Economizer and Static-Pressure Faults in a VAV Office
An 18-story Class B office built in 1988 has a central VAV system with a 250-ton centrifugal chiller, two 100-hp supply fans with inlet guide vanes, and electric reheat in 40 perimeter zones. BAS trends show AHU-2 supply static at 1.45 in. w.c. continuously; the fan runs at 100% speed from 6 a.m. to 7 p.m. and 60% at night. The outside-air damper sits at its minimum 30% during occupied hours, although design minimum is 15%. The economizer actuator was replaced last year, but the damper linkage was not recalibrated; outside-air enthalpy is below return enthalpy for 3,200 hours per year. Several west zones call for cooling and heating simultaneously in the afternoon; reheat totals 85 kW for 1,800 hours per year. Chilled-water supply is 44°F, and differential pressure across the worst coil is 12 psi versus 6 psi design. The owner will fund either a static-pressure reset and damper repair or a chiller replacement. Electricity is $0.10/kWh with an $18/kW monthly demand charge. Each fan draws 75 kW at 100% speed and 26 kW at 70% speed.
1. Diagnose the simultaneous heating/cooling and economizer symptoms. Which findings are likely causes, and which are consequences? Rank the first three actions by cost and persistence.
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The unrecalibrated economizer linkage is a likely cause: the damper is not modulating to the correct minimum or to full free-cooling position. A 30% minimum where 15% is designed increases outside-air load and can make the chiller work harder, but it does not by itself explain reheat. The high duct static pressure is the more direct cause of simultaneous heating and cooling: VAV boxes throttle to minimum to absorb 1.45 in. w.c., zones overcool, and reheat energizes. The 12 psi coil differential pressure versus 6 psi design confirms over-pressurization and excess flow.
Ranked actions:
- Recalibrate the economizer linkage and reset the minimum outside-air position to 15% - low cost, immediate, and restores free cooling.
- Implement static-pressure reset, with the setpoint reset down until the worst box damper is about 90% open - controls-only and addresses the root cause of reheat.
- Review VAV box minimums and reheat lockout; then investigate chiller staging. Chiller replacement is last because the controls faults are still present.
2. Estimate the annual energy and demand savings from reducing fan speed to an average 70% for 3,000 hours per year by adding VFDs and resetting static pressure. Each VFD costs $45,000 installed, or $90,000 for the two fans. Is the project justified at a 5-year simple payback?
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Per fan, savings = 75 − 26 = 49 kW. For two fans = 98 kW. At 3,000 hours/year, energy savings = 98 × 3,000 = 294,000 kWh/year, worth $29,400 at $0.10/kWh.
Demand savings are less certain because the monthly peak may not coincide with the 3,000 reduced-speed hours. Upper bound: 98 kW × $18/kW-month × 12 = $21,168/year. A conservative 50% coincidence gives $10,584/year.
Total annual savings: $29,400 + $21,168 = $50,568 upper bound, or $39,984 conservative. Simple payback = $90,000 / $50,568 = 1.8 years upper bound, or $90,000 / $39,984 = 2.3 years conservative. Both are well inside a 5-year hurdle. The project is justified, but confirm that the fans do not already have VFDs, that the 70% speed is achievable after static reset, and that the VFDs are not being credited with savings already captured by IGV operation.
3. The owner asks whether to replace the chiller instead. What data would you require, and what is your recommendation?
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Require: a 12-month chiller kW/ton profile at actual load and condenser-water temperature; part-load curves; current refrigerant and phase-out or safety implications; maintenance and failure history; remaining life; whether the 250-ton capacity is oversized relative to the measured peak load; and the cost and availability of utility incentives. Also require a post-controls load profile, because fixing the economizer and static pressure will reduce cooling load before any chiller decision.
Recommendation: do not replace the chiller yet. The controls faults are demonstrably present, low-cost, and likely to reduce both fan and cooling energy. Implement damper repair and static-pressure reset first, then re-measure chiller load and kW/ton for one cooling season. Replace the chiller only if measured efficiency, reliability, or refrigerant compliance still justifies it after the load reduction.