New 4-ton system, 1,600 CFM design.

Key issues:

1. Undersized return: A single 14-inch flex return is inadequate for 1,600 CFM. ACCA Manual D recommends ~1,000–1,200 CFM max for 14-inch flex at reasonable velocity. You need at least two 16x25 grilles or a larger duct (18–20 inch).

1. Flex duct installation: 8-inch flex stapled every 6 feet without proper support creates compression and restrictions. SMACNA standards require supports every 4–6 feet with the flex fully stretched and supported on both sides of each hanger. Stapling to trusses often crushes the inner liner.

1. Hard turns: Sharp bends around the ridge increase static pressure significantly. Use long-sweep elbows or increase bend radius to ≥1.5x duct diameter.

1. Supply duct sizing: Eight 8-inch runs may be undersized depending on length and fitting losses. Total effective length likely exceeds design parameters, pushing static to 1.1" w.c. (typical max is 0.8–0.9").

Immediate fixes:

• Replace 14-inch return with dual 16x25 grilles or upgrade to 18-inch flex.
• Re-support all flex ducts properly, ensuring full extension and no crushing.
• Install turning vanes or long-sweep elbows at ridge penetrations.
• Verify total effective length calculations match blower curve.

This system is likely overheating the blower and reducing efficiency. Consider hiring a certified HVAC technician to rebalance and verify airflow.

You have three compounding errors: undersized supply ducts, an undersized return, and poor flex installation.

1. Supply Ducts Undersized (Primary Issue)

• Design: 1,600 CFM / 8 runs = 200 CFM per run.
• Capacity: Per ACCA Manual D / SMACNA, an 8-inch flex duct at 200 CFM has a friction rate of ~0.18–0.22 in. w.c. per 100 ft (exceeds the typical 0.10 design budget).
• Effective Length: Your runs are 25–40 ft physical. With "hard turns around the ridge" (count as 2–3 elbows each ≈ 30–50 ft equivalent length each) and poor support (sagging adds 20–40% effective length), your Total Effective Length (TEL) per run is likely 80–120+ ft.
• Result: Supply side drop alone is 0.15–0.25+ in. w.c. per run.

2. Return Severely Undersized

• Filter Grille: 16x25 = 250 sq. in. At 300 FPM max face velocity (Manual D), max flow = 520 CFM. You need 1,600 CFM.
• Return Duct: 14-inch flex ≈ 550 CFM capacity at 0.10 FR.
• Result: Return side static is likely 0.4–0.6+ in. w.c. (filter + duct). This starves the blower and kills total airflow.

3. Installation Quality (Manual D §4, IMC §603.5)

• Stapling every 6 ft on 25–40 ft runs allows sag > 1.5 in. per span (max allowed is ½ in. per ft / 2.5 in. per 5 ft). Sag increases friction drastically.
• "Hard turns" around ridge without hard elbows or radius bends crush the liner, spiking turbulence/pressure drop.

The Math: Supply (0.20) + Return (0.50) + Coil/Filter/Grilles (0.30) ≈ 1.0+ in. w.c. Matches your 1.1 reading. Blower is likely moving ~1,100–1,200 CFM, not 1,600.

Fix (Requires Licensed HVAC Contractor):

1. Return: Replace with two 16x25 filter grilles (or one 20x30 + 16x25) fed by two 14-in or one 18-in hard-pipe/insulated flex runs. Target < 0.15 in. w.c. return static.
2. Supplies: Re-run as 9-inch or 10-inch flex (9-in handles 200 CFM at ~0.