A hot-water radiator that is cold at the top but warm at the bottom commonly has trapped air. Before bleeding it, confirm that the system is hydronic and that the radiator has an accessible bleed valve.
Check the temperature pattern
Cold at the top with warmth below points toward trapped air; cold at the bottom with a warm top can suggest sludge or poor flow. A completely cold radiator may have a closed valve, circulation problem, or no heat call.
Bleed only under safe system conditions
Turn the heating system off and let the radiator reach a manageable temperature. Open the bleed valve only slightly with the correct tool and have a container or cloth ready for water.
Close the valve when water flows steadily
Air often escapes with a hiss before water appears. Close the valve without overtightening. On pressurized systems, check boiler pressure afterward according to the boiler manual.
Repeated air buildup needs diagnosis
If the same radiator needs bleeding repeatedly, air may be entering through a leak, pressure may be wrong, or corrosion may be producing gas. Do not treat frequent bleeding as normal maintenance without finding the cause.
Sources
- U.S. Department of Energy — Thermostats — Thermostat use, temperature setbacks, and heating and cooling energy savings.