Oversized Replacement in a Tight Duct System
A homeowner replaces a 20-year-old 2.5-ton (30,000 Btu/h) split system in a 1,600 sq ft house in a mixed-humid climate (design 95 F outdoor, 75 F indoor). The existing ductwork is R-6 flex in a vented attic, with a measured external static pressure of 0.82 in. w.c. at the old blower's rated airflow. The contractor proposes a 3-ton (36,000 Btu/h) unit 'for extra capacity' because the old system 'ran all the time.' The house has single-pane windows, R-11 walls, and a blower-door result of 6.5 ACH50. The homeowner wants the new system to 'feel colder and dehumidify better.' The quoted 3-ton unit has a nominal sensible heat ratio of 0.78 at 400 cfm/ton.
1. Evaluate whether the proposed 3-ton size is appropriate, and explain what the 'ran all the time' observation most likely indicates.
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The runtime complaint is not evidence of undersizing; a 20-year-old unit running continuously in design weather is expected, and it may also reflect low charge, dirty coils, or a failing compressor. Sizing must come from a room-by-room load calculation (Manual J or equivalent), not from the old nameplate. A 1,600 sq ft house with R-11 walls, single-pane windows, and 6.5 ACH50 in a mixed-humid climate will typically calculate well below 30,000 Btu/h total, often near 24,000 Btu/h. Upsizing to 3 tons makes the sensible-to-total ratio worse: at 400 cfm/ton the 3-ton unit delivers roughly 28,100 Btu/h total and about 21,900 Btu/h sensible, so at part load it satisfies sensible demand quickly and short-cycles, leaving latent load unmet. The correct action is to run the load calculation, select the smallest unit that meets the calculated load at design conditions, and verify latent capacity against the design latent load.
2. Given the measured 0.82 in. w.c. external static pressure, what must be resolved before or during installation, and how does duct static affect the sizing decision?
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0.82 in. w.c. is above the typical 0.5 in. w.c. rating for residential air handlers and indicates a restrictive duct system. High static reduces delivered airflow, which lowers total and sensible capacity and can freeze the coil or trip limits. Before commissioning, the contractor must measure static across the air handler and compare to the manufacturer's fan table, then correct the ductwork: enlarge trunk and branch sizes, reduce flex length and sharp bends, seal leaks, and verify return path sizing. Airflow must be confirmed at the selected tonnage, typically 350-450 cfm/ton, with a target that balances sensible and latent capacity. The sizing decision cannot be finalized until the duct system can support the selected airflow; otherwise a smaller unit at correct airflow will outperform a larger unit starved of air. Document pre- and post-installation static readings.
3. The homeowner wants better dehumidification. What design and commissioning choices support that goal?
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Favor a unit with a lower sensible heat ratio, or select a variable-speed/variable-capacity system that runs at low capacity for long periods. Target airflow near 350 cfm/ton rather than 400-450 to increase latent removal, provided the coil and static allow it. Verify the condensate drain and trap, and confirm the coil is clean. Consider a dedicated dehumidifier or a reheat coil if the calculated latent load exceeds the unit's latent capacity at part load. Commission by measuring supply and return dry-bulb and wet-bulb temperatures, calculating actual sensible and latent capacity, and confirming the indoor humidity stays below 55% RH at design part-load. Explain to the homeowner that a correctly sized, longer-running system dehumidifies better than an oversized one, and document the measured results.