What bleeding a radiator actually does
Over time, small amounts of air work their way into a sealed central heating system. Air is lighter than water, so it rises and collects at the top of your radiators. That pocket of air pushes the hot water down, which is why an air-locked radiator is warm at the bottom and cold across the top. Bleeding simply releases that trapped air through a small valve so hot water can fill the whole radiator again.
It is a genuinely useful thing to know how to do. A house full of half-heated radiators makes the boiler work harder, costs more to run and never quite feels warm. Ten minutes with a radiator key often fixes it.
How to tell a radiator needs bleeding
The classic sign is a radiator that is hot at the bottom and cold at the top when the heating has been on for a while. You may also hear gurgling or bubbling, or notice that a room never warms up properly while the rest of the house is fine.
One important distinction, because it changes everything: cold at the TOP usually means trapped air, which bleeding fixes. Cold at the BOTTOM usually means sludge and debris sitting in the radiator, which bleeding will not touch and which needs a power flush instead. If yours is cold at the bottom, read radiators cold at the bottom rather than bleeding it.
The step by step
One: turn your heating off and let the radiators cool. Bleeding a hot radiator risks scalding water and steam coming out of the valve, and it is easier to feel the cold spots once it has settled.
Two: find the bleed valve. It is the small square-headed valve at the top corner of the radiator. You will need a radiator bleed key, which costs a pound or two from any DIY shop, or a flat-blade screwdriver if the valve has a slot.
Three: hold a cloth or small container under the valve to catch drips. Slowly turn the key anticlockwise, about a quarter to half a turn. You should hear a hiss as the trapped air escapes. Do not open it fully.
Four: when the hissing stops and a steady dribble of water appears with no spluttering, the air is out. Close the valve firmly clockwise, but do not overtighten it or you can damage the seal.
Five: work around the house. If you have more than one radiator to do, start with the one downstairs furthest from the boiler and work toward the upstairs ones, as air tends to collect highest.
Check your boiler pressure afterwards (the step people forget)
Letting air and water out of the system usually drops the pressure. On a combi boiler, look at the pressure gauge on the front. If it has fallen below roughly 1 bar (into the red or below the marked band), the boiler may not fire until you top it up.
Topping up is done using the filling loop, the small silver braided hose under the boiler with a valve at each end. Open both valves slowly until the gauge rises back into the normal band, usually around 1 to 1.5 bar cold, then close them again. If you are not sure where your filling loop is or the pressure keeps dropping afterwards, that is worth a call rather than a guess.
When it is not just air
Bleeding fixes trapped air. It does not fix a radiator that is cold at the bottom (sludge), radiators that need constant re-bleeding (air is getting in somewhere, which points to a fault), a boiler whose pressure will not hold, or a system where several radiators stay cold after bleeding and balancing.
If any of that sounds like your house, the honest answer is that it needs a proper look. LMB covers Bridgend, the Vale and South Wales for exactly this kind of heating fault. Message Lloyd and he will tell you what is actually going on before anyone books a visit.
