Can anyone help with combi boiler- Vaillant turbomax plus 828E (VUW GB 282/2-5) about 8 years old. runs fine all day with heat exchanger cover off but will not run with cover on ! flame dies and then faults. replaced air flow switch and pump, Fan is running ok hot water and heating ok till cover replaced then........dies. gas pressures ok. flue seems ok! any ideas? thanks.