this is how it is regarding bbus and fires left in as i said earlier BAXI dont recommend it but it can be done we take the risk if we leave it working and before someone says but glowworm do there is more chance of finding a Bermuda this was the biggest selling boiler of all time so chances are it will be one of these,you do not need to drill holes in the heat exchanger this only applies to solid fuel and thats Dodgey ground before we start.....anyway controls must be left in but disabled,gas capped,electrics dead,and drained of water,and the flue liner must be up to another 15 years use that raises another thread in its own right some of you are wondering why must the controls be left in? because the fire/bbu was designed to take it air for combustion trough the controls being in place remove them and its not as the manufacturer designed,think of a outset gas fire air reliefs are designed to be a specific size for? the flue to work properly its no different with a back boilers controls and regarding ventilation if the fire is under 7 kw (check the data badge)it does not require ventilation if the back boilers disabled