Cold at the bottom is not an air problem
If your radiator is warm across the top but cold along the bottom edge, air is not the cause. Air rises, so trapped air makes a radiator cold at the TOP, which you fix by bleeding it. Cold at the BOTTOM is the opposite pattern and it points to something heavier than air sitting in the way: sludge.
Why the bottom goes cold: sludge
Central heating sludge is a thick, dark, iron-oxide muck that builds up inside radiators and pipework over the years, mostly from the slow corrosion of metal components inside the system. Because it is heavy, it settles at the bottom of radiators. That layer of sludge blocks the hot water from flowing across the base of the radiator, so the bottom stays stone cold while the top still warms.
Left alone it gets worse. Sludge does not just cool your radiators; it clogs pipes, wears out the pump, blocks the boiler heat exchanger and drags down the efficiency of the whole system. A sludged system makes your boiler work harder and shortens its life.
What you can try yourself
There is not a lot the average homeowner can safely do about sludge beyond confirming the symptom. You can feel the radiator when the heating has been on a while to confirm the cold band is at the bottom, and check whether more than one radiator is affected, which suggests a system-wide sludge problem rather than one bad radiator.
Taking a radiator off to flush it out at a hosepipe is possible but messy, risky if you are not confident with the valves, and it does not clean the pipework or boiler where sludge also hides. For most people the honest answer is that this one needs an engineer.
Why a power flush fixes it
A power flush pushes a high-flow cleaning solution through the whole system at mains-safe pressure, breaking up and carrying away the sludge from radiators, pipework and the boiler, then adds a corrosion inhibitor to slow it coming back. Done properly it restores full flow, so cold-bottomed radiators heat evenly again, the boiler runs cooler and quieter, and the whole system is more efficient.
LMB carries out power flushes across Bridgend and South Wales. For what it involves and the signs that mean you need one, see power flush costs and symptoms, or message Lloyd with what your radiators are doing and he will tell you whether a flush is genuinely needed or whether it is something simpler.
