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The return-fan speed is controlled to maintain a constant negative pressure in the mixing box. When commissioning the controls, it is important to know how to select the constant negative-pressure set point in the mixing box to control the return-fan speed. The appropriate set point can be selected by:

1. Setting up the AHU

   Temporarily set up the unit for 100-percent-outside-air operation: Shut off the return fan, close the return damper, and open the outside-air damper.

2. Positioning the outside-air damper

   Set the outside-air damper to the selected minimum position. (An arbitrary adjustment, the damper could be anywhere from 10- to 25-percent open.) The minimum position should be high enough to avoid excessive pressure drop, but low enough to leave sufficient stroke for the outside-air dampers to modulate open during the economizer cycle. The example AHU's initial position was 15-percent open (1.5 v on a 0- to 10-v signal to the damper motor).

3. Making adjustments

   Start the supply fan and adjust the variable-speed drive to deliver the desired minimum outside-air quantity. The example unit had an initial setting of 20-percent speed. If the setting were correct, the desired 4,000 cfm would move at 1,778 fpm in the unit's 18-in.-by-18-in. supply duct.

   The example's measured duct velocity (by pitot traverse) was only 1,000 fpm (2,250 cfm). Pressure in the mixed-air plenum (Point C, Figure 1) was -0.06 in. wg. The airflow was less than the desired minimum, and the mixed-air-plenum pressure was too low to be a workable control set point. After several iterations of increasing fan speed and adjusting outside-air-damper position, a fan speed of 60 percent and a damper position of 25 percent produced a duct velocity of 1,697 fpm (3,818 cfm). The mixed-air-plenum pressure was -0.28 in. wg.

   The minimum outside-air-damper-position set point was left at 25 percent. The mixed-air-plenum-pressure set point was -0.30 in. wg. (The square root of 0.30/0.28 multiplied by the measured 3,818 cfm equals 3,952 cfm, which was acceptably close to the desired 4,000-cfm minimum outside-air quantity.)

4. Returning the AHU to normal

   Finally, return the unit to normal control. The example AHU's static-pressure profile and airflow measurements were taken at full load and compared with the fan curve as part of the unit's commissioning. The entire process took about 5 to 6 hr.

After the constant negative-pressure set point has been selected and the AHU has been returned to normal control, the relief/spill damper (Damper 1, Figure 1) modulates and tracks the outside-air damper (Damper 3, Figure 1). The relief damper should be closed when the outside-air damper is at minimum.

The return damper (Damper 2, Figure 1) and outside-air damper (Damper 3, Figure 1) are controlled via discharge air or mixed-air temperature as part of the economizer cycle. As the outside-air damper opens beyond minimum, the return damper moves toward a closed position.

Maintaining the minimum outside-air quantity is not the only concern when controlling a VAV unit in a freezing climate. At a full load of 20,000 cfm that included 4,000 cfm of 0°F outside air and 16,000 cfm of 70°F return air, the example AHU's mixed-air temperature was 56°F. Therefore, the mixed-air temperature matched the design supply-air temperature.