Potterton Netaheat Profile 60e banging when firing up?

When the boiler 1st fires up it sometimes makes a loud bang as it does so - assuming due to extra build up of gas. Any ideas what can cause this?
I though it might be either piezo not firing 1st time so extra gas has accumulated or problem with over-supply of gas.. Boiler runs well after that.
Any ideas??
Andrew T, December 2013


done
clean flame detector sensor or change it ,it stil problem change black box pcb

ltd, December 2013
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