All:

I am posting this for general information:

I have a Grant external combi boiler which uses a Riello RDB1 burner.

After approx 2-3 years it failed and needed a new photocell.

The symptom: was that the burner motor ran but the igniter did not fire.
I suspect the oil pump was spraying oil nicely as it is the smell of
heating oil that caught my attention.

This has just happened again, 2 years later, similar symptom but this
time the boiler would fire if it was cold but would not refire as part
of the normal cycling when boiler output greater than required by
radiators/water.

When using a multimeter on a new photocell, the dark reistance is
greater than 2MOhm. On the duff one the reading is 50-80KOhm. Hence the
control box decided that the boiler had already ignited and did not need
a spark. Looking at data sheets for Light Dependant Resistors (LDRs)
most of them have a working temperaure range up to 75 degrees, I suspect
that because this is a combi so the internal cabinet temperature stays
quite warm (it has a heat store, i.e. a lump of water in there) it cooks
the sensor in a couple of years.

Hopefully this will be usefull to someone in the future, my plan is
always keep a spare photocell in the house ready to go.

I note that over the 5 years I have had the boiler Grant have made a few
variants.

To limp along for a couple of days I was manually switching off the
boiler but leaving the circulation pump on until the heat store
temperature was down to 20-30 degrees, then the boiler would refire,
switch off boiler supply when it switched itself off after 5-10 minutes.
The boiler has a mains feed to power the burner side and a feed to
power the heating circulation pump, simplified description as circ pump
also switched from boiler supply for hot water.

Lawrence.