Hi Brian
The bang you are getting from the boiler is from a build up of gas in the boiler burner chamber and when it ignites you get the bang as excess gas is ignites upon first light up
This could be two one of things
firstly it could be the ignition not operating correctly to light the gas this is usually because the electrode requires cleaning to allow a better spark however if this was the case the problem would be worse than mentioned
2nd option is that is a sticking and faulty gas control valve that occasionaly does not seat correctly and allows a small escape of gas through to burned when turned off thus when lighting you get the bang usually in this case you will also be able to smell gas around the outside flue,This is not an uncommon fault with this boiler
and just to add that if when you say it clicks some time but does not light you mean
the boiler tries to start ie you hear the fan come on then there is a click and the boiler is silent then the same thing happens thus boiler cycling then this would also cause same bang however fault in pcb control board again not an uncommon fault on this unit
needless to say as this bang is caused by excess gas escape for your safety please get repaired as soon as posible,if not sooner
tony, February 2008
