Jet cowls offer an advantage over standard round cowls in that expelled air is directed vertically rather than laterally - as is common with most roof terminations. This decreases the possibility that exhaust fumes present a nuisance to neighbouring buildings. Using a jet cowl often means a vertical duct can be shorter than it would otherwise need to be with a standard cowl.
Rainwater is caught in a dish within the cowl and is fed out through a drainage tube.
(Sizes 100mm to 150mm no longer include the tube; the inner dish is inverted and rainwater is directed into the outer channel and drainage holes)
Jet cowls are designed to connect directly to spiral duct tube.