Second-floor hot bedrooms: oversized equipment versus undersized ducts
A homeowner in Worcester complains that the second-floor bedrooms are 5°F warmer than the first floor in July. The existing system is a 3-ton, 13 SEER split with a PSC blower, a 16 x 8 in. main trunk, twelve 6-in. branch runs, and a single 14-in. flex return. The equipment is in the basement; the longest supply run is 38 ft with four 90° elbows. A contractor measured 0.92 in. w.c. external static pressure with a clean 20 x 25 x 1 filter, 1,080 cfm total airflow, and an 18°F temperature split. A room-by-room Manual J calculation gives a total sensible load of 26,400 Btu/h and latent load of 5,400 Btu/h. The homeowner wants a larger 4-ton unit to push more air. The ductwork is buried in soffits and cannot be substantially enlarged.
1. Is the 3-ton unit correctly sized? Support your answer with the load and airflow numbers, and state what equipment selection you would recommend.
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The total load is 26,400 + 5,400 = 31,800 Btu/h, or about 2.65 tons, so a nominal 3-ton unit is not obviously undersized. But the measured airflow is only 1,080 cfm, and the 18°F split gives roughly 1.08 x 1,080 x 18 = 21,000 Btu/h of sensible capacity, well below the 26,400 Btu/h sensible load. The system is failing because it is moving too little air, not because it needs 4 tons.
I would not recommend a 4-ton unit. At 1,600 cfm it would need much more duct capacity than exists and would likely increase static pressure, reduce airflow further, and worsen humidity. I would select a 2.5- to 3-ton two-stage or variable-speed unit whose expanded sensible capacity at the actual entering conditions and 1,080 cfm meets the load, and I would verify the selection with manufacturer performance data, not nominal tonnage alone.
2. The duct system is at 0.92 in. w.c. and the return is one 14-in. flex. What is the likely cause of the high static and how would you correct it without a major rebuild?
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The 14-in. flex return is a likely major restriction. At 1,080 cfm, a 14-in. round duct is carrying roughly 1,000 ft/min, which is high for a flex return and creates excessive pressure drop. The 16 x 8 in. trunk is also small for 1,080 cfm, and four elbows plus flex add loss. The 20 x 25 filter is not the main problem at that airflow if it is clean and properly sized.
Without a major rebuild, I would add a second return path from the second floor, replace accessible flex with hard duct and long-radius fittings, and enlarge the return drop to at least 18 in. or equivalent rectangular area if the chase allows. I would also inspect for crushed flex, disconnects, and closed dampers. If the trunk cannot be enlarged, reduce airflow to the load and use a variable-speed blower, but do not simply increase blower speed to overcome a restrictive duct system.
3. What commissioning measurements prove the correction, and what if the customer insists on the 4-ton unit?
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Commissioning should include external static pressure with the final filter, total airflow by a reliable method, room-by-room supply airflow, return-path static, temperature split, refrigerant charge by subcooling or superheat as applicable, and duct leakage if accessible. The target is not a single static number; it is achieving design airflow without excessive static and with acceptable room temperatures.
If the customer insists on a 4-ton unit, I would document in writing that the existing ducts cannot support 1,600 cfm and that the installation would violate the equipment's airflow requirements. A 4-ton unit on this duct system would likely run high static, low airflow, cold coil, and short cycling or high humidity. I would offer a load-based 2.5- to 3-ton selection, duct corrections, and a written commissioning report, and decline to install the larger unit without the required duct upgrades.