In 1970, when I started in the heating industry, there was a battle going on between the folks who believed in catching and controlling the air in a hydronic system by holding it inside a plain-steel compression tank, and those who thought it was best to catch the air and just get rid of it using automatic air vents and diaphragm-type compression tanks.
My father and I worked for the Bell & Gossett representative in the New York City metro area at the time, so we had to subscribe to the Air Control school of thought because that’s what B&G preached back then.