Is there an officially stated reason for that? The wall-vented balanced flue has become pretty much the norm for retrofit boilers here these days.
None that I know of... it used to be legal in vienna but not in Lower Austria (i.e. in some cases just the next town), now it's just the other way round... gas code is made by the utility company and they have some rather weird requirements. A lot of them seems to be centered around driving out any old gas appliances as soon as they have any right to force it (usual code reuirement: as soon as anything is changed all non-compliant appliances have to be replaced). It doesn't really seem to work out - almost all gas related fatalities that made it to the media in recent times were due to lack of regular servicing and cleaning of combi boilers or water heaters and in each account the names of the victims seemed to indicate Eastern european immigrants who have a reputation for working around codes often.
Side note on Ireland: as you might know, natural gas is odorized, so a leak can be detected easily. It seems every country uses different substances to achieve this and Ireland obviously uses the same stuff as Italy uses for bottled LPG... so all the cheap hostel kitchens really reminded me of Italy holidays (with the constant replacing of bottles all the kitchens in Tuscany I ever saw had a slight gas smell to them).
Ireland obviously jumped the natural gas wagon even later than Austria where the town gas production already declined in the 1960s, rather natural gas was refined to town gas, and from 1970 to 1978 pretty much the entire distribution was changed over to natural gas. At the same time almost all old gas appliances were scrapped as most manufacturers didn't provide natural gas jets for old their burners.
