Perimeter Overheating: Second Chiller or Broken Economizer?
A 1,800 m² two-storey office (1998) has one VAV air handler served by a 60 kW air-cooled chiller. South and west perimeter zones overheat from March to May and again in October while interior zones are comfortable, and the chiller runs on many mild days. The BAS shows the outside-air damper held at its 20% minimum; the economizer actuator linkage was found disconnected. With cooling on, AHU fans plus chiller draw 45 kW; with the compressor off they draw 15 kW. Local bin data show about 1,300 hours per year between 8 °C and 18 °C with a cooling load. The owner holds a $140,000 quote for a second 60 kW chiller to 'fix the capacity problem.' A handheld CO2 meter reads 1,150 ppm in a conference room whose BAS sensor reads 620 ppm. Repairing the economizer is quoted at $4,200.
1. Should the owner buy the second chiller? Judge the evidence and state what you would check before any capital is committed.
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The symptom pattern does not support a capacity problem.
A genuine undersized chiller produces high space temperatures everywhere, worst at peak ambient, with the chiller at 100% and leaving-water temperature drifting. Here:
- Interior zones are comfortable while perimeter zones overheat, and the problem appears in mild months, not at design conditions.
- The building has a disconnected economizer holding minimum outdoor air - a known, quantified defect worth $4,700/yr (see next task) sitting in front of a $140,000 quote.
What perimeter-only, mild-weather overheating usually means
- Distribution: VAV boxes at minimum flow, failed or mis-set reheat, boxes stuck shut, duct static reset too low, supply-air temperature too warm.
- Simultaneous heating and cooling: reheat running in cooling season (very common - check the trend), or a perimeter heating control fighting a cooling demand.
- Solar/orientation gain with no zone-level compensation.
Checks before capital
- 2-week trend of zone temperature vs flow vs damper position vs reheat valve for two perimeter boxes.
- Entering/leaving chilled-water temperatures and chiller % load at peak - is the chiller actually saturated?
- Supply-air temperature setpoint and static pressure reset schedule.
- Verified airflow at the worst box (hood or balometer).
If the chiller is not saturated at peak, the money belongs in the boxes and controls, not in a second chiller.
2. Quantify the economizer repair and identify the conditions under which it would not pay back.
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Sizing the saving
- Cooling-mode draw 45 kW; compressor-off draw 15 kW → compressor + condenser ≈ 30 kW
- Available hours: 1,300 h/yr
- Energy: 30 × 1,300 = 39,000 kWh/yr ≈ $4,680/yr at $0.12/kWh
- Simple payback: 4,200 / 4,680 = 0.9 years
- NPV, 10 years at 12%: (4,680 × 5.650) − 4,200 = 26,442 − 4,200 = +$22,200
Add demand savings if the economizer reduces the summer peak - check the interval data before claiming it.
When it would not pay back
- Humid climate: if control is dry-bulb rather than enthalpy, a working economizer can import moisture, raise latent load and offset the compressor saving. Verify the changeover logic.
- No relief path: if the building cannot relieve the extra outdoor air, pressurisation and door/humidity problems appear and the economizer gets disabled in practice.
- Low-load coil freeze or overcooling: the minimum damper position and low-limit protection must be functional.
- The 1,300 hours is an estimate - confirm it against chiller-run hours in the BAS trend, not just bin data.
Even with a 50% haircut on the hours, the payback stays under 2 years. Repair it - but repair the linkage, the changeover logic, and the relief path together, or the fix will be bypassed within a season.
3. The CO2 sensor reads 620 ppm against 1,150 ppm handheld. What is the consequence, and what must be programmed when the economizer is restored?
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Consequence: demand-controlled ventilation is effectively disabled, and the room is being under-ventilated while the BAS reports a healthy number.
- A reading of 620 ppm against 400–420 ppm outdoor air implies only ~200 ppm rise, so the DCV loop believes the space is near-empty and holds the damper at minimum.
- The true 1,150 ppm corresponds to roughly 15 cfm/person of outdoor air - near the minimum of ASHRAE 62.1 for an office, not the comfortable design condition the sensor implies.
- NDIR sensors drift; many rely on automatic baseline calibration, which fails if the space is never truly unoccupied. If there is no night-time purge reference, the sensor must be recalibrated or replaced on a schedule.
Programming requirement when the economizer is restored
The outdoor-air damper position must be the maximum of three demands, not a fixed minimum:
- the ventilation minimum (occupancy-based or DCV),
- the economizer call for free cooling,
- any pressure/relief constraint.
Otherwise the economizer and DCV fight each other: DCV closes the damper, the economizer opens it, and either the space overheats or is over-ventilated. Also alarm on the CO2 sensor disagreeing with a portable reference at each re-commissioning, and trend damper position + CO2 so the fault is visible next time.