Testing, adjusting, and balancing (TAB) verifies that airflow and water flow through a building's HVAC equipment match the design values on the mechanical drawings, while commissioning verifies that the entire building system, controls, sequences, and equipment together, performs the way the owner's project requirements describe across every mode it will ever operate in. Most commercial buildings, and virtually every healthcare or critical environment project, need both: TAB establishes the physical numbers commissioning then tests under real operating conditions.
TAB is a measurement and adjustment discipline. A certified technician, credentialed through NEBB, AABC, or TABB, measures actual airflow at every diffuser, VAV box, and air handler, and actual water flow at every coil and terminal unit, then adjusts dampers and balancing valves until those measurements match the design flow rates on the drawings. The final TAB report documents design versus measured flow for every point in the system.
Commissioning does not duplicate that measurement work. A commissioning provider relies on the TAB report as one of the inputs it verifies, alongside controls programming, equipment startup documentation, and functional performance testing of sequences that TAB alone never touches, such as how an air handler responds to an economizer changeover or how a room's pressure relationship holds up when a door is opened.
Commissioning, following the process ASHRAE Guideline 0-2019 describes, starts earlier and runs longer than TAB. It begins with the owner's project requirements (OPR) and basis of design (BoD) during pre-design, continues through design review and construction observation, and finishes with functional performance testing that exercises every system mode: full load, part load, unoccupied setback, emergency power transfer, and failure scenarios TAB was never scoped to check.
| Scope Element | TAB | Commissioning |
|---|---|---|
| Measures actual airflow/water flow against design values | Yes | Relies on TAB's results |
| Adjusts dampers and balancing valves to design flow | Yes | No |
| Reviews design documents against owner's project requirements | No | Yes |
| Functionally tests control sequences and failure modes | No | Yes |
| Verifies system performance across full and part load | No | Yes |
| Produces signed test reports for AHJ/code compliance | Yes | Yes, for its own scope |
| Governed by | NEBB, AABC, or TABB procedural standards | ASHRAE Guideline 0 / Standard 202 |
Any project where the mechanical system includes controls sequences beyond simple on/off operation needs both. A VAV system with economizer control, a hospital with pressure relationships to maintain between rooms, or any building pursuing LEED certification (which requires fundamental commissioning under its own credit structure) all need TAB's flow verification and commissioning's sequence verification, because neither one checks what the other checks.
A simple exception exists on very small projects with no meaningful controls sequence, where TAB alone may satisfy code without a separate commissioning scope. That is uncommon on commercial work of any real size, and it is never the case on healthcare, laboratory, or critical environment projects, where AHJs and standards like ASHRAE/ASHE 170 expect both.
The IECC's commercial provisions, and the state energy codes built on them, generally require both a signed TAB report and commissioning documentation for covered systems, though the specific threshold varies by jurisdiction and building size. A project relying only on a TAB report to satisfy its energy code commissioning requirement is a common and costly mistake, because most energy codes distinguish the two explicitly and require separate documentation for each.
Because TAB and commissioning verify different things and rely on each other's outputs, sequencing them correctly on a project schedule (TAB substantially complete before functional performance testing begins) matters as much as scoping them both in the first place. Aero Performance Group provides testing and balancing and commissioning services as coordinated scopes rather than separate vendors handing off paperwork, which keeps the TAB report and the commissioning functional test results consistent with each other.
If a project scope currently only names one of the two, or the mechanical specification is unclear about which certification body's standard applies, request a scope review before bid documents go out.
Yes, though some project owners and AHJs require independent third-party verification for one or both, meaning the commissioning agent cannot be the same firm that installed the mechanical system, and in some cases cannot be the same firm that performed TAB either. Checking the project specification's independence requirements before assigning scope avoids a disqualification late in the project.
LEED's Fundamental Commissioning and Verification prerequisite requires commissioning for all LEED projects, and most mechanical specifications on LEED projects separately require TAB as a standard part of HVAC installation, regardless of the LEED credit structure. A LEED project without a documented TAB report generally cannot support its commissioning functional test results, since commissioning verifies performance against design values that TAB is what actually establishes in the field.
TAB typically happens in a defined window near the end of construction, once systems are started up and ready for balancing, and can often complete within a few weeks depending on building size. Commissioning spans the entire project, starting during design and continuing through a warranty period after occupancy, so its overall timeline is considerably longer even though its most intensive field activity (functional performance testing) also clusters near the end of construction.
Commissioning has no verified flow data to test sequences against, which means functional performance testing may pass procedurally while the underlying airflow and water flow are still wrong. A commissioning agent who discovers this mid-project typically has to send the mechanical contractor back to perform TAB before functional testing can resume, adding cost and schedule delay that proper sequencing would have avoided.
Retro-commissioning re-verifies an existing building's control sequences and system performance against current operating needs, which may or may not include re-balancing airflow and water flow depending on what the retro-commissioning study finds. If retro-commissioning identifies flow problems, a TAB rebalance typically follows as a corrective action rather than being part of the retro-commissioning scope itself.
TAB follows procedural standards published by NEBB, AABC, or TABB, whichever the project specification names. Commissioning follows the process framework in ASHRAE Guideline 0 and ASHRAE/IES Standard 202, which describes commissioning phases and responsibilities rather than field measurement procedures.