Retro-Commissioning a Building With a Stuck Economizer
A 95,000 sq ft distribution center is pursuing an energy performance target after a utility audit showed an energy use intensity of 78 kBtu/sq ft, about 22 percent above a peer benchmark. The audit identified three measures: an economizer repair on four rooftop units, LED retrofits in the warehouse, and a rooftop PV array. The economizer dampers on all four units are seized open, so the units bring in full outside air year-round. The building has a demand charge that peaks between 2 and 6 p.m. in summer. The owner has $150,000 available this year and wants the largest visible reduction in energy use intensity. The LED project costs $85,000 with a 3.5-year simple payback. The economizer repair costs $22,000 and the audit estimates $14,000 in annual savings. The PV array costs $310,000 and would require financing.
1. How should the manager sequence these measures, and what analysis supports the sequence?
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The manager should rank by cost-effectiveness and by interaction effects, not by visibility.
- Economizer repair first. At $22,000 with $14,000 in annual savings, simple payback is about 1.6 years - far better than the LED project. More importantly, the seized-open dampers are a functional defect: the units are conditioning full outside air year-round, which inflates both energy and peak demand. Repairing them restores design intent and also reduces the load the other measures must offset. Fixing a defect before investing in efficiency is the correct order.
- LED retrofit second. At $85,000 with a 3.5-year payback, it is still attractive and reduces internal load, which in turn reduces cooling demand. It is a sound second tranche.
- PV last. At $310,000 it exceeds the available capital, requires financing, and its savings depend on rate structure and net-metering terms. It is a longer-horizon decision, not a first-year move.
Budget fit: $22,000 + $85,000 = $107,000, leaving roughly $43,000 within the $150,000 envelope for commissioning, metering, and contingency.
Analysis to support it: verify the audit's savings estimates with actual interval data where available, confirm the economizer repair scope includes controls and damper actuators (not just hardware), and model the demand-charge impact separately from the energy impact, since the owner's stated goal (energy use intensity) and the demand charge tell different stories.
2. The owner insists that PV gives the most visible sustainability result. How should the manager respond, and what measurement plan should accompany the chosen projects?
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The manager should reframe the conversation around verified results rather than visible technology.
Response to the owner:
- PV is the most visible measure but the least cost-effective per dollar this year, and it cannot be funded within the current envelope without financing. Deferring it does not abandon it; it sequences it.
- The economizer repair is a prerequisite for credible savings claims. If the building is conditioning full outside air, any reported PV production is partly offsetting a self-inflicted load. Fixing the defect first makes every later measure look better and makes the energy use intensity number honest.
- Present a two-year path: repair and LED now, measure and verify, then size PV against the reduced load and the actual rate structure. A right-sized array on a corrected building is a stronger sustainability story than a larger array on a defective one.
Measurement plan:
- Establish a weather-normalized baseline from 12 months of utility data before construction.
- Install or use interval metering on the four RTUs and the main service to separate economizer savings from LED savings.
- Track energy use intensity monthly against the baseline and the peer benchmark, and track summer demand peaks separately.
- Verify economizer operation post-repair with trend data (damper position versus outside-air temperature) rather than assuming the repair worked.
- Report results in a form the owner can use for sustainability disclosure, with stated assumptions and normalization method.
The manager's obligation is to protect the owner from spending on visible measures while a correctable defect continues to erode savings.