What balancing actually is
Hot water takes the path of least resistance, so without adjustment it tends to rush through the radiators closest to the boiler and barely reach the ones furthest away. Balancing means adjusting the small brass valve at one end of each radiator, the lockshield valve, to slightly restrict the fast radiators and even out the flow, so every radiator gets its fair share and warms at a similar rate.
It is different from bleeding. Bleeding removes trapped air; balancing shares out the flow. A system can need both.
What you need and the basic method
Balancing is fiddly but doable with patience. You need a way to measure radiator temperature, ideally two thermometer probes or a digital thermometer, and to remove the plastic cap on the lockshield valve. In short: bleed all radiators and top up the pressure first, then turn the heating on and note the order the radiators warm up. The first to heat is usually closest to the boiler.
Working from that fastest radiator, you slightly close its lockshield valve, then measure the temperature difference between the flow (valve) end and the return (lockshield) end, aiming for a consistent drop of around 11 to 12 degrees across each radiator once settled. You repeat down the line, opening the furthest radiators more and restricting the nearest, until the whole system heats evenly.
When it is not a balancing problem
Balancing helps when radiators are all working but heat unevenly. It will not fix a single radiator that is cold at the top (that is air, so bleed it), a radiator cold at the bottom (that is sludge, covered in radiator cold at the bottom), a failed pump, or a system so sludged that no amount of balancing helps.
Getting it done properly
Balancing a whole house well takes time and a bit of trial and error, and it is easy to end up chasing your tail. If you would rather have it done right, or if balancing does not fix the uneven heating, it usually points to something else worth an engineer looking at. LMB covers Bridgend and South Wales for heating that will not warm evenly. Message Lloyd with what your radiators are doing.
