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My employer (small commercial HVAC company) took a job replacing a steam boiler. It a Peerless 210-8-S, atmospheric burner, about 1,000,000BTU/H. Apparently the new boiler will be almost identical to the old, and the bid was to replace the boiler only. Not a great situation to be in, I know.
I haven't see all the piping or radiators in the building, but it's a two-pipe system with TRVs and thermostatic traps on all radiators. The mains appear to be vented via crossover traps. I don't know how many, how big, or where all of them are located, but there are two in the boiler room (main leaves, two supplies circle the perimeter of the building in opposite directions and end up back in the boiler room).
I'm trying to figure out why there would be a main vent (Hoffman 75) and a vacuum breaker installed directory on the boiler. I'm thinking perhaps the crossover traps are not large enough (or failed at some point) causing pressure to build and shut the boiler down prematurely. Does that theory hold water? Could water expansion in the boiler be building pressure? Obviously the vent closes as soon as there's any steam being made, so I don't see how the vent on the boiler would do much of anything.
What other reasons might there be for that vent? I was surprised to see it there, but my steam experience is limited so I thought I'd ask the experts.
Next time I'm on site, I'll take picture of everything.