Condensing boiler

[Coherers]

Horizontal flue ... to outside wall (3M) would be a pain and would discharge into a narrow alleyway between houses, but probably clear of opening windows.

My neighbour had a boiler installed a few years ago, with the flue discharging into the quite wide side passage between houses. Although the flue was installed according to the relevant British Standard (BS 5440?) / manufacturer recommendations etc, we frequently get the fragrance of "combustion products" in the second-floor bathroom coming in via the window. This has been a sore point ever since. What we wanted the neighbour / installer to do was to extend the flue up the wall to discharge the vapour plume above the windows. However, to no avail. Litigation has been considered!
Pity I don't have you as a neighbour, Ed !

Jeremy