When a radiator stays completely cold while the rest of the heating appears to work, it usually feels like the problem is isolated and stubborn. The boiler runs, other rooms begin to warm, yet one radiator shows no sign of life at all. In UK heating systems, this behaviour is rarely random and almost never means the radiator itself has failed.
A radiator that does not heat at all is usually being denied hot water altogether. The issue is nearly always related to flow, pressure, or control somewhere in the system. When the wider heating performance feels inconsistent at the same time, it can help to step back and assess the setup using the house cold diagnostic.
Why a radiator can stay completely cold
For a radiator to heat, hot water must enter, circulate through the panel, and exit freely. If any part of that process is interrupted, the radiator will remain cold regardless of how long the heating runs. The key difference between a lukewarm radiator and a fully cold one is that, in this case, water is either not reaching the radiator at all or is being stopped almost immediately.
This narrows the cause to a small group of problems related to valves, pressure, circulation, or internal blockage.
When hot water never reaches the radiator
If both pipes feeding the radiator remain cold while nearby radiators heat normally, the radiator is effectively isolated from the system. This usually happens when flow is restricted before the radiator itself. Control valves are the most common reason, particularly thermostatic valves that are no longer opening fully.
Radiators affected this way often appear completely dead rather than partially warm. A similar pattern is discussed in why some radiators only heat on full, where flow only becomes possible under extreme demand.
Pressure that is too low to support circulation
System pressure plays a critical role in how far hot water can travel. When pressure drops below the normal operating range, the boiler may still run and nearby radiators may still warm, but those further away or on higher floors can be left without enough force to receive water.
This often shows up as one or two radiators staying completely cold while others behave normally. Restoring stable pressure usually brings those radiators back into circulation.
Air trapped before the radiator, not inside it
Bleeding a radiator only removes air trapped within the radiator itself. In some cases, air becomes lodged in the pipework leading to the radiator instead. When that happens, the radiator can bleed normally yet still receive no hot water.
This type of air lock blocks flow entirely, creating the impression of a failed radiator when the restriction actually sits just upstream.
Internal blockages stopping flow altogether
Sludge does not always cause partial heating. In more severe cases, it can block the entry points of a radiator so effectively that water cannot circulate at all. This is more common in older systems or where corrosion protection has been neglected.
A radiator affected this way often remains cold even though the pipe feeding it becomes hot. That pattern indicates that hot water is reaching the radiator connection but cannot move through the panel itself. This behaviour is explored further in why radiators can stay cold at the bottom.
Circulation limits elsewhere in the system
When circulation is weak overall, radiators at the end of the system tend to suffer first. Pumps can lose effectiveness gradually, and imbalance across the system can favour some circuits while starving others. Over time, this causes certain radiators to fall further behind each heating season.
These issues tend to develop slowly, which is why a radiator may work intermittently before eventually staying cold altogether.
Why one cold radiator affects the whole house
A single cold radiator disrupts the balance of the entire heating system. The boiler runs longer to compensate, rooms heat unevenly, and energy use increases without delivering comfort. Restoring proper flow to every radiator allows the system to stabilise and operate more efficiently.
How individual radiator performance fits into the wider picture of comfort, heat retention, and running cost is explained in How to Keep a UK Home Warm for Cheap.
When a radiator stays completely cold, it is almost always signalling a flow or circulation issue rather than a fault with the radiator itself.
