3-Ton Packaged Unit Delivering 935 CFM at 1.10 in. w.c. Static
A 3-ton packaged gas/electric unit with a fixed-speed PSC blower is rated 1,200 CFM at 0.50 in. w.c. external static pressure. In the field, a digital manometer reads −0.42 in. w.c. in the return plenum and +0.68 in. w.c. in the supply plenum with the unit running and the filter in place. A flow hood sums 935 CFM across the supply registers. The filter is a new 1-inch pleated MERV 13. The return is a single 18-inch flex run serving a 1,600-square-foot house; the supply trunk is 16-inch hard duct with long flex branch runs. Supply air measures 56°F and return air is 76°F dry bulb / 62°F wet bulb. Suction pressure is 108 psig with 46°F suction line temperature on R-410A. The homeowner says the unit runs all afternoon and cannot hold 75°F on a 95°F day, even though the supply air 'feels cold.' A previous company sold a charge adjustment that did not help. The coil is clean and the blower is on the high tap.
1. Calculate the external static pressure and the delivered airflow as a percentage of nominal. Using fan-performance logic, decide whether the blower or the duct system is at fault, and name one additional test that separates them.
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Calculations:
- External static = 0.42 + 0.68 = 1.10 in. w.c. - more than double the 0.50 in. w.c. rating.
- Airflow = 935 / 1,200 = 78% of nominal, or about 312 CFM per ton against a 400 CFM/ton target.
- Delta-T = 76 − 56 = 20°F.
- Superheat = 46 − 33 ≈ 13°F (108 psig ≈ 33°F saturation).
Who is at fault: A PSC blower on high tap has a published fan curve. At 1.10 in. w.c. that wheel cannot move 1,200 CFM - the 935 CFM measured is essentially what the curve predicts. The blower is doing its job; the duct system and filter are imposing resistance the fan was never selected for. An 18-inch flex return for 3 tons is undersized (and flex has high friction), and a 1-inch MERV 13 pleat is a high-resistance filter in a slot designed for a basic filter.
Separating test: Pull the filter and re-read static and airflow. If static drops and airflow rises meaningfully, filter/return restriction dominates. If static stays near 1.10 with the filter out, the duct system itself is the constraint. Also measure static in the return boot versus the return plenum to locate where the loss occurs.
2. Explain how the high static produced these symptoms and why the previous 'charge adjustment' could not have fixed them. Include the effect on capacity and on the compressor.
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Chain of cause and effect: At 78% airflow, the coil sees less air than design. The refrigerant side still tries to evaporate a full 3 tons, so suction pressure drops and the coil runs colder than design - which is why the supply air feels cold (56°F) even though the house never catches up. The 20°F split looks acceptable, which is the trap: a temperature split alone hides low airflow because a starved coil produces a large split over too little air.
Capacity: Sensible capacity is roughly proportional to airflow × temperature rise. At 935 CFM instead of 1,200 the sensible output is down on the order of 20%+, so the unit runs continuously and loses ground on design days. Total capacity also falls as suction pressure drops and the compressor's mass flow decreases.
Compressor effect: Lower suction pressure means a higher compression ratio, higher discharge temperature, and reduced refrigerant mass flow for cooling the motor - all of which shorten compressor life and can nuisance-trip overloads.
Why charging was wrong: Superheat was near normal (13°F) because the TXV was doing its job; there is no charge fault to correct. Adding refrigerant to a low-airflow system raises head pressure without restoring capacity, and charging charts are invalid when airflow is off by more than about 10%. Fix airflow first, then evaluate charge.
3. State the corrective work in priority order and the measurements you would require to sign the job off.
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Corrective work, priority order:
- Reduce filter resistance: move to a low-pressure-drop filter appropriate to the cabinet, or upgrade to a larger media cabinet with more face area. Never solve airflow by removing filtration entirely.
- Enlarge or add return capacity: replace the single 18-inch flex with a properly sized return (or two returns) sized for 1,200 CFM at reasonable velocity and low friction; keep bends and flex length to a minimum.
- Straighten supply restrictions: shorten long flex branch runs, eliminate sharp turns and crushed sections, and confirm the trunk and branch takeoffs are sized for the delivered CFM.
- Only then revisit charge and blower tap - the correct tap is the one that lands rated CFM at corrected static, not automatically the high tap.
Sign-off measurements: external static at or near the 0.50 in. w.c. rating (wet coil, filter in place), airflow 1,150–1,200 CFM (about 400 CFM/ton), delta-T in the 18–21°F range at 62°F wet bulb, superheat/subcooling per the manufacturer's chart, and a room-side check that the space holds setpoint on a design day. Record static before and after - a homeowner-visible number is the best defense against another 'charge adjustment.'