How can I find (and test) my steam system main vent?

Sorry, I read it twice because I thought it did not make sense. I read it a third time on my phone and realized I got it wrong. I have dyslexia.

Your questions are very good

No water hammer at A unless incorrect near boiler piping exists. Typically only cold water that comes into contact with steam will cause hammer…so imagine C is improperly pitched and has a sag. In between heating cycles, that standing water will cool, and when steam comes across it, it may cause hammer (the steam rapidly collapses and the rush of air/steam to fill that void picks up the water and flings it against the pipe).

Yes, all condensate from the main and all radiators flows from C to A and continues down back to the boiler.

Any steam that condenses in C or B will cause a local area of low pressure that will allow more steam to fill it. People sometimes say "pulls in more steam" but really the steam is being pushed to fill the low pressure area…you can't pull steam.

Also, if the main vent is stuck closed, then B and maybe also C will not get hot, b/c the air inside A/B/C will not let steam pass. So, if B and C is cold, that means main vent is not opening?

Exactly. B and C staying cold is a key indicator the vent is failed closed. In that case, all the air of the main must escape through any working radiator vents, and that takes a long time, delaying heating.

If main vent is stuck open, then what happens? Steam reaches C, and then B, so both B & C get hot, and A will stay cold since all steam is escaping from B. But A stays cold when main vent correctly closes itself also. So what's the difference is B is open or closed? ie: Why does main vent at B need to close?

If B doesn't close, then you lose a lot of steam out of it, which means you will have to replenish the water in the boiler very much more often than you want. The introduction of fresh water into a boiler greatly increases corrosion of its iron. Preventing steam and water leaks is a prime maintenance item of any residential steam system.

I've brought your labels here to page 2:

maybe easy to see, maybe not.

Certainly easier to see when you cut away some of that sheetrock

Yes. I would expect A to not get as hot, or to take longer to get hot because it just gets hot from condensate, not from steam (if I understand your system correctly)

I like your advice, @Steamhead, but why would you test from cold when during the heating season and especially with insulated pipes, a maybe better test is with the pipes warm.

With the pipes cold, a much smaller vent will seem big enough because the steam will travel much slower since it has to heat the pipes much more.

It feels strange to argue against my usual position that massive venting isn't really that helpful.

He meant that air and steam are both gasses, which they are… all three are fluids (there are some remarrkably weird things which are fluids, by the way, technically speaking. But I don't think we need to go there?) Air is sufficiently far from it's condensing point to behave pretty much like an ideal gas. Steam, at the pressures and temperatures we play with, isn't — so it does some very odd things at times. But for most purposes it's a reasonably well behaved gas…

it’s not an absolute pass/fail.

As @Steamhead described earlier in this thread, it’s a comparison between steam filling your main with no vent vs steam filling your main with the vent in place.

Your vent is definitely a Gorton #1. The larger #2 is dramatically larger, almost comically larger