Troxler ICO 4740, Manual Of Operation And Instruction, Page: 51 / 171

ICO 3-17
Figure 10: External Duct Cover (Open Design)
A design that is not open (e.g., one that incorporates louvers)
will create back pressure on the exhaust line (see the figure
below).
Figure 11: External Duct Cover (Closed Design)
NOTE
If a screen is added to keep out birds, etc., it needs to
be cleaned periodically or soot will build up that will
eventually cause back pressure.