National Oilheat Research Alliance 19 Airflow at registers and grilles can also be calculated with anemometers or velometers by measuring the average airflow velocity and multiplying it by the free area of the register or grille. The free area of a grille can be found in the manufacturer's literature. If the manufacturer’s information is not available, a good “rule of thumb” is to estimate the free area as approximately 60% of the overall grille. For example, a 10" x 6" grille provides 42 sq. in. of free area. Duct Leakage Testing Increasingly, local codes are requiring duct leakage testing at the time of a new complete system installation (Figure 17). Leaks in ducts can add to the operating cost of any heating or cooling system. For example, drawing in cold air from an attic, which is then subsequently heated by the furnace, will reduce the temperature rise and require the furnace to run longer. (Figure 18). Leaks in the supply ducts will reduce the heated air delivered to the living place. Reducing duct leakage therefore increases total system efficiency. Prior to testing, the system should be started to confirm airflow at the return and that the system is supplying air to all registers. Once these preliminary checks are made, the duct leak test can be done. Duct leakage is determined by blocking all duct outlets, pressurizing the duct and measuring the amount of air that is required to maintain that pressure. As air leaks out of the Figure 17 Figure 18
NORA Advanced Air Flow
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