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Venting Radiator Risers

As part of a master venting project, I want to properly vent the top of each riser. The building is a multi-family pre-war with 1-pipe steam. Each bathroom has a blind riser. There are one and two bedroom units.

Can I assume that all the radiators branch off from the blind riser, so that I can rely on venting the top of the blind riser alone?

How common is it that some or all radiators have their own riser that much be vented by the top radiator (presumably by adding a vent immediately prior to the radiator valve)? Note that in the basement, I can see at least 40 risers going into the ceiling and the building has 10 unit on each floor and thus 10 blind risers.

And if I cannot rely on the blind riser branching to all the radiators, it there any way to guesstimate which radiators are not branched off the blind riser (perhaps based on the number of risers in the basement; or perhaps only the closest radiator to the blind riser)?

Thank you,


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