Generally speaking, the most common problem area found in kitchen exhaust systems is horizontal ductwork. While some systems consist entirely of vertical ductwork, which allows for lesser buildup of grease and residue, in most cases there is at least some length of ductwork running horizontally. This creates greater surface area where grease can more readily collect, in places where it is more difficult to remove.
If you are unsure of the configuration of your system, there’s an easy way you can check for yourself whether it contains horizontal ductwork. Determine the number of exhaust fans your system has on the roof. Then remove the filters from your kitchen hood and see how many plenum duct openings you have. If you have more duct openings than fans, then you have horizontal ductwork joining the passages on their route to the roof.
Horizontal Ductwork Diagram