Central heating packed up

Hi - hope someone can give me a pointer or two. I have a conventional open vented heating system with a cylinder, pump, air separator and 3 port valve in the airing cupboard and conventional boiler in the kitchen.

Suddenly the heating, which has been working, isn't. The hot water still is working. There is just no call for heat at the boiler as the red light is on steady. The thermostat, which is only a month old is up high (and as I say has been working OK). The pump is OK. Controller seems OK too. Have bled rads, not much air in them.

My guesses are - thermostat or other wiring problem, port valve problem (although the lever seems to move as it should).

Any other theories or things a plumbing novice can try?
many thanks
Rob

The motor in the 3 port valve has more than likely had it.

Hi Rob, as said the 3 port needs investigating, read the FAQ's, I had similar probs, removed actuator as described, by pressing black button on the side, and found I couldn't move the spindle with my fingers, bit of wd40, and wiggle back and forward(with pliers) did the trick, beware it only moves a few degrees, don't force it... good luck

You need to follow the circuit with a test meter!

Tony