(The following is taken from the third edition of HVAC Systems Testing, Adjusting & Balancing, published by the Sheet Metal and Air Conditioning Contractors' National Association.)

The TAB procedures for basic air systems found in Chapter 13, "General Air System TAB Procedures," are the foundation for the testing, adjusting and balancing of the air-distribution system. There are, however, certain different or additional procedures that should be used when balancing other than single-duct, constant-volume air systems.

Even though some of the duct systems addressed in this section are considered obsolete by the HVAC industry, TAB firms may encounter them when re-balancing or retrofitting systems in older buildings.

Variable-air volume systems

There are many types of VAV systems, but they can fall into two basic categories: bypass systems and reduced airflow or turndown systems. They further can be connected to single- or dual-supply air ducts, have constant or variable airflow on the primary side of the VAV boxes with variable airflow on secondary or distribution side. The variable airflow rate can be up to 100 percent of full flow. Figure 14-1 is a system layout for a typical VAV system.

Figure 14-1. A typical variable-air volume (VAV) system.

Terminal units

VAV terminal units (VAV boxes) can also be classified as pressure-independent and pressure-dependent. A pressure-independent device has a volume regulator that will maintain the proper airflow regardless of the terminal inlet static pressure. A pressure-dependent device will allow the airflow to vary in accordance with the inlet static pressure.

Diversity factor

Usually VAV systems are designed with a diversity factor, which means that the supply fan airflow (cfm or L/s) capacity is less than the sum of the airflows of all the terminal devices. If the diversity factor is not given, it can be approximated by dividing the supply air fan maximum airflow by the sum of the airflows of all VAV terminal units and converting the decimal number to a percentage.

Primary-air volume control

The supply air fan installation for a VAV system is similar to that for constant-volume systems, except that the fan-air volume must be varied. Inlet or discharge dampers and variable-speed drives or motors may be used to control the system airflow. A static-pressure sensor, usually located about two-thirds of the way from the fan to the end of the duct system, senses the supply air duct static pressure and sends a signal back to the apparatus controlling the fan airflow volume.

Verify that the controls are set to maintain a constant static pressure at the sensor location, as the system airflow varies.

Systems with combination return/exhaust air fans require special attention by the TAB technician. Building pressure will vary if the return-air fan volume does not vary closely with the supply air fan volume. The three common methods used are: building-static control, open-loop control and closed-loop control.

Building-static control

Building-static control senses differential pressures between a typical room and outdoors, and increases the volume-air handled by the return/exhaust air fan as building pressure increases.

Figure 14-2. Open-loop fan-volume control.

Open-loop control

Open-loop control uses an adjustable span and start point on the supply air and return-air fan controls to sequence the return-air fan operation with the supply air fan (Figure 14-2). This system requires close attention by the TAB technician. If the system load varies significantly among major zones the supply air fan serves, resistance in the return-air system may not vary in direct proportion to resistance in the supply air system.

Closed-loop control

The closed-loop control senses changes in the volume of air the supply air fan delivers and uses a controller with a second input proportional to the return-air fan flow to reset the return-air fan.

Square-root factors should be applied where the flow is measured as velocity pressure, enabling comparison of linear signals. Controlling flow eliminates the effects of different fan or vane characteristics. The controller can be set to maintain the difference in the airflow required between the supply and return-air fan to maintain building pressure and accommodate auxiliary exhaust systems. If this difference is also the minimum amount of outside air the system requires, the flow controller on the fans will maintain the minimum ventilation rate, regardless of variations in outside-air and return-air damper characteristics.

Multiple-point Pitot tubes or flow-measuring stations should be used to sense velocity pressure at the fans on VAV systems, since the velocity profile will vary with flow. Single-point Pitot tubes will give inaccurate readings of total airflow. The calibration of these flow stations should be verified by the TAB technician.

(For information on ordering SMACNA's HVAC Systems Testing, Adjusting & Balancing, write 4201 Lafayette Center Drive, Chantilly, VA 20151-1209; call (703) 803-2980; fax (703) 803-3732; see http://www.smacna.org on the Internet.)