Every steam system is an open system. Water expands 1,700 times when it changes to steam, and that shoves the air out. Steam condenses inside the pipes and radiators and shrinks to 1,700th its volume, and that brings the air back. Steam systems breathe – just as we do.
You already know that where there is air, steam will not go. Steam and air have different densities (steam is lighter than air). Steam acts like a plunger, shoving the air ahead of itself. When it condenses, the steam leaves the insides of the pipes all wet. The air enters and reacts with the wet steel and iron, forming rust.
Now, the steam moves so quickly through the pipes that it carries the rust right to the air vents. The vents clog and call in sick. They’re not going to work today. When this happens, the other vents have to pick up the slack and work even harder than they usually do. They’re really blowing air now, and you can hear that air moving at high velocity. Some people think that’s a good thing. They hear the air and say, “Yep, she’s doing a great job!” But when you’re able to hear the air moving from the vent, what you’re listening to is the sound of high-velocity air that’s shoving even more rust and debris from the pipes into the vents. You shouldn’t hear vents venting. It’s not a good sound.
When the main vents clog (and these handle most of the air in the system), people often replace them with plugs. Plugs don’t vent. They plug. Hence their name.
Want to learn more? Get a copy of Greening Steam: How to Bring 19th-Century Heating Systems into the 21st Century (and save lots of green!).