When installing a replacement steam boiler with a gravity return there is one dimension that is normally overlooked which is called the “A” dimension. The “A” dimension is the distance from the boiler manufacturer's normal water level (NWL) to the lowest steam carrying main. The distance can exceed the 28” minimum vertical distance but should not be less than then 28".

"A" Dimension Wet Return

"A" Dimension Dry Return

Returning Water to Boiler

The idea of the 28" measurement is a column of water 28” high equals 1 pound per sq. in. pressure. Let's assume the steam boilers operating pressure is a maximum of 2 psi. To get the return water back into the boiler through the Hartford loop, we must exceed 2 psi minimum in the return piping. When the boiler is operating at 2 psi, the steam is going out to the system condensing back to 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 2 PSi of steam pressure in the boiler. To clarify, we have about 1.5 pounds pushing down on the return water and 2 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 2 psi pressure 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.