Air is the natural enemy of steam heating.
In a closed piping circuit, steam and air don’t mix uniformly.
Steam is light, expansive, and condensable. Sky air is non-condensable, dense, and sluggish.
When steam enters a cold cast-iron radiator, the air inside must have an exit path.
If the radiator’s vent valve fails to open, the incoming steam compresses the air into the upper columns, forming an invisible, iron-hard gas bubble. Air has a heat flow rate less than one percent of iron. Because of that, the trapped pocket acts as a heat insulator.
You can pump a thousand pounds of live steam against the radiator inlet. The iron will remain as cold as a gravestone in January.
Giles climbed the wide stone stairs of the Grand Hospice in District Two, his boots clicking softly on the clean cut stone treads.
Beside him walked Toby Karr. It carried a brass check tray with three small brass bleed keys and a clean cotton wiping rag. Sister Teresa led the way, her white linen wimple pinned tight, her rosary beads clicking against her hip.
