One possible chain of events, with big flue leak into air intake:
- boiler starts up, fan runs on purge cycle,
- after n seconds, gas valve opens, ignition cracker operates and burner fires,
- flue gases recirculate and fire goes out due to lack of oxygen,
- boiler tries to restart and fan runs for n second purge. But because the air / gas/ combustion products mixture is recirculating, only a small amount of extra air from outside gets mixed in.
- gas valve opens again, cracker operates.
But this time the combustion chamber and flue are still full of unburned gas / air mixture. BANG!
- boiler starts up, fan runs on purge cycle,
- after n seconds, gas valve opens, ignition cracker operates and burner fires,
- flue gases recirculate and fire goes out due to lack of oxygen,
- boiler tries to restart and fan runs for n second purge. But because the air / gas/ combustion products mixture is recirculating, only a small amount of extra air from outside gets mixed in.
- gas valve opens again, cracker operates.
But this time the combustion chamber and flue are still full of unburned gas / air mixture. BANG!
