Sylvan, you are entitled to your opinion. You do your installations the way you learned.
Hot water rises because a given column of cold water is heavier than the same volume of hot water. So the cold water drops to displace the hot water upwards.
A heating system is a continuous circuit of water. The circulator moves water faster than gravity can drive it through small tubes or pipes. The circulator is needed only if the customer wishes to control the flow of water in a timely manner, use tube sizes too small to permit sufficient flow by gravity (usually for the purpose of saving material costs), and use small tube sizes to reduce surface emission losses and water volume heating times.
If the circulator moves water toward the expansion tank, the water pressure will not increase at the outlet of the circulator. The water pressure will decrease at the inlet of the circulator. This is not desirable because lowering a pressure lowers the boiling point. With a circulator making a possible 5 psig pressure drop this is not serious in a properly filled and pressurized system. With a 4 psig pressure at the top of the return loop at the top of the home, a 5 psig drop in pressure would only lower the boiling point slightly below 212F.
However a problem can occur at an air vent at that point as the pressure can drop below atmospheric and permit oxygen in air to enter the system. This could actually increase the number of bubbles passing through the circulator. The danger is also increased if someone installs a high-head circulator, both lowering the boiling point further and pulling air in. A circulator capable of a ten-pound pressure could lower the boiling point to 190F and create a 6-psig negative at the air vent. The drop in boiling point is a cause of cavitation – striking the impeller with steel-like water hitting after the collapse of steam bubbles.
To prevent these problems, all major circulator manufacturers have suggested installing the circulators on the supply side pumping away from the expansion tank as is done on commercial boiler systems. As long as low-head pumps are used properly, the majority of times you can ‘get away’ with placing the circulator on the return. You will notice that residential circulators are now shown placed on the supply side by boiler manufacturers in their literature. It is good to follow manufacturer’s recommendations.
Air is always in water in a system. When fresh water comes into a boiler it is about 5% air by volume. By heating the bubbles and making them larger, they gather at the hottest location and the largest vessel. It is not possible to get rid of the remaining 2% by volume. This air (actually nitrogen after the oxygen has rusted something) will go in and out of solution as the pressure changes as the water goes up and down through the system. In today’s pipe diagrams, either the boiler or a separate air scoop collects the bubbles of air to be vented before they get to the circulator. The expansion tank is usually at that location as well.
You may follow the lead of the “dead men”, but remember they put gravity systems in because there were no dependable circulators to run at a reasonable or available electrical source in their day. They had no experience with circulator systems. It is not prudent to ask advice from the inexperienced or the dead. And it takes a lot of patience to listen to the long story of the experienced “old live men”.