Intermittent problem with Potterton Puma 80E

My Potterton combi boiler (Puma 80E) has been behaving itself...until now.

Symptoms are:

1. Working pressure has increased from a steady 1.5 bar to 2.5 bar over the last two weeks

2. DHW works fine...boiler does not fire for CH reliably. I have checked the house thermostat settings (not too low). I have switched mains power off and on again, which has "shocked" things back into life, but surely this isn't right...? The boiler will run for a few minutes, then switch itself off and not fire again, although this is not always the case, i.e. sometimes it does, sometimes it doesn't - even with central heating set to constant.

3. Switched system off for 20 minutes. Bled radiators. Some air, but not a lot. No effect on system pressure.

Any ideas? An engineer should be on the way later today, but just wanted to get some input

Do you have a dripping HOT tap - even slightly.
These have a very sensitive diverter valve on them.

Thanks for the reply.

We did have a problem with a dripping hot tap a couple of years back and the plumber went round re-washering the basin and bath hot water taps. However, there are no dripping hot taps now.

Any idea why the system operating pressure has started rising? Should I be worried? PRV has not activated and there is no drip from exit pipe.

This boiler has started to do our heads in...

Check the pressure in the expansion vessel. Car type (schrader) valve.
Should have air in it approx 0.8 bar, if water comes out - its fecked

if the pressure is increasing either

the fill loop is passing

dhw heat ex is fecked

preaaure should be 1 1.5 cold

These are all getting a bit old so have several things showing signs, all mendable.

The pcb is likely to be giving trouble, quite possibly intermittent. You could check the solder joints on the top left corner, but they get failures all over.
The Pressure vessel pressure, as above (see faqs). 1 to 1.5 is normal. There isn't a heat exchanger which can leak from tap water to heating on these, but you could try disconnecting the filling loop.

And the verdict is....

Faulty diverter valve/wax capsule. Replacement ordered for fitting Thursday.

PCB fault was ruled out because of intermittent nature of fault. According to the engineer, a PCB problem would have manifested itself constantly.

He had good words about this forum though...

Thanks to all who replied.