Harold Kestenholz
A hot water loop off the bottom of the boiler was always meant to serve a baseboard or radiator loop below the water level of the boiler. When there is an attempt to heat water above the boiler water level, there are usually problems of the kind you describe.
As the water is at or near steam temperature (212F) the pump is subject to water temperatures often beyound its design. Steam pockets develop as the water drops back down from the upper floor, many times creating banging. Any air entrained in the steam settles out at the high point in the loop as the water cools causing pumping rate problems.
A better solution is to interpose a heat exchanger between the boiler water and the baseboard loop (with a small expansion tank.) There are now relatively inexpensive flat plate exchangers available from boiler manufacturers that can solve most of the problems encountered when using the boiler water above the boiler water level. This makes the baseboard loop a sealed hot water system usable at any height above the boiler with milder baseboard temperatures.
