Potterton Apollo 50B

[JAPlum]

Hi there.
I have a apollo 50B and out of the blue,the boiler has started making a loud ticking noise after the boiler has been running for about 1,2hours?for about 10-15 sec and then the boiler cuts of and you have to switch the programmer on again.
The pump is running fine,it happens in high or low setting,also the 3 port valve is working correctly to?
Please can anybody help?

 

[bengasman]

Open vent, so good chance of crud/blockage/dying pump