Bleeding a radiator is the controlled venting of air that has accumulated at the high point of a sealed wet central heating emitter, exploiting the fact that entrained air, being less dense than water, migrates to and collects at the top of the emitter and displaces the circulating water that would otherwise transfer heat there. The bleed valve is a manually operated vent at the top of the emitter whose opening releases the trapped gas until liquid presents at the orifice, indicating the air pocket has been fully displaced; the observable symptom of the fault is a thermal gradient in which the upper region of the emitter remains cold while the lower region heats normally. This belongs to plumbing and domestic heating maintenance within building services, and it interacts with system pressurisation theory: venting gas from a sealed system necessarily reduces system pressure, so restoring the design fill pressure is a required consequence of the operation.
Bleeding a radiator is the controlled venting of air that has accumulated at the high point of a sealed wet central heating emitter, exploiting the fact that entrained air, being less dense than wate…