I have a baxi eco 80 combi which if i turn the hot water tap on reset the boiler will fire up and work,but if i move the setting to ch and hot water then turn the tap off the boiler still tries to ignite but then cuts off .i have had an engineer up who as been at it two days and last night fit a new pcb board at a cost to me of £102 but stilll it wont work.There`s the ignition but no boiler on ch but it works on hot water....any ideas?