When a radiator is hot on one side and cold on the other, the uneven pattern often looks confusing but is usually very specific. In UK heating systems, this behaviour nearly always points to restricted sideways flow rather than a problem with heat generation.
A radiator should heat evenly from the inlet side across to the outlet side. When one half stays cold, hot water is entering but failing to travel fully across the internal channels. If several radiators behave unpredictably at the same time, it can help to step back and assess the wider system using the house cold diagnostic.
How heat is meant to move across a radiator
Hot water enters through the inlet valve, spreads horizontally through the radiator panels, then exits through the outlet side. For this to happen evenly, water must flow freely through the full width of the radiator.
When resistance builds on one side, heat concentrates near the entry point and fades before reaching the opposite panel. This is why one side can feel hot while the other remains completely cold.
Restricted flow across the radiator
The most common reason for one-sided heating is restricted flow. Water reaches the radiator but cannot move across efficiently. This usually happens when the outlet side offers too much resistance, slowing circulation across the panel.
In some homes, similar resistance forms closer to the centre of the radiator rather than the edge, creating a different uneven pattern. That behaviour is explained in why radiators can be cold in the middle.
Sludge settling unevenly inside the radiator
Sludge does not always block radiators evenly. In many cases it settles toward one side, especially near the outlet, where flow is weakest. This creates a radiator that behaves as if it has two separate halves.
Radiators affected this way often heat slowly, cool quickly, and never distribute warmth evenly. Although sludge is more commonly associated with cold lower sections, partial build-up can interfere with sideways circulation just as effectively.
Valve behaviour limiting sideways circulation
Control valves influence how strongly water moves through a radiator. When a valve fails to open fully or restricts flow internally, water may enter but lack the force to travel across the panel.
This creates a hot inlet side and a cold outlet side, even though the radiator appears to be connected normally.
Pressure and circulation strength
System pressure affects how forcefully water moves through pipework. When pressure is low, water tends to favour the shortest and easiest routes. Radiators further along the circuit or with tighter internal paths are more likely to show uneven heating.
In these cases, one side may heat while the other never fully fills with hot water.
When air interferes sideways instead of upward
Air does not always collect neatly at the top of a radiator. In some situations, it becomes trapped along one internal channel, interrupting flow to part of the panel while leaving the rest unaffected.
This can produce one-sided coldness even when the radiator bleeds normally. A related pattern, where air prevents vertical circulation instead, is explained in why radiators stay cold at the top.
Why uneven radiators affect the whole system
A radiator that only heats on one side reduces overall system efficiency. The boiler runs longer to compensate, rooms warm unevenly, and energy use rises without improving comfort.
How individual radiator performance fits into the wider picture of comfort and running cost is covered in How to Keep a UK Home Warm for Cheap.
When a radiator is hot on one side and cold on the other, it is almost always signalling restricted circulation rather than a fault with the radiator itself.
