The idea of the 28" measurement is that a column of water 28" high equals 1 pound per sq. inch pressure. Let's assume the steam boiler's operating pressure is a maximum of 2 psi. To return the water to the boiler through the Hartford loop, we must maintain a minimum pressure of 2 psi in the return piping. When the boiler is operating at two psi, steam is released into the system, condensing back into water, and encountering flow resistance in the piping. As steam moves through the system, we can drop about 8 oz. We do not have enough pressure to overcome the two psi of steam pressure in the boiler. To clarify, we have approximately 1.5 pounds of pressure pushing down on the return water and two psi at the boiler. Now let's add a 28" column of water as condensate stacks in the vertical return at the end of the steam main. The steam in the steam main is condensing and stacking in the vertical return. We will get enough condensate return at 2.31" per 1 psi to raise the pressure above two psi at the boiler. Since high pressure always goes to low pressure, the return water pushes back into the boiler until the pressure equalizes as the stacked condensate drops.

If you don't have the proper 28" "A" dimension, condensate water will back up into the horizontal steam carrying main and cause water hammer. This is a thunderous knocking sound from the steam system. It sounds like someone hitting the pipes with a large hammer.