Webasto DUAL TOP RHA 100 Installation Instructions Manual page 32

Combustion air supply
NOTE
The intake opening for combustion air must be located so that it cannot
become clogged with dirt or snow. See Fig. 26.
Fig. 26: Avoid the line becoming clogged with dirt
NOTE
The intake opening for combustion air must not point in the direction of
travel. See Fig. 27.
Fig. 27: The line must not end pointing towards the front of the
vehicle
30
Combustion air intake hose should point downwards from the heater. If
this is not possible, make a condensate drain hole with a diameter of 4 mm
at its lowest point. See Fig. 28.
Fig. 28: Prevent the formation of condensate
Dual Top