Bizarre central heating problem

Simon (Dark Angel) · 2009-03-12T12:27:31+00:00

Right, my central heating system consists of a Potterton Suprima 30 balance flue boiler and EP2000 programmer, with a Myson MPE322 3-port valve and a Myson compact CP35 pump.All was fine until yesterday, however now the boiler refuses to fire up when it's supposed to come on and the pump just seems to run continuously, even when the heating/water isn't programmed to be on.The only way to stop it is to turn the power off to the system at the fused spur plate, otherwise the pump will just continue pumping cold water round the system.Any suggestions? I did have a similar problem about 2 years ago, which I was told was probably due to the 3 port valve head (which also kept jamming itself onto either heating or DHW and required you to flip the ports override switch to get it to move). I changed the head and all seemed well, but now the system has started playing up again, though the head hasn;t been jamming like before.Could it be the 3-port valve head again, or perhaps something to do with the boiler or programmer?Thanks in advance, hope I've not left anything off the description (I'm quite handy with a screwdriver, but plumbing and heating is not exactly my forte).