Welcome! Here are the website rules, as well as some tips for using this forum.
If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Donnelly System? Air vents where?
Finally finding some time to try and learn more about my steam system in our house. After looking around this amazing forum and looking through The Lost Art of Steam Heating I believe we have a Donnelly System that has been monkeyed with over the years. We have a two pipe system. Supply is at the top and return down low on the radiators. Every radiator except one (it has a Hoffman 17C) has a bulbous 90 degree elbow on the return with a "C" stamped on it (see photo). Cracking one of these open I found a check valve (see photo) which makes me believe it is a Donnelly system and so the question is how to optimize it.
Most radiators have air vents and I assume I should remove all of those? What is the best way to plug the air vent hole? A short properly sized bolt?
Also attached is a rough schematic of the mains and returns with a call out of where air vents are currently located. Black lines are mains, gray lines are returns - I did not include any radiators but hopefully it is enough detail to get the point across. The system air vents are Hoffman 76 vacuum valves. The ones on the mains are broke (ones on returns still work). I suspect I should upgrade the air vents on the returns and want to confirm that I should replace the broken vents on the mains.
Also attached are a few photos of the near boiler piping with attempts at labels scribbled in red.
Any recommendations or information would be appreciated. It is fascinating learning about these old systems and I hope to dial ours in before the pending heating season.