Sizing a Replacement System for a Tight, Well-Insulated House
A 2,400 sq ft single-story house in Climate Zone 4A has been retrofitted with spray foam insulation and new double-pane low-E windows. The homeowner complains that the existing 5-ton single-stage air conditioner short-cycles and does not dehumidify well. A Manual J load calculation using the latest ACCA approved software yields a total cooling load of 24,000 Btu/h and a sensible load of 18,000 Btu/h at the design conditions of 95°F dry-bulb and 74°F wet-bulb outdoors, and 75°F dry-bulb and 63°F wet-bulb indoors. The duct system is in the conditioned space and has a measured static pressure of 0.5 in. w.c. at the air handler. The homeowner wants a system that will be comfortable and energy-efficient, and is open to either a single-stage or two-stage unit. The contractor must select equipment using Manual S.
1. Calculate the required sensible heat ratio (SHR) and determine the total capacity and sensible capacity that the selected equipment must provide at the design conditions. Explain how you would use Manual S to select an appropriate unit.
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First, compute the Sensible Heat Ratio (SHR): SHR = Sensible Load / Total Load = 18,000 / 24,000 = 0.75. This means 75% of the cooling load is sensible and 25% is latent. The equipment selected must have a capacity that meets or slightly exceeds the total load (24,000 Btu/h) and a sensible capacity that meets or exceeds the sensible load (18,000 Btu/h) at the design conditions. However, Manual S requires that the equipment's sensible capacity at the design conditions be within 100% to 115% of the sensible load (or per ACCA guidelines, the total capacity within 100% to 115% of total load, and sensible within 100% to 115% of sensible load). A unit that is oversized in sensible capacity may not remove enough latent heat. Therefore, look for a unit with a rated total capacity of about 24,000–27,600 Btu/h and a sensible capacity of about 18,000–20,700 Btu/h at the design conditions. Use the manufacturer's expanded performance data to find a unit that matches the SHR of 0.75 as closely as possible. A two-stage unit may be beneficial because it can run at lower capacity most of the time, improving dehumidification and comfort. Also, verify that the selected unit's airflow is compatible with the duct system (0.5 in. w.c. static pressure is acceptable for many residential systems, but check the fan curve). Finally, ensure the unit's latent capacity (total minus sensible) is at least 6,000 Btu/h (24,000 - 18,000) to handle the latent load.
2. The contractor considers a 3-ton single-stage unit with a rated total capacity of 36,000 Btu/h and a sensible capacity of 26,000 Btu/h at the design conditions. Using Manual S criteria, evaluate whether this unit is acceptable. If not, what would you recommend?
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The 3-ton unit has a total capacity of 36,000 Btu/h, which is 150% of the total load (36,000/24,000 = 1.5). The sensible capacity is 26,000 Btu/h, which is 144% of the sensible load (26,000/18,000 = 1.44). Manual S typically requires that the selected equipment's total capacity be no more than 115% of the total load and the sensible capacity no more than 115% of the sensible load (some versions allow up to 125% for certain conditions, but 115% is a common threshold). Here, both exceed 115%, indicating significant oversizing. Oversizing leads to short cycling, poor dehumidification, and comfort issues. Therefore, this unit is not acceptable. Instead, recommend a 2-ton (24,000 Btu/h) unit or a two-stage unit that can operate at a lower capacity. A two-stage 2-ton unit might have a low-stage capacity of about 70% (16,800 Btu/h) and a high-stage of 24,000 Btu/h, which would better match the load and provide longer runtimes for dehumidification. Also, check the sensible heat ratio of the unit: if the unit's SHR is higher than 0.75, it may not remove enough moisture. The contractor should select a unit with an SHR close to 0.75, possibly with a lower sensible capacity or enhanced dehumidification features.