TAB Explained: Testing, Adjusting, and Balancing
TAB is the formal process of verifying that HVAC systems perform to their engineering design. This guide explains what TAB covers, how the process works, who performs it, and what the final report documents.
What Is TAB?
Testing, Adjusting, and Balancing (TAB) is a systematic process for verifying and documenting that air and hydronic (water) HVAC systems are installed and operating in accordance with design specifications. TAB is performed by trained technicians using calibrated instruments, and the results are documented in a formal TAB report.
The three components of TAB:
- Testing — measuring actual performance of fans, coils, pumps, and terminal devices and comparing to design values
- Adjusting — making changes to damper positions, fan speeds, pump speeds, and valve positions to bring performance in line with design
- Balancing — proportioning airflow or water flow across a distribution system so each branch receives its design share
Scope of TAB Work
A complete TAB scope typically covers:
Air Systems
- Air handling unit (AHU) performance — CFM, static pressure, RPM, amperage, motor power
- Supply, return, and exhaust airflows at all terminal devices (registers, grilles, diffusers)
- Outside air, return air, and exhaust air damper positions and proportions
- Variable air volume (VAV) box minimum and maximum airflows
- Fan coil units, terminal reheat coils, induction units
- Exhaust fans (toilet exhaust, kitchen hood, parking garage)
- Building pressurization verification
Hydronic Systems
- Pump performance — GPM, head pressure, RPM, amperage
- Control valve flow verification
- Coil entering and leaving water temperatures at design conditions
- Chilled water, hot water, condenser water system flows
- Pressure differential across critical elements
The TAB Process
- Pre-TAB review — TAB agent reviews design drawings, specifications, and equipment submittals to understand design intent and record design values for all components.
- System readiness verification — Confirms the system is complete and ready: filters installed, coils clean, dampers operable, controls energized, and controls sequences active.
- Preliminary testing — Fan and pump performance is measured as-found. Controls sequences are observed to understand how the system operates before adjustments begin.
- Proportional balancing — Air (or water) flows are adjusted from the extremities of the distribution system inward, proportionally redistributing flow without starving any branch.
- Final measurements — All points are re-measured after adjustment to verify compliance with the ±10% tolerance (or project-specified tolerance).
- Documentation — All measurements are compiled into the TAB report.
ℹ Proportional balancing explained
Proportional balancing is a systematic approach where the technician measures all outlets, identifies the one with the lowest ratio of measured/design, and adjusts all others toward that ratio by closing their dampers proportionally. This preserves total system airflow while redistributing it correctly.
TAB Certification Bodies
Two primary organizations certify TAB professionals and firms in the United States:
| Organization | Focus | Key Standard |
|---|---|---|
| NEBB (National Environmental Balancing Bureau) | Certification of TAB firms; sets procedures for TAB work | NEBB Procedural Standards for TAB |
| AABC (Associated Air Balance Council) | Certification of TAB firms; emphasizes independence | AABC National Standards for TAB |
| ASHRAE | Publishes technical standards referenced by both | ASHRAE Guideline 1.1 (HVAC&R TAB) |
TAB Report Structure
The TAB report is the deliverable that proves the system was balanced. A complete TAB report includes:
- Project identification (name, address, mechanical engineer of record, TAB firm, date)
- Equipment schedules with nameplate data vs. measured performance
- Air distribution summary tables (design CFM, measured CFM, % variance at every outlet)
- Hydronic system summary (design GPM vs. measured GPM per branch)
- Remarks noting any conditions that prevented compliance
- Certification statement signed by the certified TAB agent
The mechanical engineer of record reviews the TAB report and may request corrections or additional measurements before approving it for project closeout.
TAB vs. Commissioning
TAB and commissioning are related but distinct processes:
- TAB verifies component-level performance — that fans, coils, and registers hit their design numbers. TAB does not test control sequences, set control points, or verify that the system responds correctly to changing loads.
- Commissioning (Cx) verifies integrated system performance — that the HVAC system, controls, and envelope interact correctly to maintain design conditions across a range of operating scenarios. Commissioning includes controls functional testing, sequence of operations verification, and seasonal testing.
On projects that require commissioning, TAB is typically a prerequisite — the system is balanced first, and then the commissioning agent tests the integrated performance.
FAQ
Who performs TAB work? +
TAB work is performed by certified TAB agents — typically firms that are certified by NEBB (National Environmental Balancing Bureau) or AABC (Associated Air Balance Council). Individual technicians may hold certifications from these organizations as well.
When is TAB required? +
On commercial construction projects, TAB is typically specified in the mechanical specifications and required as a condition of substantial completion. Some building codes require TAB documentation as part of the permit closeout process.
What is the difference between TAB and commissioning? +
TAB verifies that individual system components (fans, coils, registers) perform to design specifications. Commissioning is a broader quality assurance process that verifies the integrated performance of the entire building — including controls, sequences, and system interactions. TAB is often a component of commissioning.
Can a mechanical contractor perform their own TAB? +
In many specifications, TAB is required to be performed by an independent third-party firm — not the contractor who installed the system. This ensures objectivity. However, some smaller projects may allow the installing contractor to self-certify.