Air trapped in the top of radiator (cold radiator, at the top)
Solution: Bleed out the air (and re-pressurise the system if required)
Poor or no flow to radiator
Possible faults: faulty valve, system not balanced correctly, air lock, blocked pipes, underpowered boiler
Sludge in radiator system (as shown in picture below)
Problem: over time the system has corroded internally and the rust is carried around the system and often deposited at the bottom of radiators resulting in cold spots. Very basic solution: Remove radiator and manually flush with a hosepipe. The disadvantage to this radiator fix is that you are only flushing one component of the system. Best solution: ‘Power flush’ the whole system using a dedicated machine. Call James if you need help with this.