Rooftop VAV With Simultaneous Hot and Cold Zones and an Economizer at Minimum
An office building has four 25-ton packaged rooftop units with variable-frequency supply fans, VAV boxes with hot-water reheat, economizers, and CO2-based demand-controlled ventilation. Supply fan speed is controlled to a duct static pressure setpoint of 2.0 in. w.c. measured about two-thirds of the way down the main trunk. On a mild, dry October afternoon (outdoor 58°F), the fourth-floor south zones call for cooling continuously while north zones call for heating. The fan runs at 100% and the static pressure transmitter reads 1.4 in. w.c. The economizer actuator reports 10%, its configured minimum, with the outdoor damper nearly closed. A conference-room CO2 sensor reads 1,150 ppm while that zone's box sits at minimum airflow. The engineer has confirmed the return damper is open, the relief path is clear, and every box has been calibrated to its scheduled minimum and maximum airflow.
1. The fan is at 100% yet static pressure is below setpoint, and zones are simultaneously hot and cold. What failure pattern fits these readings, and how would you confirm it?
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On a properly sized system, a fan at 100% should drive the sensor well above 2.0 in. w.c.; reading only 1.4 means either the sensor is lying or air is escaping between the fan and the sensor. Both produce the same zone symptoms, so separate them first.
Confirm the sensor: compare the transmitter output against a calibrated manometer at the same tap, check the tubing for kinks, water, dust plugs or a leaking connection, and confirm the tap is perpendicular to airflow and not in a turbulent elbow. A plugged high-side port reads low, which drives the VFD to maximum and over-pressurizes the duct near the fan.
If the sensor is accurate, the fan is genuinely producing only 1.4 in. w.c. at full speed. Look for a duct failure downstream of the fan but upstream of the sensor: a disconnected flex collar, a torn liner, a fire/smoke damper that has partially closed, or a failed volume damper. Air dumped into a return plenum or ceiling void explains starved south zones, which sit furthest away and go to maximum without achieving design airflow - hence continuous cooling calls and overheating. The north zones are a second problem: they sit at minimum airflow receiving 55°F air, and their reheat valves are apparently not opening, so they overcool and call for heat. Verify reheat valve stroke and actuator wiring at the north boxes before concluding anything about the refrigeration circuit.
2. Why is the economizer at minimum at 58°F dry-bulb, and what would you check?
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At 58°F and dry, outdoor air is a free cooling source, so the damper should be wide open with mechanical cooling off. Being at minimum means the economizer is either disabled, faulted, or being told not to economize.
Check in order:
- Command versus feedback. Command the actuator to 100% from the controller and watch the feedback. No movement means a failed actuator, a lost 0–10 V signal, or a stuck linkage; movement with stale feedback means a bad feedback potentiometer.
- Changeover logic. Verify the enthalpy or dry-bulb changeover setpoint and the outdoor air sensor calibration. A drifting or mis-mounted OA sensor reading 70°F+ will lock out free cooling on a 58°F day.
- Configuration. Confirm economizer enable, minimum position, and whether a building pressure or relief interlock is preventing opening. Some sequences hold minimum when the fan is at maximum and relief is deemed inadequate.
Note the interaction: with mechanical cooling available this alone would not overheat the south zones, but combined with starved airflow it removes the capacity margin and forces the compressor stages to run continuously, which is exactly what the maximum-fan, low-static condition reflects.
3. The conference room holds 1,150 ppm CO2 with its box at minimum. Is demand-controlled ventilation working? Explain the sequence of checks and the likely causes.
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No - with DCV active, the zone should have driven above minimum as CO2 rose past its setpoint (commonly around 800–1,000 ppm). Sitting at minimum at 1,150 ppm means one of the links in the chain is broken.
Check in sequence, from sensor outward:
- Sensor. Verify calibration and location - a sensor mounted in the return duct rather than the breathing zone, or in a dead corner, reports the wrong value. Confirm with a handheld reference.
- Signal. Trend the CO2 value arriving at the BAS point. A live sensor with a dead point is a wiring or point-mapping fault.
- Program. Confirm the DCV routine is enabled for that box and that the reset formula produces a setpoint above the box minimum. A common error is a reset that writes to a maximum-flow setpoint with the box in a fixed-minimum mode.
- Box capacity. Check that the box's scheduled maximum can actually deliver the ventilation the room needs; a box sized only for its cooling load may have no headroom.
- Airflow delivery. A starved box cannot ventilate even with a correct setpoint, so the static/duct problem above must be resolved first.
Also check the design occupancy used for the ventilation calculation; a conference room retasked for more seats will read high even with a healthy system.