Cold at the bottom is different from cold at the top
When the top of a radiator is cold but the bottom is warm, trapped air is a common explanation and bleeding may help. A radiator that is warm at the top but noticeably cold across the bottom suggests something else. Sludge or heavy deposits can settle in the lower waterways and reduce circulation through that part of the panel. Compare the problem radiator with others in the house. One isolated cold radiator is different from several radiators showing the same pattern.
Sludge forms gradually
Corrosion products can circulate around a heating system and collect in low-flow areas. The black material often called magnetite can settle in radiators and pipework, particularly where inhibitor levels have not been maintained. The result is patchy heating and slower warm-up. A radiator does not need to be completely blocked to feel cold at the bottom. The pattern may become more obvious in winter when the system runs for longer and the difference between flowing and stagnant areas is easier to feel.
Bleeding is not the cure for every cold radiator
Bleeding releases trapped air from the top of a radiator. It does not remove sludge from the bottom. Repeatedly bleeding a sealed system can also lower the boiler pressure if the system is not topped up correctly afterwards. Before doing anything, check whether the radiator has air at the top or simply poor circulation below. If the whole radiator stays cool, a closed valve, stuck thermostatic pin or balancing issue may be more likely than sludge.
What can be checked without dismantling the system
Make sure both radiator valves are open and compare the temperature of the pipes entering each side. If one pipe is hot and the other remains much cooler, flow through the radiator may be restricted. Look at whether other radiators heat evenly. Do not loosen large valve nuts or remove a radiator unless you understand how the heating circuit is isolated and drained. Heating water can be dirty and can escape quickly once a pressurised connection is opened.
Possible professional remedies
A plumber or heating engineer may remove and flush an individual radiator, improve balancing or recommend wider system cleaning if several radiators are affected. Chemical cleaning and powerflushing are different approaches and neither should be sold automatically without evidence that the system needs it. If an old radiator is heavily restricted or corroded, replacement may be more sensible than repeated cleaning. After work, the system should be refilled correctly and inhibitor restored where appropriate.
What to say when asking for a price
Describe exactly where the radiator is cold and how long it takes to warm. Mention whether other radiators show the same pattern and whether the boiler pressure is stable. A photograph will not show temperature, so a simple sketch or note about hot and cold areas is more useful. Ask whether the quotation covers one radiator only or includes investigating the wider system. That distinction can explain large differences between prices that otherwise look like they are for the same job.
Ask what evidence points to sludge
A cold lower section is suggestive of restricted circulation, but it does not mean every heating system needs a full powerflush. Ask what the temperature pattern across the radiator shows and whether other radiators have the same symptom. If only one old radiator is affected, removing and flushing that unit may be a more proportionate first step. If several radiators are poor and the system water is heavily contaminated, wider cleaning may make sense. The contractor should be able to explain why the proposed method suits your system. After any cleaning work, compare how quickly the radiator warms and whether the cold band has disappeared. The result should be judged by improved circulation, not simply by the fact that dirty water came out during flushing. A noticeably more even surface temperature is the useful test afterwards.
This page is general guidance about radiator cold at bottom; the cause in an individual home still needs to be established from the actual installation.