Radiator / heating symptoms / trapped air
Why is my radiator cold at the top?
Start with the observable symptom. This page explains possible causes; it is not a diagnosis.
A radiator cold at the top but warmer below often has trapped air. Bleeding may help, but let the radiator cool, follow the manufacturer’s instructions and check boiler pressure afterwards; repeated air or pressure loss needs professional attention.
Start with the pattern.
The upper part of the radiator stays cool while the lower part heats.
Possible causes, not a diagnosis.
- Air is trapped in the radiator.
- The system pressure may need checking after bleeding.
- A wider circulation or boiler issue is possible if several radiators behave the same way.
Check what is safe to observe.
- 01
Compare the pattern across several radiators while the heating is running.
- 02
Check the boiler pressure without opening the boiler casing.
- 03
Listen for hissing or gurgling, and look for any visible leak around valves.
Take the smallest useful step.
- Switch the heating off and let the radiator cool before bleeding.
- Use a radiator key and open the bleed valve only as described in the manual.
- Close the valve when air stops and water begins, then check pressure as instructed.
- Do not bleed a hot radiator or open the valve fully.
- Do not repeatedly top up a system that keeps losing pressure.
- Call a heating engineer if pressure drops, water leaks or air returns quickly.
- Use an appropriately licensed gas professional in the UK (Gas Safe registered engineer) for work inside a gas boiler.
Professional requirements and rental responsibilities vary by location. Use the qualified local professional or housing authority that applies where you live.
UK example: Gas Safe RegisterWhy this page says that.
This page separates an observable symptom from possible causes and keeps the recommended action within a safe household boundary.