Reading a recent thread on venting reminded me of a lesson I learned long ago, from a great uncle of mine who had worked for Thomas Edison at one point. It seems that Mr. Edison needed to know the volume of a new light bulb he wanted to use. So he set the problem to some of the young bucks in the lab and waited And waited. And presently came back in and asked what the holdup was. "Well, Mr. Edison, this is a very complicated shape and it takes time to figure it out"! Mr. Edison picked up the bulb, filled it with water, and poured the water into a graduated cylinder and that was that.
What's the connection with venting -- specifically main venting? Well... consider exactly what it is one is trying to do: get steam rapidly (as rapidly as can be) and evenly throughout the length of the various steam mains before -- if possible -- any of the radiation begins to produce much heat, so as to have even heating. But steam mains are almost inevitably "very complicated shapes". Not only do they twist and turn, they are varying diameters and varying lengths and varying amounts of insulation and varying... "very complicated shapes".
I know in this day of computers and all I'm regarded as hopelessly old-fashioned -- somewhere around the dinosaur age, I expect, but... is there a simpler approach? May I suggest... we know we need to have adequate main venting. In fact, it's usually possible to say that it's very hard to have too much main venting, except on economic grounds. And there are rough rules of thumb (most of which are, incidentally, overkill). My suggestion is this. First, put a low pressure gauge on the system -- the header is a good place, but it could be on the boiler though that can be jittery. Then put main vents on the system where and as they belong, sized by some handy rule of thumb (depends -- is this one pipe? Two pipe with steam main vents? Two pipe with crossover traps? Do they go on the steam mains? Do you need them on the dry returns (two pipe of any flavour, yes indeed you do)?). Then fire the puppy up and watch the pressure gauge. Does it rise to no more than a few ounces and more or less hold while the radiation starts to wam evenly? Fine. You're done (unless you really went bonkers with the vents -- you might want to try reducing the venting?). Is there a main which is slow? Find out why -- uninsulated? Insulate. Flat pitch on a counterflow line? Fix the pitch. Not enough venting? Try adding some. Does the pressure continue to rise? Definitely not enough venting. Add some -- evenly on the system (one of the beauties of crossover traps -- all the vents are at the boiler and close to hand to play with!).
It may take some fiddling -- perhaps a couple of hours worth, even -- but when you've got it to the point that all the mains are seeing steam at the ends within a few minutes of each other and the pressure is holding flat or climbing very very slowly... then if you're on one pipe you can worry about radiator vents (you want slow vents on the radiators to start with...). On two pipe, you can start closing inlet valves on spaces which are too warm.
Stone age? Yeah, sorry about that. No apps. But it works...
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch