As I was just opening my eyes this morning I heard a steady hiss coming from downstairs. Anyone who’s got single pipe steam heat knows the sound, especially early in the morning when the boiler is working hard to warm the house from its overnight setback temperature. That’s the sound of pressure from the steam displacing air inside the pipes. It’s normal. That’s what an air valve on a steam radiator is supposed to do. But if it’s loud enough that you can hear it a floor away, you’ve likely got a problem. It often means the valve is stuck