New Install Fails at Commissioning: Mine vs Supplied vs Existing Decision Tree

Why this matters

A brand-new installation that fails at start-up is one of the most expensive moments in field work, because three parties are now exposed: your workmanship, the manufacturer who supplied the equipment, and the existing site you tied into. Whoever owns the fault owns the rework and the warranty claim. If you assume your own work is fine and chase the equipment, you may miss a wiring error you made. If you assume the equipment is defective and return it, you may lose a day to an RMA for a unit that was fine. Commissioning failures demand a disciplined three-way isolation: mine, supplied, or existing. This applies to HVAC start-ups, plumbing fixture and pump installs, electrical panel and circuit additions, and appliance hookups.

Symptom presentation

The unit will not start, trips immediately, runs but underperforms, leaks, or throws a fault code on first power-up. Emotionally, the pull is to blame the new box because it is the new variable. Statistically, first-start failures split across all three sources, and the cheapest to verify is usually your own connections.

The customer sees only "the new thing you installed does not work," so the burden of proving where the fault lives sits with you regardless of who actually caused it.

Quick checks

Before isolating, confirm the obvious commissioning prerequisites:

• Power and supply present and correct: voltage, phase, polarity, water pressure, gas pressure, or fuel as applicable, measured at the unit, not assumed from the panel.
• Your terminations: every connection you made is tight, correct, and to spec. Re-torque, re-check polarity, verify line/load orientation.
• The supplied unit's own indicators: fault codes, status LEDs, factory test reports, and shipping locks or transit bolts still in place.
• The existing system you tied into: was it working before you touched it, and is the interface (drain, vent, return, breaker, supply) sized and configured for the new load?

Isolation tree

Verify in cheapest-first order: your work, then the supply you provided to it, then the unit, then the existing site.

1. Did you make a workmanship error?

   • Re-verify your terminations, polarity, orientation, fastener torque, and that nothing was left disconnected or reversed. This is the lowest-cost check and a common first-start cause. If found and fixed, you are done.

2. Is the supply you delivered to the unit correct?

   • Measure voltage, phase rotation, water/gas pressure, and airflow at the unit's input. A unit fed wrong supply will fault even when both the unit and your terminations are perfect. Wrong supply is still your scope if you provided that feed.

3. Is the supplied unit itself defective?

   • Only after your work and your supply check clean should you suspect the box. Confirm with the manufacturer's commissioning procedure: factory fault codes, a known-good comparison if available, and the supplier's diagnostic line. Document serial, code, and your verified-clean inputs so the warranty claim is clean.

4. Is the existing system the limiter?

   • An undersized existing circuit, a partially blocked existing drain or vent, an existing valve left closed, or an existing return that cannot move the air will make a perfect new unit fail. Test the existing interface independently of the new unit.

5. Is it an interaction, not a single owner?

   • Sometimes the new unit's draw exposes a marginal existing condition that was tolerable before. Name this explicitly as a combined fault; it changes who pays and what the fix is.

Confirming diagnosis

Confirm ownership by substitution and isolation:

• Mine: correcting your termination or supply clears the fault. The fault was in your scope.
• Supplied: with your work and inputs proven correct, the unit still faults on its own diagnostics. The unit owns it; pursue warranty with documentation.
• Existing: the new unit runs correctly once the existing interface is corrected or temporarily bypassed for test. The existing system owns it, which is a change-order conversation, not free rework.

The trap is stopping at the first plausible cause. Confirm by making the fault go away with a targeted change, not by argument.

Cross-trade examples

The three-way split shows up identically across the trades:

• HVAC start-up: a new condenser trips on first call. Mine could be reversed line/load or a miswired contactor; supplied could be a factory-defective board or a unit shipped with a transit issue; existing could be an undersized breaker or a return that cannot move the air. Check your terminations and supply first, then the unit's codes, then the existing interface.
• Plumbing fixture or pump: a new pump cavitates or a new fixture leaks. Mine is a fitting or orientation error; supplied is a defective seal or impeller; existing is a closed valve, wrong inlet pressure, or undersized supply line.
• Electrical addition: a new circuit faults at energize. Mine is a termination or polarity error; supplied is a defective breaker or device; existing is a shared neutral, an overloaded panel, or a feed problem upstream.
• Appliance hookup: a new appliance will not start. Mine is the connection; supplied is the unit's own fault code; existing is an inadequate circuit or supply.

The order is always the same because it is cheapest-first: prove your own work and the supply you provided before you blame the box or the building.

Next steps

Document which of the three you proved, with the measurements that prove it, before you call the supplier or write the change order. Photograph factory fault codes and your verified inputs. Where the existing system is the cause, present it as a scope finding with options rather than absorbing it silently. Where the unit is defective, your clean documentation is what gets the RMA approved without a fight.

References

• ISO 14224, Collection and exchange of reliability and maintenance data, on failure attribution and as-found condition recording.
• ACCA Standard 5 (HVAC Quality Installation) commissioning verification practices for new equipment start-up.
• NFPA 70 (National Electrical Code) and NFPA 70B for verification of supply, terminations, and circuit adequacy on new connections.
• PHCC service practices on fixture and system commissioning and tie-in verification.