By: Nicole Imeson
Air and water balancing, or testing, adjustment, and balancing (TAB), fine-tunes a system's performance to match the flow rates in the design documents and equipment submittals. "Balancing is a key milestone in the commissioning schedule. Among the many benefits, the TAB process confirms the design intent, checks that ductwork is installed, and ensures diffusers and VAVs with hydronic reheat coils connect to the right systems," says Luke Buis, owner of Inland Technical Services Ltd, a building commissioning and TAB firm in Kelowna, BC. "Balancing not only improves occupant comfort, but it's also an indicator of construction and design quality and building efficiency. It's crucial for meeting greenhouse gas emissions goals."
Commissioning ensures that buildings, systems, and equipment are planned, designed, installed, tested, operated, and maintained to meet the owner's documented functional requirements. Since mechanical systems move air and water throughout a building, which is invisible during walkthroughs and cannot be measured by the eye, TAB plays a crucial role in functional testing. "Balancing is an extension of verifying how everything in a building is running," explains Buis.
New vs Existing Building Commissioning
TAB involves more than measuring air and water flow rates; it requires activating all mechanical systems to test effectively. This process also "weeds out many control sequences, ensuring systems run concurrently as intended," says Buis. The BCxA best practices for new construction commissioning (NCCx) suggest witnessing start-up and TAB before beginning functional testing. CxPs can use TAB reports to identify system issues and mark the milestone for starting functional testing.
In an ideal world, TAB would finish during original construction, and the system would maintain consistent operation with the same setpoints, damper settings, and valve positions. This would mean no need for ongoing rebalancing or surveys. However, this ideal rarely happens. Operators, contractors, and occupants often tweak the building's operation.
For existing building commissioning (EBCx), an air and water flow survey should come first to identify current issues. "A TAB report serves as a benchmark of how a building operated at a specific time," says Buis. If the owner has an original TAB report, the new survey can be compared to highlight changes or problem areas.
Commissioning providers (CxP) should review the TAB plan to ensure it is thorough, and discrepancies are resolved or tracked in the commissioning master issues log, and all stakeholders understand the time needed for balancing in the project and commissioning schedule. "The more complex the building, the more detail the commissioning team should add to the balancing plan," explains Buis. Like commissioning and other construction tasks, balancing is a long, complex process often overlooked in scheduling.
How Balancing Works
A balancing technician, usually an independent, third-party contractor, takes initial readings of the system's flow rates and compares them to the specifications. This comparison often reveals differences between the design and actual performance. Exact matches to the design flow rates are challenging, and most engineering guidelines allow a variation of +/- of 5-10%, depending on the system.
Airflow rates are balanced by first measuring the air coming from each grille and diffuser using a flow hood and pitot tube traverses and comparing it to the design and equipment specifications. The dampers and fans are adjusted to match these specifications, ensuring each room gets the right amount of air. The contractor also checks for any blockages or leaks in the ductwork that could affect airflow. The system is cycled between heating and cooling mode and minimum and maximum flow to ensure it works properly. Most importantly, the tab process will identify the proper controlling pressure for a variable volume system.
In a water system—hydronic heating and cooling or domestic hot water recirculation—flow is measured by inserting test probes into the balance valve ports and measuring the pressure difference across the valve. Using manufacturer information for the specific valve, the pressure drop is converted to flow. Valves and pumps are adjusted to match these required flow rates, ensuring each zone receives the correct amount of water. Similarly to air systems, hydronic systems are cycled between heating and cooling mode and minimum and maximum flow to ensure it works properly. Most importantly, the tab process will identify the proper controlling pressure for a variable volume system. Effective balancing saves pump and fan energy usage.
Ensuring each coil gets the correct flow rate is crucial for proper heating in each zone. A low flow rate means not enough heat, while a high flow rate can limit flow to other coils further from the central plant. With delivery delays and equipment availability issues, alternate equipment from specified is common. Even if the flow rates between specified and alternate coils are close, slight differences may occur to achieve the same output. Design reviews also help catch missing volumes, unit conversion issues, and discrepancies between field totals and central unit outputs.
With advances in equipment and affordable flow measurement tools, operators can now measure and adjust real-time flow rates. However, these tools often need field calibration to work correctly. They must adjust for actual ductwork configurations, system pressures, and airflow ranges. Adjustable fan speed units with ECM motors usually need software tweaks from the manufacturer's representative. If the unit arrives on-site with a lower-than-specified flow rate, the manufacturer and balancer must return to adjust the fan speed, adding time and unexpected costs to the project.
Thermal Comfort and Energy Conservation
Balancing helps with both thermal comfort and energy conservation. If an area lacks proper heating or cooling, occupants feel uncomfortable. "Incorrect flow rates can also create negative pressure, such as drawing in too much outside air and affecting the building enclosure," adds Buis. Exhaust-to-outdoor air ratio deficiencies increase heating and cooling loads beyond the design, causing more discomfort. Over-flowing equipment also wastes energy. Properly setting outside air dampers on central air handling units ensures enough ventilation without raising heating and cooling loads.
In the US, projects often specify National Environmental Balancing Bureau (NEBB) certified TAB firms, but such firms are rare in western Canada. NEBB certification involves formal training and an exam, while certified firms need references and must follow NEBB guidelines. Even if verification isn't required, witnessing the balance on-site and spot-checking readings can help ensure accuracy. If this isn't included in the scope, reviewing the initial and final readings in the TAB report can indicate if the system has been adjusted.
TAB ensures HVAC systems work as intended, providing thermal comfort and energy efficiency. By fine-tuning air and water flow rates to design specifications, TAB verifies mechanical systems perform correctly and addresses issues that could cause inefficiencies and discomfort. As part of commissioning, TAB validates construction quality, develops a benchmark for performance, and supports sustainable building operations, making it essential for achieving top system performance and occupant satisfaction.