TAB is the last check before an HVAC system is handed over. Someone measures what the system actually delivers, adjusts it until it matches the design, and writes it down. For an HVAC contractor or general contractor, this is the work that stands between you and a signed-off final inspection.
What Test, Adjust, and Balance means
The three words are three jobs, done in order.
- Test: measure what the system really does. Airflow at every outlet, total airflow at the fan, fan speed, motor voltage and amperage, and static pressures across the system.
- Adjust: change the system until the numbers move toward the design. That means fan speed, pulley or sheave changes, and damper positions.
- Balance: share the air out in proportion so every branch and every outlet gets its design airflow, not just the ones closest to the fan.
The work follows procedural standards published by the certifying bodies, NEBB, AABC and TABB. A project specification will usually name one of them. See what air balance certification involves if the spec asks for a certified firm.
What gets measured
- Airflow at each diffuser, grille and register, usually with a capture hood. Here is how the common capture hoods compare.
- Duct airflow by traverse, using a pitot tube and a calibrated manometer where an outlet reading will not do.
- Fan speed (RPM), motor amperage and voltage against the nameplate.
- Static pressure across filters, coils and fans, and in the supply and return duct.
- Outdoor air quantity.
- For water-side systems, flow through coils and pumps. We also balance hydronic systems for water source heat pumps, chillers and boilers.
Measuring tools need to be accurate and calibrated. The accuracy of the TAB process depends on the instruments, so ask for calibration records.
How a balance goes, step by step
- Review the plans and the spec. Know the design airflow for every outlet and the tolerance the spec allows.
- Walk the job. Confirm the system is complete and running before any measurement starts.
- Set total airflow at the fan. Correct fan speed first. It is the cheapest fix and it changes everything downstream.
- Balance proportionally. Start from the branch with the lowest airflow relative to design and work outward, so every adjustment brings the whole system closer to design.
- Verify and lock. Re-measure, mark each damper position and lock it.
- Report. Record the final readings and every deficiency found.
The spec normally sets the acceptable tolerance. Plus or minus 10 percent of design airflow is a common figure, but read your own spec and do not assume.
What has to be ready before TAB arrives
Most delays come from systems that are not ready, not from the balancing itself. Before the TAB crew shows up:
- Equipment is installed, started up and running.
- Filters are in, and they are the specified type.
- Ceiling tiles, diffusers and grilles are installed.
- Dampers and test ports are installed and reachable, with fire and smoke dampers open.
- Variable frequency drives and controls are working.
- Duct leakage testing is finished where the spec requires it.
The full list is in our pre-test checklist, and this guide covers how to prepare the systems.
What TAB does not cover
A TAB contractor balances systems. They do not fix installation errors or finish unfinished work. When the crew finds a problem, such as a missing damper, a wrong fan, or an undersized duct, it goes into a deficiency report for the responsible contractor to fix, and then the area is re-checked. This is what a good deficiency report contains.
TAB is also not commissioning. They overlap, but they are different jobs. Read how TAB differs from commissioning.
How long it takes
It depends on system size and complexity. A single rooftop unit with 10 to 15 registers typically takes half a day. Larger commercial projects with several systems can take 1 to 3 days. Plan TAB into the schedule early, working back from your turnover date. Here is how to schedule it so it does not hold up turnover.
What is in the TAB report
A report that will stand up at inspection includes:
- Measured versus design airflow for each diffuser and register.
- Supply and return air quantities.
- Fan performance data: RPM, amperage and static pressure.
- System static pressures.
- Outdoor air measurements.
- Recommendations for any deficiencies found.
Reports are formatted to meet local inspection requirements and can be customized for your jurisdiction. See what inspectors look for in a TAB report.
Who should do the work
Use a firm certified by NEBB, AABC or TABB if your spec asks for one, and ask for the certificate. Air Balance Masters holds certifications from the National Comfort Institute (NCI), the National Environmental Balancing Bureau (NEBB), the Building Performance Institute (BPI) and the Association of Energy Engineers (AEE).
Need a system balanced before final inspection? Tell us about your project and get a quote, usually the same day. Call Ghazi Hussain at (908) 323-9780.
