When a radiator is warm at the bottom but cold at the top, the pattern usually points to a circulation issue rather than a fault with the radiator itself. In UK heating systems, this behaviour is extremely common and is almost always linked to air or restricted flow inside the system.
Understanding why the bottom heats while the top stays cold helps narrow the cause quickly. If several rooms behave unpredictably at the same time, it can also help to step back and assess the wider system using the house cold diagnostic.
Why the bottom heats before the top
Hot water enters a radiator at the bottom and rises upward as it circulates through the internal channels. For the top to heat properly, water must be able to move freely upward. When something blocks that movement, heat remains concentrated at the lower section and never reaches the top.
The most common reason for this interruption is air collecting in the upper section of the radiator, where water can no longer displace it.
Air trapped in the upper section
Air naturally rises within a heating system, which is why it collects at the top of radiators. When air occupies that space, hot water is pushed downward and circulation becomes uneven. The result is a radiator that feels warm near the base but remains cold above.
In most cases, removing the trapped air restores full circulation immediately. When the issue returns repeatedly after bleeding, it suggests that air is entering the system again or that circulation is already compromised elsewhere.
Pressure changes after bleeding
Bleeding releases air, but it also reduces system pressure. If pressure drops too low, hot water cannot circulate strongly enough to fill the upper sections of radiators. This often affects taller radiators or those further from the boiler first.
When pressure remains unstable after bleeding, the radiator may partially heat at the bottom but fail to fill completely.
Restricted flow through the radiator
Even with sufficient pressure, restricted flow can prevent heat from rising properly. Valves that are only partially open or systems that are not balanced evenly tend to favour some radiators while starving others.
When flow is weak, water enters the radiator but lacks the momentum to circulate fully upward. Over time, this creates persistent top-cold behaviour. How uneven flow develops across a system is explained in this guide to balancing radiators properly.
Circulation limits and partial blockages
Circulation problems do not always stop heat completely. In some cases, sludge or resistance within the system slows flow just enough to prevent full circulation. The radiator may heat partially, then cool quickly once the heating cycle pauses.
Although sludge more commonly causes cold lower sections, partial buildup can also interfere with upward circulation. Related behaviour is explained further in why radiators stay cold at the bottom.
When air keeps returning
If the top of a radiator repeatedly turns cold after bleeding, air is likely re-entering the system. This can happen through minor leaks, ageing components, or unstable pressure. While not usually urgent, repeated air entry indicates that the system is no longer fully sealed.
Monitoring pressure stability over time often reveals whether this is a persistent system issue rather than an isolated radiator problem.
Why this affects the whole house
A radiator that only heats partially forces the boiler to work harder to maintain temperature elsewhere. Heat becomes uneven, warm-up times increase, and running costs rise without improving comfort.
How radiator performance connects to overall warmth and efficiency is covered in How to Keep a UK Home Warm for Cheap.
When a radiator is warm at the bottom but cold at the top, it is almost always signalling a circulation or balance issue rather than a fault with the radiator itself.
