One Way Wall.
Icon: The icon in the element list would be a green rectangle with arrows pointing downwards.
Description: "One way wall, air and particles can only move in the direction of the arrows."
This wall would only let air, and particles to flow one way through it, which is the direction of where the arrows are pointing. Users can change this direction with Cut/Paste (just do CTRL + X, CTRV + V, press R, then click).
Uses:
The wall is green (RGB 41, 204, 82), with dark grey arrows pointing in the set direction (set like FAN).
this is actually a good idea.....
Might be easier to implement four different types of wall, one for each direction.
Probably, but 4 new walls that do virtually the same thing seems too excessive.
Since the backbone for FAN already exists, it'd probably be relativly simple to impliment it, but I've never looked at the code before (and have a slight fear of C++), so anything I say is largley irrelavant.