When one radiator stays cold while every other radiator in the house heats normally, the problem is almost always local to that radiator rather than the heating system as a whole. In UK homes, this pattern is common and usually points to restricted flow, trapped air, or a control issue affecting that single unit.
Because the boiler is clearly producing heat and distributing it elsewhere, the system itself is functioning. The challenge is identifying where heat is being prevented from entering or circulating through the cold radiator. If several rooms behave inconsistently at the same time, it can help to step back and assess the wider setup using the house cold diagnostic.
Why one radiator can stop heating on its own
For a radiator to heat, hot water must enter through the inlet valve, circulate through the panel, and exit freely. When only one radiator fails, something along that local path is interrupting the flow. This interruption is usually mechanical rather than systemic.
The behaviour of the pipes connected to the radiator often reveals whether water is reaching it at all or being stopped once it arrives.
Local valve restrictions
Thermostatic and manual valves regulate how much hot water enters a radiator. When a valve does not open fully, the radiator can appear completely inactive even though the rest of the system is operating normally.
This is particularly common with thermostatic radiator valves that have remained in the same position for long periods. Internally, the valve may no longer respond smoothly, preventing water from entering despite the dial appearing to function.
Flow limited by the return side
Even when water enters the radiator, it must be able to exit freely. If resistance is too high on the return side, circulation slows or stops altogether. This creates a radiator that remains cold while neighbouring units heat as expected.
When similar resistance affects only part of a radiator rather than the whole panel, it can produce uneven heating patterns. A related example is explained in why radiators can be cold on one side.
Air trapped within the radiator
Air trapped inside a radiator prevents hot water from filling the panel properly. In some cases, air pockets persist even after bleeding, particularly if circulation is already weak or pressure is unstable.
When a radiator bleeds normally but still fails to heat, it often means that air is not the only factor involved. This behaviour is explored further in why radiators can stay cold after bleeding.
Pressure that no longer supports circulation
Low system pressure affects how far hot water can travel. Radiators located on higher floors or at the end of longer pipe runs are usually the first to lose heat when pressure drops. In these cases, one radiator may stop heating entirely while others continue to work.
This does not indicate a boiler fault, but rather insufficient force to maintain flow across the whole circuit.
Partial internal blockage
Sludge or debris can build up inside a single radiator without affecting others. When this happens, water may reach the radiator but fail to circulate through it. The radiator remains cold, even though the connected pipework may feel warm.
This type of blockage often develops slowly and may not be obvious until circulation elsewhere improves and the imbalance becomes more noticeable.
Why a single cold radiator still matters
One non-functioning radiator disrupts the balance of the entire heating system. The boiler runs longer to compensate, rooms heat unevenly, and energy use increases without improving comfort.
How individual radiator performance connects to overall warmth and running cost is explained in How to Keep a UK Home Warm for Cheap.
When only one radiator is not working, the cause is almost always local and mechanical rather than a failure of the heating system itself.
