Look closely at the two pictures on this page. See something unusual?
What kind of radiators are these? What your looking at are two pipe radiators. See the steam inlet on one side and a steam trap on the other. In the circle
are one pipe steam radiator air vents. Now don't you ever do this! Someone was trying to solve a problem by doing this. But all they did was to create a future problem. It may even have helped their initial issue. All they did was to treat a symptom instead of fixing the problem.
If a two pipe radiators trap fails closed, it cannot pass the air out of the radiator when the steam pushes against it. The air gets trapped in the radiator and the steam can't get in. What your left with is a cold radiator. So what some knucklehead does is to add a one pipe radiator air vent to the radiator. And you know what? Its so stupid as to work. It works because the air can now get out. But heres what the knucklehead just did.
The trap was failed closed, right? So now steam is condensing in that radiator. Where is it going? How about its filling the radiator up! Now one of two events is going to happen. The radiator will fill up with water till it no longer heats. The homeowner calls the plumber who shuts off the boiler, goes up stairs to the radiator and is going to take the cover off the trap to replace the moving part that failed. This is a steam system so there should be no water in the radiator when the boilers off. He takes the cover off and gets douched by five or six gallons of water. And of course the room will have white carpet and be on the second floor so that the water can destroy the ceiling on the first floor as it pours thru the hole the pipe uses. A pretty mental picture huh? All because some jerk put a one pipe radiator air vent on a two pipe radiator. Here is another scenario. The radiator starts filling with water. The boiler says ''hey I'm running out of water''. So the water feeder turns on and feeds water to the boiler. Then the system shuts down. The water in that radiator may be able to leak back past that bad trap and now slowly causes the boiler to flood. You keep draining extra water out of the boiler. You keep wondering where the water comes from. You finally call your plumber who replaces the automatic water feeder cause he figures it must have gone bad. You pay him and the problem returns. So now your out the money for the feeder, your mad at your plumber, you hate your boiler, and you still have your problem. All because some jerk put a one pipe air vent on a two pipe radiator. But hey, the radiator got warm didn't it?
Its okay to put an air vent anywhere on the supply side of the system you desire. You can put it on the riser pipe to the radiator, just before the valve. Thats okay. But don't put it on the two pipe radiator. The radiator must be able to air lock if the trap fails closed. Think of it as the radiators way of saying ''I'VE GOT A BAD TRAP--FIX ME''.