inefficiency of the system and also worried about it getting air-bound again. Is there a more permanent way to fix the problem?