Why is my radiator cold at the bottom?

A radiator that is warm at the top but cold at the bottom commonly points to sludge, debris or poor circulation, not trapped air. Bleeding is unlikely to cure that temperature pattern.

Worcester Bosch distinguishes the two common patterns: cold at the top can indicate air, while cold at the bottom can indicate sludge or debris and may require the system to be flushed by an installer. Other circulation faults can produce uneven heating, so the surface pattern is a clue rather than a complete diagnosis.

Check the ordinary room controls first: the programmer is calling for heat, the room thermostat is high enough and the TRV is not set low. Do not start opening boiler valves or draining the system. A heating professional can compare radiators, assess circulation and decide whether cleaning, balancing or a component repair is justified.

Any work on a gas boiler itself, its gas pipework or its casing is for a Gas Safe registered engineer. HSE says water pipes and radiators can be wet work, but the final connection to a gas boiler and work on the boiler are restricted.

Give the engineer a useful whole-system picture. Note whether one radiator or several are affected, whether upstairs and downstairs behave differently, how long they take to warm and whether the system is noisy. Do not assume that an advertised powerflush is automatically the answer: the engineer should establish the cause and condition first. If a radiator leaks or a valve will not close, switch the heating off and arrange repair rather than continuing to heat it.

Sources

• Radiator fact sheet — Worcester Bosch, accessed 14 November 2026.
• Radiator cold at the bottom — Worcester Bosch, accessed 14 November 2026.
• Gas safety: who can do the work? — HSE, accessed 14 November 2026.

On Leatmere

Read heating inhibitor and sludge and powerflushing.