What do commissioning levels L1 through L5 actually mean?

They're a structured progression: L1-L2 cover component and installation checks, L3 covers subsystem functional testing, L4 covers system-level functional testing, and L5 covers integrated systems testing (IST) where multiple systems are verified working together, typically culminating in startup.

Can a project engineer handle commissioning on a smaller project?

On smaller, less complex projects it can work if the project engineer has direct functional testing experience. On larger or more technically complex systems, a dedicated commissioning engineer is worth the separate hire.

Who owns RFIs during the commissioning phase?

Typically the project engineer continues managing RFIs through commissioning, since design clarification questions still route through them, while the commissioning engineer focuses on test execution and issue identification.

Does a commissioning engineer need a PE license?

It depends on jurisdiction and project type. Many commissioning roles emphasize hands-on testing and startup experience over licensure, though some owner or regulatory requirements call for PE involvement in sign-off documentation.

When should we bring a commissioning engineer onto the project?

Ideally during design or early construction, not just at mechanical completion. Early involvement lets them shape the commissioning plan and flag design issues that would otherwise surface as testing failures late in the schedule.

Comparison

Commissioning Engineer vs. Project Engineer

These titles get used interchangeably on job postings far more often than the scopes overlap in practice. One owns getting systems to run; the other owns getting the project delivered.

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Definitions first

What each option actually means

A commissioning engineer verifies that installed systems function as designed, working through defined test levels up to integrated startup. A project engineer coordinates design, schedule, and cross-discipline execution across the life of the project. On a capital project, you typically need both, not one instead of the other.

Commissioning Engineer

Owns the verification that installed systems actually perform to design intent, working through structured commissioning levels from component checks (L1-L2) through subsystem and system functional testing (L3-L4) up to integrated systems testing and startup (L5). The job is proving performance, not designing it.

Choose it when

You're approaching mechanical completion and need functional testing planned and executed
The project requires structured commissioning levels (L1-L5) documented for handover
Integrated systems testing (IST) across multiple disciplines needs coordination
Startup and turnover documentation must satisfy owner or regulatory requirements
You need someone who can troubleshoot systems live during startup, not just on paper

Trade-offs you accept

Not typically responsible for design coordination or budget/schedule ownership
Engaged most heavily late in the project lifecycle, less useful early
Scope is verification and startup, not resolving upstream design issues, though they'll surface them

Project Engineer

Coordinates the engineering side of project delivery across its life, design coordination between disciplines, RFI management, schedule and budget tracking, and interfacing between design intent and field execution. The job is keeping the project moving and technically coherent end to end.

Choose it when

The project needs cross-discipline design coordination from early phases
RFIs, submittals, and change management need active ownership
You need one technical point of contact tracking schedule and scope across engineering disciplines
The project spans design, procurement, and construction phases, not just startup
Owner, EPC, and contractor communication needs a consistent technical interface

Trade-offs you accept

Not a substitute for hands-on functional testing or startup troubleshooting expertise
Depth in any single discipline is typically shallower than a specialist in that discipline
Effectiveness depends heavily on project management and coordination skill, not just technical knowledge

Side by side

Decision matrix

Compared on the criteria engineering leaders actually weigh, not on vendor talking points.

Commissioning Engineer vs. Project Engineer decision matrix
Criterion	Commissioning Engineer	Project Engineer
Primary scope	Verifying installed systems function to design intent	Coordinating design, schedule, and execution across disciplines
Project phase focus	Heaviest from mechanical completion through startup	Active from early design through project closeout
Key deliverable	Commissioning test records, L1-L5 sign-offs, startup reports	RFI logs, coordination schedules, design interface resolution
Testing responsibility	Owns functional and integrated systems testing (IST)	Not typically hands-on for functional testing
Design involvement	Reviews design intent to verify against; doesn't originate it	Actively coordinates and resolves design issues across disciplines
Schedule ownership	Owns the commissioning schedule specifically	Owns or tracks the overall engineering schedule
Cross-discipline role	Coordinates system-level testing across disciplines at startup	Coordinates design and RFIs across disciplines throughout
Risk owned	Systems that don't perform as designed at startup	Design gaps, schedule slippage, and coordination failures
Typical certifications	Discipline-specific plus commissioning-focused training (e.g., ASHRAE, ICC-adjacent for buildings; OEM startup training for industrial)	PE licensure often expected or preferred depending on scope
Best fit for	Late-stage verification and startup readiness	End-to-end technical project coordination

Our position

What we recommend, and when we do not

These roles fail most often when a company hires one expecting the scope of the other. A project engineer hired to run commissioning without functional testing background will struggle at L4-L5 integrated systems testing. A commissioning engineer hired to run overall project coordination will be strong at startup and thin on early-phase design management.

On most capital projects of real size, you need both, and they hand off to each other. The project engineer owns coordination through design and construction; the commissioning engineer takes primary ownership as the project approaches mechanical completion, verifying what was designed and built actually performs.

Where companies try to save headcount by collapsing these into one role, the usual failure mode is that design coordination suffers during the intense final commissioning push, or commissioning gets rushed because the project engineer is still fighting RFIs. Neither compromise is cheap once it shows up as schedule slip or a failed system test.

If you're staffing a single project and have to prioritize, match the hire to your current phase. Early and mid-project, prioritize the project engineer. Approaching startup, prioritize commissioning expertise, ideally brought on early enough to have input on commissioning planning before mechanical completion, not after.

Answers

Frequently asked questions

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Related decisions

Every comparison connects to an engagement model you can run and a discipline we recruit.

Still deciding between the two?

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