Figure 8 This type of system is the most economical to install. However, it usually has poor airflow performance because of static pressure losses at the furnace plenum. These losses are due to poor or too numerous duct fittings at the plenum. (Figure 8). Ductwork installed in unconditioned spaces must be insulated. Reducing Radial Duct System A reducing radial (Figure 9) system uses several larger ducts from the main supply plenum that feed into smaller ducts as they get closer to the supply registers. This reduces the number of connections at the supply plenum, lessens static pressure losses and reduces the amount of duct used. Extended Plenum The extended plenum duct system (Figure 10) uses a large trunk duct that does not change in size as it travels the entire length of the building. The main trunk duct is considered an extension of the plenum. Accurate design 10 National Oilheat Research Alliance is critical in all systems. Extended plenum systems however, are less forgiving than others and require particularly accurate design. The extended plenum works best if the furnace is in the center of the home. Figure 10 Recommendation for laying out an extended plenum system: • The supply and return ducts should not extend more than 24 feet from the furnace or air handler • The first branch should be at least 18 inches from the beginning of the main duct. This helps achieve the best balancing of the branch lines • The main trunk should extend at least 12 inches past the last branch connection Reducing Extended Plenum In a reducing extended plenum or reducing plenum system, (Figure 11) the main trunk line will reduce in size as branch lines are connected. Typically, plenum trunks will reduce Figure 11 Figure 9
NORA Advanced Air Flow
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