As many know, older "packaged" boilers shipped with the circulators on the return pumping into the boiler. Various reasons for this location, easy of packing and crating, cooler temperature for pump seals among them. Low head 3 piece circe developed low head so no issues with excessive ∆P.
So we then embraced the pumping away concept, locate the complete package air separator, expansion tank and circulator on the supply. Mainly we do the best air removal at the hottest point in the system. Really no advantage to the circulator and expansion tank at that location.
Fast forward to mod con technology with high pressure drop heat exchangers. The high pressure drop, along with pressure lock out switches mandates the circulator be located pumping into the boiler. The ∆P developed by the circulator adds to fill and assures the boiler performs properly. Combis and some boilers come equipped with boilers mounted inside, all are pumping into the boiler.
This seems to confuse many installers, where is the correct location for all the components, can the be separated, etc.
Obviously reading the manual clears up what the manufacturer requires for boiler circulator.
So why not pump all boilers with the circulator and expo tank on the return? Low pressure drop style, cast boilers for example would work fine, many mod cons insist on return piping, no reason fire tube design could not pipe the same.
With a low pressure drop type boiler even less potential for excessive pressure causing the relief to seep or pop.
Most of the boilers I spotted at AHR that were imports look to be former tankless HX design, small passage tube style, so they want "pumping return" and either a P/S, or hydraulic separator.
Advantages include both circulator and expansion tank are in the coolest fluid, generally anywhere from 10- 40° cooler fluid temperature. When multiple circulators are used the hydro sep or low loss header becomes the PONPC for all the various circs.
Standardization would certainly help the industry. Some examples:
Bob "hot rod" Rohr
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me