An Experimental Study and Analysis on Vent Cap Performance

Air is removed from residential buildings for different reasons such as heat, odor, or moisture removal. The air is usually removed with exhaust fans connected to duct work that lead outside the building. At the interface between the ducting and the ambient, there are vent caps that are used to allow air to flow out the building but not into it. These vent caps can significantly contribute to the pressure that the exhaust fan must overcome to remove air from the building, which means the exhaust fan must consume more energy to run. Unfortunately, vent cap performances are not currently evaluated because there is no establish method to test them. For this reason the objective of this research was to develop a vent cap performance evaluation method that may be used for a variety of vent cap types, designs, and sizes. The evaluation method was then implemented to determine the variance among vent cap designs, determine the evaluation method's repeatability, and to create a method to estimate the vent cap's performance at a specific air flow rate. An air flow chamber with an extension arm was used to test the vent caps. The extension arm is U-shaped to allow the adjustment to the three different orientations that are needed by the three vent cap types. The extension arm also has static pressure taps to measure the pressure drop across the vent cap. The pressure drop was also used to calculate a nondimensional parameter known as a loss coefficient to facilitate performance comparison. From this research it was concluded that the evaluation method can be implemented on the three vent cap types and a variety of designs and sizes. The repeatability of the evaluation method was confirmed by comparing two trials for two different products. The evaluation method was used to confirm that the variation in the products is significantly larger than the uncertainty in the measurements, which were 0.0077 in. w.c. for the pressure loss and 0.06 in the loss coefficient. It was also shown that the loss coefficients can be used to estimate the vent caps' pressure drop at high air flow rates