you can't do this the only thing you can do is adjust the burner pressure down and that will not be the solution your after on the profile there is no modulating gas valve simply burner on or off no low flame high flame unfortunatly
Richard boiler engineer
February 2010
Your plumber should be registered to mod the gas valve would contravene regs. to alter the boiler spec.
Could try weather compensation electronic control but would consider not worth the cost
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