Minimum Outdoor Air with Return-Fan Tracking and Economizer Lost Motion
AHU-5 serves a 20,000-sq-ft, 200-person assembly space. Design supply is 20,000 cfm; design outdoor air is 3,000 cfm based on ASHRAE 62.1 default occupancy of 5 cfm/person plus 0.06 cfm/sq ft. The AHU has a return fan, an economizer, and a relief damper. At minimum outdoor air with the economizer disabled, the supply fan reads 20,000 cfm by traverse, the return fan reads 17,900 cfm, and the relief/exhaust airflow is 2,100 cfm. The outdoor-air damper is 18% open; the minimum-position potentiometer is set at 2.0 V. Space CO2 is 1,100 ppm; outdoor CO2 is 420 ppm. Building pressure is +0.08 in. w.c. The economizer linkage has 5% lost motion.
1. Calculate outdoor air by mass balance and compare it with the ASHRAE 62.1 minimum for this space.
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Outdoor air = supply − return = 20,000 − 17,900 = 2,100 cfm. That is 10.5% outdoor air. The 62.1 minimum is 200 × 5 + 20,000 × 0.06 = 1,000 + 1,200 = 2,200 cfm. Measured outdoor air is 2,100 cfm, about 95% of the minimum and 700 cfm below the 3,000-cfm design. The relief reading of 2,100 cfm agrees with the outdoor-air mass balance, and the positive building pressure indicates little exfiltration. The design 3,000 cfm appears to include margin or a different occupancy assumption, but the system is still slightly short of the calculated minimum.
2. Use the CO2 readings to estimate actual outdoor air per person and discuss the discrepancy.
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CO2 generation for 200 people at light activity is about 0.0105 cfm/person, or 2.1 cfm total. Steady-state outdoor air = generation/(Cspace − Cout) = 2.1/(1,100 − 420) × 10^6 = 3,088 cfm, or 15.4 cfm/person. This is higher than the 2,100-cfm mass balance, which suggests either the CO2 sensor reads high, occupancy exceeds 200, or actual outdoor air is lower than 2,100 cfm. Check CO2 calibration and actual occupancy. CO2 is a screening tool, not a flow meter; use it to confirm the mass balance, not replace it.
3. Recommend the adjustment and verification steps, including return-fan tracking and economizer linkage.
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First verify outdoor air with a duct traverse at the intake and a return traverse; do not trust the 18% damper position. Set minimum outdoor air to 2,200 cfm for 62.1 compliance, or 3,000 cfm if the engineer requires design margin. Repair the economizer linkage lost motion so minimum position repeats. With a return fan, set tracking so building pressure remains about +0.02 to +0.05 in. w.c. and the relief damper does not backdraft. Re-measure supply, return, outdoor air, and relief after adjustment. Verify CO2 below about 1,000 ppm at design occupancy. The 2.0-V potentiometer setting is not a flow setpoint; document actual cfm.