Worcester 24.i H/W & C/H problem

will fire up but can cut out at any stage between 1min and 10 if hw needed soon after may not fire up at all .if ch needed will struggle to fire although eventually fires at some point.horrible rattling sound sometimes heard. 3year old worcester 24icombi any suggestions appreciated

horrid rattle is usually the fan bearing gone especially if on hot water as the pump only runs on central heating. The fan does not tend to run slow until it gets to an unbearable din, but the min gas pressure can drift down low ,it should be checked at every service as with all gas boilers . Instructions on how to re adjust are in the manual and should only be done by a gas eng so cant explain here. If you look in the inspection window i bet the flame is small and slow to spread across the burners it should cross light in about 1 second (thats front to back)