High External Static Pressure and Deficient Outdoor Air in a Small Office
A 2,000 cfm packaged rooftop unit serves a 1,500 ft² open office with a design occupancy of 25 people. The design external static pressure is 0.60 in. w.c. With a clean filter, the commissioning technician measures 1.05 in. w.c. across the unit casing, and the fan curve predicts about 1,650 cfm at that pressure. The outdoor-air damper is set at its minimum position, the return-air damper is fully open, and a balometer measures 120 cfm of outdoor air. The local code adopts ASHRAE 62.1 and an energy code that requires economizer capability. The duct system consists of 180 ft of 20 x 12 in. trunk and 14 ceiling diffusers; several branch dampers are fully open. Occupants complain of stuffiness and odors by mid-afternoon, especially when the space is full. The building owner wants to increase fan speed to 'push more air' because some diffusers feel weak. The design outdoor-air fraction was not documented at commissioning.
1. Calculate the minimum outdoor-air rate required by the ASHRAE 62.1 ventilation rate procedure for this single-zone office. Compare it with the measured 120 cfm and comment on the outdoor-air fraction.
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For an office, use the people and area rates: Vbz = Rp × Pz + Ra × Az. With Rp = 5 cfm/person and Ra = 0.06 cfm/ft², Vbz = (5 × 25) + (0.06 × 1,500) = 125 + 90 = 215 cfm. For a single-zone system the ventilation efficiency can be taken as 1.0, so the required outdoor intake is approximately 215 cfm.
The measured 120 cfm is only 56% of the required rate, a substantial deficiency. At the measured total airflow of about 1,650 cfm, 120 cfm is an outdoor-air fraction of only 7.3%. Even at the design 2,000 cfm, the minimum damper position would need to deliver about 10.8% outdoor air, before considering economizer movement and damper linkage tolerances. The complaint of stuffiness is consistent with the measured outdoor-air shortfall, not with an occupant load that is unusually high.
2. Explain why the measured 1.05 in. w.c. external static pressure matters. What does it do to airflow, fan energy, and outdoor-air delivery, and why is increasing fan speed not the right first step?
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The unit is operating 0.45 in. w.c. above its design external static pressure, a 75% increase. On the fan curve, that moves operation from 2,000 cfm to about 1,650 cfm, a 17.5% airflow reduction. The cause could be undersized or restrictive ductwork, closed or misadjusted branch dampers, a dirty coil, excessive fittings, or a fan speed set incorrectly. High static also raises fan power and can increase noise and duct leakage.
Outdoor-air delivery falls with total airflow when the damper is fixed. If the minimum damper was set to deliver about 10% of 2,000 cfm, it would deliver about 200 cfm at design airflow; at 1,650 cfm it would deliver about 165 cfm, and the measured 120 cfm suggests additional damper, linkage, or pressure problems. Increasing fan speed would raise static pressure further and may overload the motor or worsen noise without fixing the restriction. The correct approach is to measure a static-pressure profile across filter, coil, and duct sections, find and correct the restriction, rebalance the branches, and then set airflow and outdoor air.
3. Outline a corrective sequence and the documentation needed to close out the ventilation and static-pressure deficiencies.
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First, measure static pressure at each component: return grille, filter, coil, supply plenum, and the longest and shortest duct runs. Compare the total and component drops with design. Inspect the duct system for collapsed flex duct, excessive length, sharp elbows, closed volume dampers, and disconnected or leaking joints. Clean the coil if fouled and confirm the filter is the correct size and pressure drop. Adjust fan speed only after the restriction is corrected, using the fan curve and motor nameplate limits.
Next, rebalance the 14 diffusers to design airflow, then set the outdoor-air damper to deliver at least 215 cfm. Verify with a balometer at the intake and, where possible, by a tracer-gas or airflow measurement that accounts for damper linkage. Confirm economizer operation and set the minimum position so it cannot fall below the required rate at actual total airflow. Document design airflow, measured total and outdoor airflow, static-pressure profile, fan speed, motor amps, damper positions, and the 62.1 calculation. Include a functional test of the economizer and a recheck after any filter loading or belt adjustment.