Just dropped into one of our rentals, who have a boiler fault. Took them a week or so to let me know, and only then when I contacted them about another matter.....

Anyway, potterton Performa 24 Combi, about 10 years old.
It fires up the central heating unless it is off on teh boiler switch. Turn to hot water only, CH fires up. Likewise CH and HW. The mechanical timer makes no difference, neither does the roomstat.

Any ideas? Diverter valve?? clutching at straws, but never come accross this before (though I am certain it is not unique)
Depending on what needs doing I have the choice of a normal gasman, or a company who will fix whatever is wrong for one price (£200)
My hopes of it being the clock have been dashed

TIA
Clive

My combi had a problem when I turned the taps on the rads got warm but here was no central heating. That was the diverter valve

I had an odd problem when the microswitch in the motorised valve began to intermittently stick. It caused all kinds of odd behaviour. I guess it depends how your system is wired.

My stepson is an apprentice gas fitter I asked him and first thing he said was diverter valve.

Thanks chaps. Looks like diverter valve is the issue then (backed up by googling).

It usually is! They sometimes wake up with a little tap! At least our old boiler did- suspect it was hard water causing limescale build up. ..

To bring to a conclusion, the chap who does my safety certificates checked it over. And looked confused. Diverter valve was working correctly. Spent 20 mins on the phone to Potterton/baxi help desk, then went and got a new PCB. No, that didn't work. Back on the phone, and this time an engineer rather than a kiddie with a script/flowchart. Then an "aha!"

It was the switch that is activated by the diverter valve was stuck in the on position.

He returned the pcb, and got a new switch. Job done.

Apparently the switches are very rarely a problem. Bill was still painful......but he only charged me for an hour. I think I should have skipped Uni and become a gas engineer.