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Two-pipe steam system troubleshooting
Hello all, new member here. We recently purchased an 1870s house in NH. This is my first experience with a steam system, so I'm trying to get our heating system working as intended. We have mostly two-pipe radiators, a couple one-pipe radiators, a convector, and a couple of baseboards thrown in. The boiler is a Megasteam that looks to be about ~5 years old and is in good shape. The piping is probably less so! I can't seem to locate a main vent on the system. I've followed all of the exposed piping in the basement and there doesn't appear to be anything that looks like a vent. I'm not sure if it's somewhere else or if it's possible that there isn't one at all.
We started off with some radiators not heating up at all and other clanging and whistling. Having read through the advice here, I've replaced most of the air vents that were old and reduced the boiler pressure down to 0.5 with a diff of 1 on the pressuretrol. That has reduced the clanging and whistling, and we have more even heating across the radiators. However, the convector and downstairs baseboards still refuse to heat up and now occasionally on long heating cycles, I find steam escaping from the air vents on all the radiators at once. I don't really understand why this would happen. We've also heard what sounds like a sound of water rushing in the basement after these cycles, which I'm guessing is the sound of water being added to the system to replace the steam that is being vented?
Apologies for any naive questions. I'm not sure how to diagnose and would love advice!