Most newer high efficiency mod/con boilers require primary/secondary (p/s) piping and even some retrofit cast iron boilers will be piped p/s by choice. Cast iron boilers do not require p/s it may still be a good choice. Most cast iron boilers are installed with the industry standard manifold type systems. If the boiler requires a boiler bypass installed to protect the boiler from condensing follow the link for boiler protection.
The drawing below shows an updated standard manifold system.
I call the above drawing the "updated" standard due to some minor changes. The changes are the placement of the purge valves, ending the manifold with tees instead of elbows and a drain on the supply header and possible the return header.
Let's take a look at what the advantages we have over the manifold of yesteryear. We have a single purge drain on the boiler riser instead of one for each return pipe. This is more service friendly than the older style. The valves above the flow checks can all be closed, close the feed valve and the supply manifold valve. You can now drain off a little bit of water that is isolated between the closed valves. Now we can service the circulators, flow checks, zone valves if used, check the air charge on the expansion tank that needs to be done with "0" water pressure, air separator and air vent. When the service is complete open the fill valve first. Secondly open the supply manifold valve. Thirdly open one of the valves on any zone. Run the circulator for that zone. Since we are pumping away from the expansion tank there is no purging required. We have not allowed enough water into the piping to stop the flow. The pump will break the air into smaller bubbles by adding pressure to the zone. The smaller less buoyant air bubbles work their way to the boiler which heats the water driving the air out of the water, and than the water and air moves to the air separator to vent the air. Continue the same steps with all the remaining zones, pack up and go home. No need to purge the heating system unless you have to completely drain the heating system of all the water.
There variations of the above drawing as long as you keep the same idea. You may use a circulator and zone valves instead of multiple circulators and flow control valves. May choose circulators with internal flow checks instead of circulators with flow control valves. Maybe use circulators with internal flow check and flange isolation flanges and eliminate the flow checks and shut off ball valves. Just keep in mind that the whole idea here is ease of service.
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