Commissioning a VAV System That Passes Functional Tests but Fails in Operation
A new 200,000 ft² office building completed commissioning six months ago. All functional performance tests passed, including terminal-box minimum/maximum airflow verification and AHU static-pressure control. Since occupancy, the facilities team reports: (1) zones on the west perimeter overheat in the afternoon, (2) some interior zones are uncomfortably cold, (3) the AHU supply fan runs near maximum speed all day, and (4) reheat valves at several boxes are open in cooling weather. The BAS trend shows the supply-air static-pressure setpoint holding at 1.5 in. w.c. at the sensor, which is located two-thirds of the way down the main duct. Terminal boxes are pressure-independent with a 30% minimum. The building is pursuing ongoing commissioning.
1. Diagnose the likely root causes of the four symptoms, treating them as a connected system rather than four separate faults.
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The symptoms are one coupled failure, not four.
- Fan near max + overheating west zones + cold interior zones + reheat in cooling weather point to duct static-pressure sensor location and setpoint combined with minimum airflow settings that are too high.
Mechanism: The static sensor sits two-thirds down the main duct, downstream of most of the pressure drop to the far terminals. To satisfy the setpoint there, the fan must over-pressurize the upstream duct, so near boxes see far more pressure than needed and far boxes are starved - the classic over-pressurization/under-delivery split. The starved west-perimeter boxes (high afternoon load) cannot get enough air → overheating. Meanwhile interior boxes with a 30% minimum that is oversized for their actual load dump cold air → cold interior zones, and the system then calls for reheat in cooling weather to fix the overcooling it created. The fan runs near max because the setpoint is being chased at the wrong location.
Secondary contributor: undersized or mis-set terminal minimums (30% of a large box can exceed the interior zone's actual minimum need), and reheat enabled as a band-aid rather than a last resort.
2. Propose the ongoing-commissioning investigation and corrective actions, in priority order, with the verification for each.
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Priority order (cheapest/most diagnostic first):
Relocate or re-evaluate the static-pressure sensor. Move it to ~two-thirds of the index (longest/highest-resistance) run, or implement static-pressure reset (trim-and-respond) so the fan setpoint floats down until the critical box's damper is ~90% open. Verify: trend the critical-zone damper position and confirm it stays ~85-95% open while fan speed drops; confirm far-zone airflow meets design.
Re-commission terminal minimums. Trend each box's actual load and reset minimums to the true ventilation/load minimum (not a blanket 30%). Verify: confirm interior zones hold setpoint without reheat in cooling weather, and reheat valve position stays closed when the zone is in cooling.
Re-check west-perimeter box sizing and airflow now that static pressure is corrected; confirm max airflow is achieved at the new, lower static. Verify: afternoon peak zone temperature within setpoint tolerance.
Review reheat control logic - reheat should be a last resort, enabled only when the zone is at minimum airflow and still below setpoint. Verify: trend reheat valve command vs. zone temperature and airflow over a week; expect near-zero reheat in cooling weather.
Document in the ongoing-Cx plan with trend-based acceptance criteria and a 30-day monitoring period before closeout.