A radiator that is hot at the top but cold at the bottom often points to poor circulation or sludge and sediment collecting in the lower section. This differs from a radiator cold at the top, where trapped air may be more likely; bleeding usually does not fix bottom-cold sludge buildup.
Confirm the temperature pattern after the system has been running
Let the system reach operating temperature and compare several radiators carefully without touching very hot surfaces. If only one is cold at the bottom and hot at the top, the issue is likely localized to that radiator or its flow.
A cold bottom often fits sludge or low flow
Internal corrosion products and debris can settle in the lower section and keep hot water from crossing the full panel. A partially closed valve or poor balancing can also reduce available flow.
Bleeding is more useful when the top is cold
Air rises, so a radiator cold at the top and warmer below may respond to bleeding. Do not keep bleeding a radiator that is cold at the bottom when no air is present; that only lowers system pressure without removing sludge.
Widespread sludge calls for system cleaning and root-cause work
If several radiators show the same pattern, drained water is very dark, or the problem returns after one radiator is cleaned, the system may need professional flushing, corrosion treatment, or assessment. Do not open a boiler or work on combustion components.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on home heating, cooling, efficiency, and energy maintenance.