A building that passes drawing-review + factory-acceptance test can still fail at commissioning. The most expensive MEP failures are the ones that emerge only when systems are loaded under real operating conditions. This article catalogs 15 commissioning gotchas — drawn from a decade of Indian commercial commissioning audits — organized by discipline, with diagnostic methods + fix patterns.
HVAC commissioning gotchas
1 — AHU coil-side ΔT below design
Symptom: Plant chilled water supply 7 °C, return 9-10 °C (design 13 °C). ΔT 2-3 °C.
Diagnostic: Air-side leaving DBT, leaving RH, and CHW flow at each AHU. If air leaving is at design but CHW ΔT is low, you have over-pumping. If air leaving is above design, you have coil starvation.
Fix: Tune CHW flow at coil (TAB). Often resolved by reducing pump speed or tightening 2-way valve characterization (linear → equal-percentage).
2 — Mixed air RH not instrumented
Symptom: Room RH drifts; no diagnostic data.
Diagnostic: Always install RH sensor at AHU mixed-air section, not just return + supply. Without it, you debug blind. ₹15K instrument cost.
Fix: Retrofit Vaisala HMP110 or equivalent at coil-entering position.
3 — Drain pan trap depth wrong
Symptom: AHU not draining properly during cooling, water back-flow, microbiological growth.
Diagnostic: Drain pan trap depth must be ≥ 2 × negative duct pressure (in inches WG) per IS 8543. Most Indian AHUs commissioned with 25-50 mm trap; under positive return pressure, this seals; under negative, it leaks.
Fix: Rebuild trap to 100-150 mm depth.
4 — VAV terminals not auto-zeroed
Symptom: VAV flow control unstable; rooms over- or under-cooled.
Diagnostic: VAV airflow sensor (differential pressure) must be zeroed at zero-flow. Commissioning checklist sometimes skips this; default offset can be 50-100 CFM.
Fix: Manual zero-cal per VAV at commissioning + annual re-cal.
5 — Chiller staging not bypass-open-first
Symptom: Chiller staging trips first-running chiller on flow alarm.
Diagnostic: BMS staging sequence opens bypass first to maintain min flow on the first chiller before commanding second chiller online.
Fix: Rewrite BMS logic — open bypass valve 30-60 seconds before chiller staging command.
Plumbing commissioning gotchas
6 — Domestic water hammer at sudden valve closure
Symptom: Pipe knocking at fixture shutoff, especially solenoid-valve units.
Diagnostic: Velocity > 2 m/s in any run + solenoid closure < 0.5 sec = hammer.
Fix: Air chambers / pneumatic snubbers at high-pressure-zone supplies, especially before solenoid groups.
7 — Hot water recirculation loop running cold at far end
Symptom: Furthest fixture takes 30-90 seconds for hot water; recirculation pump not delivering.
Diagnostic: Loop temperature drop at far end. Causes: (a) recirculation pump under-sized; (b) loop oversized so velocity insufficient for thermal mixing; (c) thermostatic balancing valves not commissioned.
Fix: Re-size pump for full circulation flow; ensure thermostatic balancing valves are commissioned to deliver design loop temp ≥ 55 °C at all points (Legionella + comfort).
8 — Greywater system bacterial regrowth
Symptom: Treated water turbidity rising over 30-60 days; bacterial counts exceed CPCB.
Diagnostic: Oxidant residual (free chlorine or ORP) in treated tank monitored. Without active monitoring, residual depletes and regrowth follows. (See Greywater Commissioning for detailed walk-through.)
Fix: Install ORP sensor + auto-dose pump; setpoint 300-400 mV.
9 — Fixture supply pressure varies by zone
Symptom: Top floor weak flow, bottom floor over-pressurized.
Diagnostic: Pressure-reducing valves (PRVs) at each zone, calibrated. Without zonal PRVs, top vs bottom can differ by 3-4 bar.
Fix: Install PRVs at each zone, calibrate to 3-4 bar at fixture supply.
Firefighting commissioning gotchas
10 — Fire pump churn pressure exceeds 140 % of rated
Symptom: Pump shutoff pressure too high; piping + sprinkler heads stressed.
Diagnostic: NFPA 20 mandates churn ≤ 140 % of rated head. Some pumps deliver 145-150 % at shutoff.
Fix: Verify pump curve at FAT; if churn exceeds 140 %, change impeller or specify a different pump.
11 — Sprinkler hydraulic test pressure not held 60 min
Symptom: Hydrostatic test signed off at 5-10 minutes; later, system leaks under operating pressure.
Diagnostic: NFPA 13 + IS 13039 mandate 60-minute pressure hold at 200 psi (14 bar) or 50 psi above operating, whichever higher. Indian contractors often hold for 10-15 minutes.
Fix: Enforce 60-min test with documented pressure drop ≤ 5 psi (0.34 bar) per IS 13039.
12 — Smoke control fan reverse rotation
Symptom: Stair pressurization fan delivers reverse flow; pressure-difference test fails.
Diagnostic: During FAT, fan rotation direction verified. Site rewire often reverses; not always caught.
Fix: Direction-arrow check post-electrical commissioning + smoke test before handover.
Electrical commissioning gotchas
13 — Earth resistance > 5 Ω after construction
Symptom: Earth resistance test fails or marginally passes.
Diagnostic: Soil resistivity not measured pre-construction; pit type chosen wrong. (See Earthing IS 3043.)
Fix: Add chemical earth pits with bentonite + carbon backfill in high-resistivity soil; ensure 4-electrode Wenner method test methodology.
14 — LED panel UGR exceeds task limit
Symptom: Glare complaints from perimeter desks.
Diagnostic: UGR computed at design; site reality differs because of reflectance, mounting height variation, panel orientation.
Fix: UGR measurement at commissioning using SLL Lighting Guide LG7 method; adjust panel placement or specify louvered fixture.
15 — Cable tray earthing discontinuous
Symptom: Earth fault current path interrupted at tray joint.
Diagnostic: Continuity test across each tray joint + at every change of direction. Without explicit bonding jumper, joint connection alone is insufficient.
Fix: 6 mm² Cu bonding jumper at every tray joint, terminated at MEB.
How to organize the commissioning
The fix for all 15 is the same: a multi-week, discipline-specific commissioning plan that doesn’t compress to two weeks at handover.
| Phase | Duration | Activities |
|---|---|---|
| Pre-commissioning | 6-8 weeks before handover | FAT review, drawing-vs-as-built check, instrument calibration |
| Functional testing | 4-6 weeks | System start-up + functional verification (each system) |
| Integrated testing | 2-3 weeks | Cross-system interaction (BMS + fire alarm + electrical + HVAC) |
| Performance testing | 2-4 weeks | Operating under load; data logging; tune-up |
| Handover | 1 week | Documentation transfer + operator training |
That’s 15-22 weeks of structured commissioning — and it’s the only way to catch the 15 gotchas above before they become operational complaints.
From the Field — Engineer’s Notebook
The single most under-appreciated commissioning instrument is a portable BMS data-logger (₹2-3 lakh) that captures every parameter across every system at 5-min intervals for 2-4 weeks. Most commissioning checklist items can be diagnosed from the data; without it, you depend on operator observation + facility manager note-taking. We carry one to every major handover and have caught 60+ issues over 5 years that would have been operational complaints in the first 3 months otherwise.
5 takeaways
1. Coil-entering RH sensor is mandatory (item 2).
2. Drain pan trap depth ≥ 2× negative duct pressure (item 3).
3. Chiller staging sequence open-bypass-first (item 5).
4. NFPA 20 churn ≤ 140 % (item 10).
5. Earth resistance test methodology (4-electrode Wenner) (item 13).
Pairs with: Cooling Load to AHU Selection, Greywater Commissioning Bengaluru, Earthing IS 3043