The short answer
Where a radiator is cold tells you the fault: cold at the top means trapped air and needs bleeding; cold at the bottom or middle means sludge and needs flushing; whole rooms cold while others cook means the system needs balancing. One dead radiator is usually a stuck valve.
What the law requires
The diagnosis is written on the radiator
Run your hand across a misbehaving radiator while the heating is on, and it will tell you what is wrong. Heat rises and air floats, so a radiator hot at the bottom but cold across the top has an air pocket sitting where water should be: bleeding releases it in two minutes. Sludge sinks, so a radiator hot at the top but cold at the bottom or in a central patch has magnetite sediment blocking the flow across its lower waterways: that needs flushing, of the radiator or the system. A radiator entirely cold while its neighbours work has a valve problem, most often a thermostatic radiator valve (TRV) whose pin has stuck shut, a classic after the summer months of disuse. And when the radiators themselves are fine but the far rooms never warm while the near rooms roast, no component is broken at all: the system is unbalanced, and the water is taking the easy route.
How to bleed a radiator (the cold-at-the-top fix)
Turn the heating off and let radiators cool; bleeding hot radiators sprays hot water.
Find the bleed valve at a top corner and fit a radiator key (a flathead screwdriver works on some modern valves).
Hold a cloth beneath and turn the valve anticlockwise a quarter to half turn. Hissing is the air escaping.
Close the valve the moment water appears steadily.
Check the boiler’s pressure gauge afterwards; bleeding lowers system pressure, and you may need to repressurise to 1.5 bar. Our boiler losing pressure guide shows how.
If a radiator needs bleeding every few weeks, the air is coming from somewhere: a small leak drawing air in, or corrosion generating gas inside the system. Repeated bleeding is a symptom to investigate, not a routine to accept.
The cold-at-the-bottom problem: sludge
The black sediment that settles in radiator bottoms is magnetite, the product of steel and water slowly rusting together, accelerated in London by hard-water scale. One affected radiator can sometimes be removed and flushed individually, a modest job. Cold bottoms across several radiators mean the sludge is system-wide, and the honest fix is a system clean: chemical cleanse for lighter cases, power flush for established ones, then inhibitor and ideally a magnetic filter so it does not return. Sludge is also the silent boiler-killer, working on the heat exchanger while it works on your radiators, which is why cold-bottomed radiators and a kettling boiler so often arrive together; our noisy boiler guide covers that half of the story, and the power flush page covers costs and the cases where a flush is not needed.
Half the house cold? Describe which radiators and where they are cold, and we will usually name the fix on the phone.
One radiator dead, or whole rooms cold?
For a single stone-cold radiator, check its valves first. A TRV stuck after summer can often be freed: remove the plastic head (usually a collar that unscrews by hand) and gently press and release the exposed pin; if it springs back, replace the head and you are done, and if it stays down, the valve needs an engineer’s attention. Check the lockshield valve at the other end has not been closed during decorating. If both valves pass and the radiator stays cold, the blockage is in the radiator or its pipework, engineer territory. For the whole-rooms-cold pattern, the fix is balancing: adjusting the lockshield valves across the house so flow is shared fairly instead of racing through the shortest circuit. It is methodical rather than difficult, transforms the worst-affected rooms, and at £95 to £160 for a full-system balance it is among the cheapest whole-house improvements in heating; the full service is covered on our central heating repair page.
Radiators, quick answers
Why is my radiator cold with the heating on and hot water fine?
On combis, if every radiator is cold but taps run hot, suspect the diverter valve; our no hot water guide explains the same component from the other side.
Do I need to bleed radiators every year?
A healthy sealed system barely needs it. Annual bleeding of the same radiator means air is getting in, which wants tracing.
Does cold-at-the-bottom always mean a power flush?
No. One radiator can be flushed alone, and lighter cases respond to a chemical cleanse. An honest assessment sorts the £120 job from the £450 one.
How much does radiator repair cost in London?
TRV replacement from £75, single-radiator flush £95 to £140, full balance £95 to £160, system cleanse £120 to £250, power flush £350 to £600, all fixed-quoted after diagnosis.