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Radiators Cold At The Bottom
The top of the radiator is hot but the bottom stays cold — sludge has settled inside the panel.
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What's happening?
This is the opposite pattern to an air lock and it has the opposite cause. Steel radiators corrode internally over years, and the iron oxide that forms is heavier than water. It settles at the bottom of each panel as black magnetite sludge, and once the layer is deep enough the hot water simply flows over the top of it. Bleeding will do nothing, because there is no air. The same sludge circulates round the system, blocks the narrow waterways of a modern heat exchanger, seizes pumps and jams motorised valves, so a cold-bottomed radiator is rarely a lone symptom. The fix is cleaning the water, not treating the radiator, and then keeping the water clean with inhibitor and a filter.
What usually causes it?
- Magnetite sludge settling in the bottom of the panel over years of internal corrosion
- Inhibitor never added, or lost during drain-downs and never replaced
- Repeated topping up of a leaking sealed system, bringing fresh oxygen in each time
- Mixed metals in the system accelerating corrosion, particularly with old steel and newer aluminium
- A system that has never had a magnetic filter fitted to catch circulating debris
- Debris and flux left in the pipework from installation and never flushed out
- Radiators left cold for long periods, letting deposits settle and harden
Is it dangerous?
Not dangerous, but it is the most expensive fault on this page to ignore. The same sludge that is blocking the radiator is going through the boiler's heat exchanger, and a blocked exchanger is a large bill. Sludge is also a common cause of kettling and of pump failure. If the water that comes out when you drain a valve is black rather than clear, the system is telling you it needs cleaning now rather than at the next breakdown.
Can I fix it myself?
Very little, and that is honest rather than unhelpful. You can confirm the diagnosis by feeling across the panel with the heating running: hot top, cold bottom, no improvement after bleeding. You can check whether the radiator valves are open and whether the thermostatic valve head has seized. Chemical cleaners sold for DIY use can loosen sludge, but on a badly sludged system they mostly move it somewhere worse, which is why a professional clean is followed by a flush and a filter rather than by hope.
When should you call a heating engineer?
Call a heating engineer when more than one radiator is cold at the bottom, when the water is visibly black, or when cold-bottomed radiators come with kettling, a noisy pump or radiators that never get properly hot. It is not urgent this week, but it is worth doing before winter rather than during a breakdown, and before it takes the heat exchanger with it.
What will the heating engineer actually do?
- Confirm sludge by checking flow and return temperatures across each radiator rather than by feel alone
- Power-flush or chemically clean and flush the system to remove magnetite and debris
- Remove and flush individual radiators that are too heavily silted to clear in place
- Fit a magnetic filter on the return to the boiler and demonstrate how it is serviced
- Dose the system with corrosion inhibitor to the correct concentration and record it
- Rebalance the radiators and re-check flow temperatures across the whole system
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