Five-Ton Rooftop Unit at Reno Altitude Against a 72,000 Btu/h Load
A 2,400 sq ft retail suite in Reno, elevation about 4,500 ft, has a calculated cooling load of 72,000 Btu/h total: 61,200 Btu/h sensible and 10,800 Btu/h latent, for an SHR of 0.85. The owner wants to reuse a 5-ton rooftop unit rated 60,000 Btu/h at AHRI conditions, 1,600 cfm nominal, and 0.50 in. w.c. external static. The existing duct system was sized for 1,600 cfm, but the return is one 20 in. flex duct with two tight 90° elbows. The installing contractor says the 5-ton unit is “close enough” because Reno is dry and the load is mostly sensible. Design conditions are 93°F DB / 59°F WB outdoors and 75°F DB / 50% RH indoors. The owner wants your written opinion before paying for a load-calculation review or ordering equipment.
1. Is the nominal 5-ton capacity adequate for the calculated load? Show the comparison and explain the altitude effect.
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On nominal ratings alone, 5 tons is 60,000 Btu/h against a 72,000 Btu/h load, a shortfall of 12,000 Btu/h, or about 17%. That alone makes the unit questionable. The altitude issue makes it worse: the 60,000 Btu/h rating is a sea-level-style nominal rating, and at 4,500 ft the air density is roughly 17% lower. At the same cfm, the mass flow across the coil is lower, so sensible capacity falls unless the equipment is selected using altitude-corrected performance data. “Reno is dry” does not rescue the selection because the sensible load is 61,200 Btu/h and the coil’s sensible capacity is exactly what altitude erodes. A 6-ton nominal unit is not an automatic fix either, because it too must be corrected to 4,500 ft and checked at the actual entering-air conditions. The correct conclusion is that nominal tonnage is not a substitute for an altitude-corrected selection at the design conditions.
2. What does the 20 in. flex return suggest about the available static pressure, and how should that affect the equipment decision?
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At 1,600 cfm, a 20 in. round duct has about 2.18 sq ft of area, so the average velocity is roughly 734 fpm before allowing for the two tight elbows and flex corrugation. That is not automatically fatal, but flex with two 90° elbows has much higher friction than smooth round duct, and the return static will consume part of the 0.50 in. w.c. external-static budget. If the return and filter assembly use most of that budget, the fan will operate at lower airflow, which further reduces capacity and can freeze a coil or cause high supply-air temperature. Before accepting the unit, require a static-pressure calculation and a fan-curve check at 1,600 cfm and the actual external static. If the return cannot be improved, the equipment selection must use the airflow the fan can actually deliver, not the nominal 1,600 cfm.
3. What load-calculation, equipment-selection, and duct-verification steps should be completed before purchase?
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Require a room-by-room load calculation using actual envelope, window, occupancy, and ventilation inputs for the suite, not a square-foot rule. Then perform an altitude-corrected equipment selection at 93°F DB / 59°F WB outdoors and 75°F DB / 50% RH indoors, checking total and sensible capacity at the actual cfm. Verify latent capacity as well, even though the latent load is only 10,800 Btu/h, because a unit selected only for sensible capacity can still have a wet coil or humidity problem. For the duct system, calculate the return and supply static at 1,600 cfm; confirm the fan can deliver that airflow at the resulting external static. If the existing return cannot be modified, reduce the design airflow or select equipment that performs at the lower airflow. Put the assumptions, altitude correction, and static budget in the written opinion so the owner can compare equipment bids on the same basis.