"obclare" wrote in message
m...
I have a problem with an Ariston a/23 combi boiler that has beaten
British gas and a handful of gas engineers.
The system fires up and immediately fires down again and fires up
again immediately and so on. Anybody any ideas? Various bits have been
replaced but to no avail. Very occasionally the system works.

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If this was a carburettor in a car I would say that there was a bit of loose
grit periodically blocking the main jet. Your problem might be similar - a
bit of loose grit or scale might be blocking the the main jet / injector.
Have you tried removing and cleaning the main injector / burner?

Cic.