thanks for those links, now I know for sure I don't wanna buy one.
from the installation manual
The total height of exhaust pipe should be more then 2.3 meters (92") and the horizontal length shall not exceed 3 (120")meters. There shall be a vertical ascending flue pipe and the hole should not be less then 250mm (10") above the exhaust pipe cover. The gap between the flue pipe and the hole should not be filled with cement, otherwise repairing would become difficult.
I do not have enough room to install a 2.3 meter long exhaust pipe, my buses inside height in the center is only 1.8 meters and the heater would be mounted on the outside wall close to the roof height, so to comply I would have to install about a 2 meter piece of flue pipe sticking up above my roof. Thats going to look funny.