Duct Static Pressure Budget for a New 2.5-Ton System
A contractor is designing a new duct system for a 1,600 ft² single-story house with a 2.5-ton variable-speed air handler rated 1,000 cfm at 0.50 in. w.c. external static. Manufacturer pressure allowances are 0.10 in. w.c. for the filter, 0.18 for the wet coil, 0.03 for supply grilles, and 0.03 for return grilles. The longest supply path has an equivalent length of 180 ft; the longest return path has 120 ft. The contractor proposes a 16 in. round main trunk at 1,000 cfm and 6 in. round branch ducts at 120 cfm each. The homeowner wants quiet operation, and the contractor promises a design friction rate of 0.10 in. w.c. per 100 ft. The layout includes four 90° elbows and a 12 ft flex connector on the longest supply run.
1. Calculate the available static pressure for the duct system and the friction rate for the longest supply-plus-return path.
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Start with the air handler’s rated external static pressure of 0.50 in. w.c. Subtract the pressure drops that are not part of the duct run: filter 0.10, wet coil 0.18, supply grilles 0.03, and return grilles 0.03. The available static pressure for the duct system is 0.50 − 0.10 − 0.18 − 0.03 − 0.03 = 0.16 in. w.c. The longest total equivalent length is 180 ft supply plus 120 ft return, or 300 ft. The design friction rate is therefore 0.16 × 100 ÷ 300 = 0.053 in. w.c. per 100 ft. That is much lower than the contractor’s promised 0.10 in. w.c. per 100 ft. The proposed layout does not support that promise unless equivalent length is reduced or available static pressure is increased.
2. Evaluate the proposed 16 in. trunk and 6 in. branch velocities.
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A 16 in. round trunk has an area of about 1.40 ft², so 1,000 cfm gives roughly 716 fpm. A 6 in. round branch has an area of about 0.196 ft², so 120 cfm gives roughly 611 fpm. Those velocities are not automatically unacceptable for residential work, but velocity alone does not prove the design meets the friction-rate budget. At the calculated 0.053 in. w.c. per 100 ft, a 16 in. trunk at 1,000 cfm is likely too small, especially with four elbows and a flex connector. A 6 in. flex branch at 120 cfm may also exceed the low friction rate. The contractor should use a duct calculator with the actual equivalent lengths and flex multipliers, and likely select a larger trunk and 7 in. branches or reduce the equivalent length with fewer or gentler fittings.
3. If the contractor insists on the 16 in. trunk, what is the likely result and what alternatives would you recommend?
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The likely result is higher-than-designed static pressure, lower delivered airflow, more noise at the air handler and registers, and possible coil freezing or high-temperature-limit trips. The blower may also draw more power or fail to maintain the requested airflow. Rather than simply increasing blower speed, reduce the system resistance. Recommend an 18 in. or 20 in. trunk if space permits, shorter and straighter flex runs, metal elbows instead of sharp flex bends, a larger filter grille or media filter, and a return path with lower pressure drop. Then verify the design with a room-by-room load and a duct calculation using the actual fitting equivalent lengths.