ASD (revised 12/20) 18 ANSI/AARST SGM-SF 2017 (with 1220 revisions) Soil Gas Mitigation Standards for Existing Homes 6.3.7 Sizing for gutter downspout duct materials A natural reduction to airflow capacity shall be accounted for when using gutter downspout in accordance with Section 6.2.5 Exception 2. Because crimped downspout flange connections reduce airflow capacity, gutter downspout materials employed shall be: a) no less than 3 x 4 inch downspout material (75 x 100 mm) to meet capacities stipulated in Section 6.3.1 for 3-inch (75-mm) ID pipe; and b) no less than 4 x 5 inch downspout material (100 x 127 mm) to meet capacities stipulated in Section 6.3.2 for 4-inch (100-mm) ID pipe. When even larger air volume needs are indicated, duct size shall increase respectively. Gutter downspout material that is less than 3 x 4 inches (75 x 100 mm) shall not be permitted unless, in accordance with Section 6.3.3 , PFE Analysis indicates that the necessary airflow for the entire system is equal to or less 40 cfm (1.1 m 3 /min). 6.3.8 Air velocities Informative advisory 1 — Maximum airflow speed Air speed/velocity within duct piping should be less than 2,000 feet per minute (fpm) (610 m/min) to prevent condensed vapor from being drawn upward within piping rather than naturally falling by virtue of gravity to its intended destination (e.g., soil). In addition, air rushing noise will usually reach an objectionable threshold at ASD exhaust locations with air velocities of about 1,600 fpm (488 m/min) or faster. • Air velocities of 1,600 fpm (488 m/min) result when air is driven at: a) 80 cfm (2.3 m 3 /min) through: 3-inch (75-mm) pipe or 3 x 4 inch (75 x 100 mm) downspout b) 140 cfm (4.0 m 3 /min) through: 4-inch (100-mm) pipe or 4 x 5 inch (100 x 127 mm) downspout c) 315 cfm (8.9 m 3 /min) through: 6-inch (150-mm) pipe or 5 x 6 inch (127 x 152 mm) downspout d) 35 cfm (1.0 m 3 /min) through: 2-inch (50-mm) pipe or 2 x 3 inch (50 x 75 mm) downspout Informative advisory 2 — Minimum airflow speed Air speed/velocity at the system exhaust that is much lower than 300 feet per minute (fpm) (92 m/min) should be considered a benchmark that might indicate: — more work to enhance PFE may be needed to achieve mitigation goals; — enhanced expectations of exhaust icing or freeze-up during extreme cold weather; — expectations of premature fan failure, if the cause is poor PFE ; or — a high vacuum fan is not aiding PFE but may cause adverse effects, such forcing liquid from U- tube manometers or forcing sand under the building out the exhaust of a system. • 300 fpm (92 m/min) equates to: a) 20 cfm (0.6 m 3 /min) through a 3-inch (75-mm) pipe; b) 26 cfm (0.7 m 3 /min) through a 4-inch (100-mm) pipe; c) 60 cfm (0.7 m 3 /min) through a 6-inch (150-mm) pipe; and d) 9 cfm (0.2 m 3 /min) through a 2-inch (50-mm) pipe.