Gents
I hope this will help someone in the future, The problem is solved but I'm not sure why.
It was the pump.
I was sure it was working because the spindle was spinning (through the vent cap) and sounded good with the screwdriver to my ear. So sure was I, that changing it hurt, but...
Once the main burner goes out it stays out. If you switch the whole thing off and leave it for a while (about an hour) it will go through the sequence again.
Corgiman
Thanks for the input, according to Potterton it has a spark generator and a PCB albeit basic on this model. Strange that you've got something that doesn't exist in your van.
The above boiler starts up on room stat demand, the fan runs, the pilot lights and about twenty seconds later the main burner comes on. All is well for four about minutes or so (timed) Rads just start to get luke warm. Pump would seem to be working ( ie screwdriver to ear against body of pump...
Thanks for your time and trouble gents, been out the trade twenty four years now and obviously out of my depth. Corgi on way tomorrow.
Should the need arise. any ideas for a reasonably priced direct replacement I don't want to dismantle the house to change pipework, flues etc.
The boiler is a Potterton Netaheat MkIIF 16-22 with working Grunfos circulating pump. Boiler about twenty years old and working well til Thursday.
open vented system with expansion tank in loft (full) Venting highest rad produces greyish water at pressure. It does send hot water through all...
WITH ROOM STAT SET FOR DEMAND AND BOILER STAT MID RANGE WHEN THE BOILER SWITCHES ON THE PILOT LIGHTS, TWENTY OR SO SECONDS LATER THE MAIN BURNER IGNITES. RUNS FOR THREE OR FOR MINUTES THEN MAIN BURNER SHUTS DOWN. IT THEN SITS WITH FLUE FAN RUNNING AND PILOT LIGHT ALIGHT WITH NO MAIN BURNER WHILE...