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One pipe system, radiator vents
I bought my first house in June. Before that my sum total knowledge of steam heat was knowing not to touch the radiators after suffering a minor burn at an aunt's house when I was 4.
About a month ago I figured with winter approaching I wanted to educate myself a bit on steam so I Googled a bit, watched a clip from Ask This Old House, and decided to replace the all the radiator vents with new ones from Home Depot. Also went around made sure all the radiator valves were fully open.
Cranked up the heat to make sure they worked and not a single vent closed. The radiators get screaming hot fairly quick. There is no water hammer anywhere. But the vents whistle when releasing the air and once the steam arrives, which is when I thought they were supposed to close, they just start steaming like a kettle and after a few minutes start spitting water.
Did I happen to get 4 faulty valves? What else might explain this? I thought that once the vents got hot, the diaphragm would push the pin up to close the vent. What could cause them to stay open?