I understand your post but you seem to think that by getting the system "working" it will be to spec and safe. I'm not having a go but I would seriously contemplate getting someone else in to rectify the situation as the installers haven't filled me with confidence a) on their install and b) on their ability to find the issue. We take safety very seriously on this site and would hate for our members to come a cropper when this can be avoided. All I would say is please don't trust the installers that they are qualified I have seen more builder thrown in dogs dinners unvented cylinders than I care to mention. Ask to see there certification if they are qualified they won't mind, I know I wouldn't.