Where a radiator is cold tells you exactly what is wrong with it. Cold at the top with the bottom warm means trapped air is sitting in the highest point of the radiator, and bleeding it is usually a five-minute fix. Cold at the bottom with the top warm is the opposite problem — sludge or debris has settled in the radiator, and that needs flushing rather than bleeding.
Cold at the top
Water circulates through the radiator, but trapped air rises to the highest point and displaces water there, leaving that section cold while the rest heats normally. This is the most common radiator complaint and the easiest to fix.
- Turn off the heating and let the radiator cool
Bleeding a hot, pressurised radiator risks a scalding spray of hot water.
- Locate the bleed valve
A small square-socket valve near the top of the radiator, usually at one end.
- Open it slowly with a bleed key
Hold a cloth beneath it. You will hear a hiss of escaping air, followed by water once the air has cleared — close the valve as soon as water appears.
- Check the system pressure afterward
Bleeding radiators lowers boiler system pressure slightly. Check your boiler’s pressure gauge and top up if it has dropped below the manufacturer’s recommended range.
Cold at the bottom
This pattern means sludge — a build-up of rust and mineral debris inside the central heating system — has settled at the bottom of the radiator, blocking water flow through that section. Bleeding will not fix this, since there is no air to release; the problem is solid debris sitting in the water.
| Signs it is sludge, not air | What to do |
|---|---|
| Bleed valve releases only water, no air, yet the bottom stays cold | This confirms sludge rather than trapped air |
| Several radiators in the house show the same cold-at-the-bottom pattern | Points to system-wide sludge rather than one radiator |
| Radiator takes a long time to heat up, or needs the flow turned up unusually high | Circulation is restricted by debris |
The fix is a power flush of the central heating system — a professional service that forces high-velocity water through the pipework and radiators to clear accumulated debris, usually followed by adding an inhibitor to slow future buildup.
The whole radiator is cold
If a single radiator is entirely cold while others work fine, check that its valves — both the manual valve and the thermostatic radiator valve (TRV) — are open. A stuck or fully closed valve, rather than air or sludge, is a common cause of one radiator being completely cold when the rest of the system is fine.
When to call a heating engineer
Have a professional carry out a power flush if sludge is suspected, especially if multiple radiators are affected — this is not a DIY job. Also call an engineer if bleeding does not resolve a cold top, if system pressure keeps dropping after bleeding, or if a valve is stuck and will not open.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
