THE SHORT ANSWER
THE TWO JOBS
People use the words interchangeably and then wonder why bleeding did not fix a cold back bedroom. They fix different problems.
Five minutes, one radiator
What it fixes
Air trapped at the top of a radiator
You need
A radiator key and a cloth
Heating
Off and cooled down first
What to do
Open the top valve slowly until water appears
Afterwards
Check boiler pressure, top up if under 1 bar
If it recurs
Air is getting in. Worth investigating
A couple of hours, whole house
What it fixes
Some rooms warm, far rooms never warm
You need
Two thermometers and patience
The principle
Water takes the easiest path unless you stop it
What to do
Adjust the lockshield valve on each radiator
The target
About 11 to 12 degrees drop across each radiator
Cost
Nothing in parts. It is all time
THIS IS WHAT SLUDGE LOOKS LIKE
Heat rises and dirt sinks, which is why the cold patch tells you the fault. Air collects along the top. Sludge settles along the bottom. A radiator that is cool all over is usually getting less flow than the ones nearer the boiler.
THE THREE FIXES
1
Turn the heating off and let it cool, put a radiator key on the valve at the top corner with a cloth underneath, turn slowly until air hisses out and water appears, then close it. Check the boiler pressure afterwards and top up if it has dropped below 1 bar. If a radiator needs bleeding repeatedly, air is getting in somewhere and that is worth investigating.
2
Bleeding will not touch it. Debris has settled in the bottom of the radiator and is blocking flow. Sometimes a single radiator can be taken off and flushed, but if several are doing it the whole system needs a power flush and a magnetic filter fitting to stop it coming back.
3
Water takes the path of least resistance, so radiators nearest the boiler get more than their share. Balancing means adjusting the lockshield valve on each radiator so the flow is shared properly. It is fiddly and takes a couple of hours across a whole house, but it costs nothing in parts and it is usually the difference between a cold back bedroom and a warm one.
YOUR SYMPTOM
Cold across the top, hot underneath
Hot at the top, cold along the bottom
Downstairs warm, upstairs or far rooms cold
Dirty water when you bleed, and kettling
COMMON QUESTIONS
Why is my radiator cold at the top but hot at the bottom?
Why is my radiator cold at the bottom but hot at the top?
How do I bleed a radiator?
What does balancing radiators mean?
Why are my upstairs radiators cold when downstairs is fine?
Do I need a power flush?
BEFORE YOU GO
More of the same, written by the engineers who do the work.