Suburban Furnace exterior vent types (with pic)

I put appliance manufacturer's properly sized bug screen prohibitions in the same class as Onan's prohibition of exhaust addons like the Gen-turi. It's a "just in case" CYA clause that gives them an out when someone DIY's too fine a screen or adds too restrictive an exhaust extension, and then makes a warranty or damage claim. In Onan's case, their own extensions are longer and more restrictive than the Gen-turi elbow. Think "lawyer speak"...