Why is my radiator cold at the bottom?

The position of the cold area is diagnostic. Air rises, so trapped air makes the top cold. Iron oxide sludge sinks, so it makes the bottom cold. Treating one as the other wastes an afternoon, which is why this distinction matters more than any technique.

Safety

What you'll need

• Radiator bleed key
• Towels and a container
• Adjustable spanner (optional)
• Hosepipe access (optional)

Step by step

1. Confirm the pattern by feeling the whole radiator when hot.Cold bottom, hot top means sludge. Cold top, hot bottom means air. Cold all over means it is not getting flow at all.
2. If the top is cold, bleed it — that is a five-minute job.Bleed key, quarter turn, wait for water instead of air, close. Then check boiler pressure afterwards.
3. For a cold bottom, first try turning off every other radiator.Forcing the full flow through one radiator sometimes shifts light sludge. Worth ten minutes before anything more involved.
4. To flush it, turn the heating off and let everything go cold.Completely cold. This is not a job to rush, and the water is genuinely hot enough to scald.
5. Close both valves, noting the lockshield position.Count the turns as you close the lockshield valve so you can restore the exact balance afterwards.
6. Drain the radiator into a container, then remove it.Expect black water that stains permanently. Protect the floor thoroughly — more thoroughly than feels necessary.
7. Flush it outside with a hose until the water runs clear.Through both ends alternately. What comes out will be startlingly black and that is the point of the exercise.
8. Refit, reopen the valves to their original positions, bleed and repressurise.Then check the boiler pressure and top up if needed.

Tips

• If several radiators are affected, the whole system is sludged. A professional power flush plus an inhibitor and a magnetic filter is more effective than doing them one at a time.
• A magnetic filter on the return pipe catches magnetite before it settles. It is the single best preventative measure and is cheap relative to the damage sludge causes.
• Corrosion inhibitor should be topped up whenever the system is drained. Without it, sludge simply reforms.

Common mistakes

• Bleeding a radiator that is cold at the bottom — There is no air at the bottom. Bleeding releases nothing and drops the system pressure for no benefit.
• Draining a hot system — The water is hot enough to cause serious scalds, and hotter water carries more sludge onto your floor.
• Not counting the lockshield turns — You lose the system balance, and afterwards some rooms will heat much faster than others.

Questions people ask