Boiler making a bit of a loud "bang" sound when it fires up

Hello! It could be a number of things causing the problem. I would advise you get it checked and serviced by a gas engineer. It could be that it needs some adjustment and a good clean, but might be something that could be a danger to yourselves. Best get it checked.

Hi it could be a few things gas valve or spark electrodes or spark ignition box. would advise to get a gas engineer to check it out as an elimination process will need to be gone through hope this helps

I agree with all the other engineers It needs to be sorted ASAP it will need checking as the other Engineers said with out looking it could be multiple things but it sounds like an explosive ignition.

My advice would be to cancel any pre-set timer demands for heating and water at the boiler immediately, either by way of a total shutdown of it's power supply at the boilers spur (power switch) Or by canceling all and any timer demands on the boilers timers it's self. For safety reasons, Under no circumstances do you want the boiler trying to fire up/ignite until it's been serviced/repaired & commissioned by a registered gas safe engineer. a worst case scenario. The combustion ratio could be offset and so could be producing fatal levels of (products of combustion) (carbon monoxide).