Found fault on Ariston DIA 20
It was the Venturi it had a parcial blockage,i removed plastic pipe from Air pressure switch to venturi,I connected a manometer hose to Venturi a quick blow to clear the obstruction I reconnected original plastic pipe
And the boiler works fine now. (Ps.the burner stays on a lot longer now than it did previously only shut off now when temp is satisfied)
David, January 2014
