Water Hammering- One Pipe Gravity Return

I am speaking as a layperson that grew up with steam;

It would save you a great deal of work if you hired a mechanical engineer or licensed steam plumber to look at

this place and then do a heat study.

It is more important to ask if they are willing to actually spend good money to fix it by replacing any bad

piping, adding main vents or replacing the current vents with larger vents, adding a pair of low water cut off

switches to bring the boiler up to code, install new steam controls, shimming radiators to increase condensate

drainage if needed, replacing old radiator vents with new ones, replacing/moving the condensate return pipes

to create properly sloped pipes, adding a main vent menora on the top floor if possible-if needed, adding a

double drop header on the boiler if there is enough ceiling height.

One of the first things on the list would be to do the heat study then wand and clean the boiler if there is a

mud leg to drain any sediment otherwise one would have to be added if there are 2 tappings in the boiler

sump and clean the burner(s) and do a combustion test.

Unless I'm completely misreading that sight glass — quite possible — that boiler is wildly overfilled. Now the water line marked on the boiler doesn't correspond to the low water cutoff installation, which is higher, so that will govern. Try starting off with the water line about even with the TOP of LWCO casing and see how that works.

A couple of thoughts.

First, so long as the wet returns — your "condensate lines" are below the boiler water line, the pitch on them is irrelevant. What is relevant is that they must be clear; they are the only place in a steam system where gunk can build up, and a clogged wet return can make for difficulties.

Second, there should be generous main vent at the end of every steam line. From the overall appearance of the size, I'm going to be that even two Gorton #2s won't be overkill. Now a further thought: if you have multi-story risers, there is no harm and a good deal of benefit to putting a main vent at the top of each riser as well.

Third, while the wet return pitch isn't critical, the pitch of the steam mains and any horizontal runouts is. The runouts must all pitch back to the steam main, or to a drip to the wet return, and the steam mains must pitch — full length — to a drip (preferably) or back to a drip near the boiler.

You mention a concern about pressure. Several things here. First, even though this appears to be a pretty big setup, I doubt very much that you need to or want to run the boiler at more than 2 psig. To concern on not enough pressure to push the condensate back. There is one place which concerns me, looking at your diagram: there is a "dry return" coming from loops 2 and 3. Since this is one pipe steam, this isn't a true dry return, but a steam main extension — and thus subject to some or full boiler pressure. The problem is that it is very close to the boiler water level which you call out, and at certain firing conditions it may be just low enough that condensate can back up into it — or try to. And this can cause some really bad water hammer. This comment, though, does go back to the first comment on the apparent boiler water level. Is that reference dimension to boiler water level to where the water level should be, or where it is?

On the line which doesn't heat. Is there a valve on that line? I see lots of valves! If there is, are you sure it's open? Trace how far the heat gets to find out what might be stopping the steam.