CH startup problem on Potterton promax 15HE

Hi folks, can anyone assist with some advice on a recent problem with the above boiler which keeps rearing it's head every now and again. The boiler is around 4yrs old, Potterton promax 15HE controlled through a Potterton EP2002 programmer.

Whilst the hot water system is doing fine, turning the central heaing on has turned into a bit of a lottery. The room stat clicks and you can hear a switch inside the boiler click but then thats it. the fan doesn't start, the boiler doesn't fire. Conversely if you switch CH and HW on at the same time and then turn the HW off, the CH generally seems to keep working?

I had a CORGI guy out today and he was scratching his head. he took the cover off, placed an electrical tester on the live switch wire inside and it was giving a live when the CH was switched on but then nothing... no boiler activity. When the same terminal was receiving a live from the HW side it was firing every time. Essentially he said the programmer sends the live signal to the same terminal in the boiler so couldn't understand why it wasn't working.

He told me it would be better if the boiler was broke then he would know what to fix! i'M £40 lighter but still no further forward....

any ideas gratefully received....

The promax incorporates a flow switch so the boiler only fires if it detects sufficient flow. A partial blockage in the heating circuit could cause this problem.

Nice name!


Did he measure the actual voltage?