The purpose of a stack discharge fitting is to keep rain out of a discharge stack. Although commonly used, cone caps have negative effects on the exhaust airflow path. Placing an inverted cone inside a cone cap helps somewhat but not usually enough. A vertical stack head (Figure 3) 25 mm (1") larger in diameter than the stack and four diameters long is recommended. It keeps almost 100% of rain out (rain usually falls at an angle) at virtually no dynamic loss other than that caused by the velocity head, C=1.0. Stack discharge diffusers are discussed below.