My understanding of the bleed valves is that they are normally open devices which close when heated. The cycle goes something like this: at rest: radiator full

My understanding of the bleed valves is that they are normally open devices which close when heated. The cycle goes something like this: at rest: radiator full of air, valve open steam begins flowing: steam is displacing air from radiator, bleed valve begins heating up bleed valve closes: radiator is now full of steam, some of which condenses and ideally runs back down pipe, if your building hasn't shifted too much over the years. any condensed steam is replaced with new steam from pipe boiler stops: steam is no longer coming up the pipe. As the steam in the radiator condenses we suck air back in through the bleed valve. radiator is cooling. bleed valve opens: at some point the radiator cools enough that the bleed valve opens. We are now ready to begin the cycle again. Additionally, I think there is a time delay component in the bleed valve to let it get the air good and purged before it closes.