When a radiator is warm at the bottom but cold at the top, the room never heats properly. The boiler may be running and the pipes may feel warm, but the heat is not circulating through the radiator as it should. In most UK homes, this behaviour has a clear cause and is usually fixable without major work.
The key is understanding what is preventing hot water from filling the upper section of the radiator.
Trapped air is the most common cause
Air naturally rises within a heating system, which means it collects at the top of radiators. When air occupies that space, hot water cannot circulate fully through the panel. The result is a radiator that warms at the bottom but stays noticeably cold at the top.
Bleeding the radiator releases this trapped air and allows water to fill the entire unit. A proper bleed is important — air can take longer to clear than expected, and the radiator may not respond until the flow is fully restored.
Once the air is released and water flows steadily, the radiator should begin heating evenly.
Incorrect flow can stop heat reaching the top
If bleeding improves the radiator only temporarily, or makes little difference, the issue may be flow-related rather than air alone. The lockshield valve controls how much water moves through the radiator. If it is set incorrectly, circulation can be uneven.
When flow is too restricted or too aggressive, hot water may pass through the lower channels without filling the radiator properly. A small adjustment to the lockshield often restores balanced heating.
This is part of system balancing, which is explained step by step in this guide to balancing radiators properly.
Sludge can disrupt internal circulation
Sludge does not always block the bottom of a radiator completely. In some cases, it settles in a way that interferes with circulation inside the panel, preventing hot water from reaching the top.
Signs of this include repeated cold spots after bleeding, slow warm-up times, or radiators that cool down faster than others. In these cases, flushing the radiator may be required to restore proper flow.
Unstable boiler pressure makes radiator behaviour unpredictable
Consistent pressure is essential for even circulation. When boiler pressure drops or fluctuates, radiators may behave differently from day to day, particularly those further from the boiler.
If pressure does not remain stable, radiators often become the first visible symptom. The underlying cause and fixes are covered in this guide to boiler pressure loss.
Repeated air build-up points to imbalance
If air returns frequently after bleeding, the system may be drawing air in due to uneven flow. Radiators closer to the boiler can dominate circulation, leaving others under-fed.
Balancing redistributes flow so all radiators heat evenly and release air more consistently. Once balanced, radiators typically retain heat more predictably and require less frequent bleeding.
When further investigation is needed
If the radiator remains cold at the top after bleeding, adjusting flow, and confirming pressure stability, a deeper blockage or system issue may be present. This is less common, but it does happen in older or poorly maintained systems.
In most homes, however, this problem is resolved by addressing air or flow imbalance.
Why fixing this improves whole-house comfort
A radiator that heats unevenly affects more than just one room. It forces the boiler to work longer, disrupts balance across the system, and makes heating feel inconsistent.
How radiator performance, airflow, and heat retention fit together is explained in How to Keep a UK Home Warm for Cheap.
Once circulation is restored, radiators typically return to predictable, even heating — and rooms warm faster with less energy.
