What are acceptable airflow tolerances for TAB?

ASHRAE 111 (Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems) and most project specifications require final measured airflows within &pm;10% of design for supply, return, and exhaust air in commercial applications. Critical spaces such as hospital operating rooms, negative-pressure isolation rooms, and laboratory fume hood exhaust systems typically require tighter tolerances of &pm;5% or better. Outdoor air quantities (for ventilation compliance with ASHRAE 62.1) are held to &pm;10% but must never be less than design minimum — under-ventilation is a code violation.

How do you balance a VAV system when all boxes can be at any position?

VAV balancing is performed with boxes commanded to maximum airflow via BAS/DDC override, treating the system as a constant-volume system for balancing purposes. The supply fan is set to a fixed speed at or near design maximum static pressure. The TAB contractor measures airflow at each VAV box (using an inline flow sensor reading from the DDC controller, verified by an independent pitot traverse or flow hood at the terminal), proportionally balances the system so the highest-resistance branch is the limiting branch, then re-measures to confirm all boxes are within &pm;10% at maximum. Minimum position balancing is verified separately at minimum airflow setpoints.

When should TAB be performed in the construction schedule?

TAB should begin only after: all HVAC equipment is installed and operational; all ductwork and piping is installed, insulated, and pressure-tested; all terminal units (VAV boxes, fan coil units, diffusers, grilles) are installed; all mechanical rooms are complete and accessible; the BAS/DDC system is online and capable of commanding equipment; and the building envelope is substantially complete (exterior doors and windows closed). Scheduling TAB too early — before BAS commissioning or during active construction — results in re-work and inaccurate final readings. Plan 3–4 weeks for TAB on a typical mid-size commercial building.

What is the difference between TAB and commissioning?

TAB is a component of the broader commissioning (Cx) process but has a more specific, technical scope: TAB measures and adjusts airflows and water flows to design quantities and documents equipment performance data. Commissioning (per ASHRAE Guideline 0 and 1) is a comprehensive quality assurance process that also verifies control sequences, interlocks, alarms, failure modes, trend logging, and owner training — in addition to relying on TAB data as input. On most commercial projects, a TAB contractor performs the TAB scope, while a separate commissioning provider (CxP) oversees the functional performance testing. Specify both separately in the project specifications.

Engineering·8 min read·May 28, 2025

🔧 TAB: Testing, Adjusting, and Balancing HVAC Systems

A complete guide to HVAC TAB (Testing, Adjusting, Balancing) — the NEBB and AABC certification process, instrumentation requirements, airflow and waterflow balancing methods, and TAB report documentation.

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What is TAB and Why is it Required?

Testing, Adjusting, and Balancing (TAB) is the systematic process of measuring and adjusting airflow and water flow rates in HVAC systems to match design quantities, verify equipment performance, and ensure proper system operation. TAB is not optional commissioning: it is required by ASHRAE Standard 90.1 Section 6.7 for commercial buildings, by IBC construction documents, and by most AHJ plan-check departments as a condition of occupancy.

Topics covered

TAB testing adjusting balancingNEBB certificationAABCHVAC balancingairflow balancinghydronic balancingpitot tube traverseflow hoodproportional balancingVAV balancingTAB reportSMACNA