An HVAC system is designed on paper to deliver specific airflows to each room and specific water flows to each coil. When it is built, it almost never does so without adjustment. Ducts and pipes are installed slightly differently from the drawings, fittings create more or less resistance than calculated, and equipment performs a little differently from its catalogue rating. Testing, adjusting and balancing (TAB) is the process of measuring what the system actually delivers and adjusting it until it matches the design intent.
TAB is one of the final steps before a building is occupied, and it is also needed whenever a system is modified or occupants report persistent comfort problems. A balanced system is more comfortable, quieter, uses less energy and supports proper ventilation and pressurization.
The three parts of TAB
- Testing means measuring the quantitative performance of equipment and systems: airflows, water flows, temperatures, pressures, fan and pump speeds, and electrical data such as motor amperage.
- Adjusting means changing dampers, valves, drives and controls to regulate flow rates and pressures.
- Balancing means proportioning flows within the distribution system so that each outlet, terminal unit or coil receives its design quantity within the allowed tolerance.
The work is governed in North America by ASHRAE Standard 111, which covers measurement, testing, adjusting and balancing of building HVAC systems, and by the procedural standards of the TAB certification bodies. See TAB certification bodies: AABC, NEBB and TABB for how those organizations differ.
What a typical TAB scope includes
Air systems
- Total airflow of air handling units, rooftop units, make-up air units and exhaust fans.
- Outdoor air, return air and relief air quantities.
- Airflow at each supply diffuser, return grille and exhaust register.
- VAV and constant volume terminal unit minimum and maximum airflows.
- Fan speed, static pressure profile across the unit, and motor electrical data.
- Building and room pressure relationships where specified.
Hydronic systems
- Flow through chillers, boilers, heat exchangers and pumps.
- Flow through each coil, terminal unit, radiation circuit and branch.
- Pump differential pressure, impeller size or speed and motor data.
- Water temperatures entering and leaving equipment.
Related work
TAB contractors frequently also verify control system sensors and setpoints, test duct leakage, measure sound levels or verify kitchen and fume hood exhaust, depending on the contract. These additions should be explicit in the specification.
How the process works
TAB follows a logical sequence. Skipping steps usually means repeating work later.
- Design review. The TAB agency reviews drawings and specifications to confirm that balancing devices, test ports and access are shown where needed. Missing balancing dampers or flow measuring devices are much cheaper to add before construction than after.
- Pre-balance inspection. Before measurement starts, the agency confirms that the system is complete: ductwork sealed, filters installed, dampers open, coils clean, strainers clean, air purged from piping, controls operational and fans rotating in the correct direction.
- Total flow measurement. Fan and pump total flows are measured and adjusted first, because branch balancing is meaningless if the total is wrong.
- Proportional balancing. Branches and outlets are adjusted so that each receives its share of the total flow. Methods such as the ratio or proportional method minimize the number of passes required and avoid unnecessary throttling.
- Final adjustment. Fan drives or variable frequency drive (VFD) maximum speeds and pump impellers or speeds are set so that the system meets design with the least possible throttling, which saves energy.
- Verification and marking. Final settings are recorded and balancing devices are marked so that their positions can be restored.
- Reporting. A TAB report records design and measured values for every item, instruments used and any deficiencies.
Air balancing procedures and instruments and hydronic balancing methods cover these steps in more detail.
Tolerances
Project specifications set the allowable deviation from design. Typical values are shown below, but the specification governs.
| Item | Typical tolerance |
|---|---|
| Air handling unit total airflow | Within -5% to +10% of design |
| Individual supply and exhaust outlets | Within plus or minus 10% of design |
| Outdoor air quantity | Within plus or minus 10%, never below the code minimum |
| Coil and terminal water flow | Within plus or minus 10% of design |
| Pump total flow | Within plus or minus 5% to 10% of design |
Critical spaces such as operating rooms, isolation rooms, laboratories and cleanrooms often have tighter tolerances and require verification of pressure differentials and air change rates under the applicable standard, such as CSA Z317.2 for health care facilities.
TAB and commissioning
TAB and commissioning are related but different. TAB measures and adjusts flows. Commissioning verifies that the entire system, including controls and sequences, meets the owner's requirements across operating conditions. The commissioning agent typically reviews the TAB report and spot-checks a sample of readings to confirm accuracy before accepting it. Functional testing of the controls depends on a balanced system, so TAB has to be complete first.
When TAB is needed in existing buildings
- After renovations, tenant fit-outs or changes in space use.
- After replacing fans, pumps, coils or terminal units.
- When occupants report persistent hot, cold, drafty or stuffy spaces.
- When doors whistle, slam or are difficult to open, which indicates pressure imbalance.
- As part of retro-commissioning or energy projects, to confirm that reduced flows still meet ventilation requirements.
Frequently asked questions
Who should hire the TAB agency?
Many owners prefer to hire the TAB agency directly or through the design consultant rather than as a subcontractor to the mechanical contractor. This keeps the agency independent of the party whose installation it is checking. Either arrangement can work if the scope is clear.
How long does TAB take?
It depends on building size and system complexity. A small office fit-out may take a few days; a hospital or laboratory can take many weeks. Delays in construction completion are the most common reason for TAB schedules slipping.
Is balancing a one-time activity?
No. Systems drift as dampers are moved, filters load, controls are changed and spaces are reconfigured. Periodic rebalancing is good practice, especially after renovations.
Key takeaways
- TAB measures actual air and water flows and adjusts the system until it matches design within specified tolerances.
- ASHRAE Standard 111 and the AABC, NEBB and TABB procedural standards govern how TAB is performed.
- A complete installation and a pre-balance inspection are essential before measurements begin.
- Balancing proportionally and then reducing fan and pump speeds minimizes energy use.
- TAB supports commissioning but does not replace it. Our air balancing and water balancing services cover new and existing buildings.
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