Cold at the top, warm at the bottom is the classic sign of trapped air, and bleeding the radiator — releasing that air — is a genuinely simple, five-minute fix with the right key. Air enters a heating system gradually over a heating season and collects at the highest points, including the top of radiators, where it blocks hot water from reaching that section.
Confirm it is really air
Feel the radiator from top to bottom with the heating on. Cold at the top and warm below points at trapped air. Cold at the bottom and warm at the top instead usually points at sludge build-up settled in the base of the radiator, which is a different problem — a power flush, not bleeding, addresses that.
What you need
A radiator bleed key — an inexpensive, widely available small square-socket key — fits the bleed valve found at the top corner of most radiators. Some modern radiators use a flat-blade screw slot instead and can be opened with a flat screwdriver. You will also want a cloth and a small container to catch drips.
How to bleed a radiator
- Turn the heating off
Let the system cool somewhat before starting — bleeding a scalding-hot radiator risks a burn from the water or steam that escapes.
- Locate the bleed valve
It is the small valve at the top corner of the radiator, usually opposite the thermostatic valve.
- Hold a cloth under the valve
Some water typically escapes along with the air once the valve opens, so have a cloth and a small container ready.
- Turn the key slowly, a quarter turn
You should hear a hissing sound as trapped air escapes. Keep the key or screwdriver ready to close it quickly.
- Close the valve the moment water appears
Once the hissing stops and a steady trickle of water appears instead, close the valve promptly — do not let much water out, just enough to confirm the air has cleared.
- Wipe away any drips
Clean up around the valve once closed.
- Repeat for every radiator that feels cold at the top
Work through the whole house systematically if more than one radiator is affected, which is common after air has built up over a season.
Check the system pressure afterward
Releasing air and a small amount of water lowers the boiler’s system pressure. Check the pressure gauge on your boiler after bleeding radiators, and top it up using the filling loop if it has dropped below the manufacturer’s recommended range — most boilers display this range directly on or near the gauge. Running a boiler with low system pressure for an extended period can cause it to lock out or run inefficiently.
How often this is needed
Bleeding once or twice a year, typically at the start of the heating season, is normal and expected as part of routine maintenance — see our seasonal maintenance guide for where this fits alongside other heating-season preparation. If you find yourself needing to bleed the same radiators repeatedly within a short period, that points at a different, ongoing problem rather than the normal gradual air accumulation bleeding is meant to address.
When it points to something else
If bleeding does not resolve a cold radiator, if a radiator is cold at the bottom rather than the top, or if you need to bleed radiators frequently within weeks of each other, the cause is more likely sludge build-up, a failing valve, or a system that is continuously drawing in air somewhere — all worth a heating engineer’s assessment rather than repeated bleeding.
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.
