DDC-VAV Terminal Box Controller Malfunctions On Minimum Airflow Operation

Setting of the DDC controller can be an involved and a time-consuming process. Most controllers, which have to be adjusted in the field at the terminal box, are adjusted utilizing a portable operator's terminal or laptop computer while verifying the flow with traverses and verifying pressures with magnehelic gauges.

The controller then is adjusted to make up for the difference existing between the readings indicated on the magnehelic gauges and the DDC transmitter. And finally, the unit is put through all cycles to verify that the settings did not change while other adjustments were made.

Operating most controllers below .03 D P, " H2O and 600 fpm is risky. And, in many cases, creates operating problems. Many designs call for a minimum primary cfm air flow value which develops a pressure differential at values far below .03 D P, " H2O. Many controllers set between .01 and .03 " H2O have a tendency to chase the set point, causing serious variations in flow.

Other controllers set at those values may not control at all. To enable the controllers to control the terminals, they must be set at a higher minimum cfm value or have major sheet metal modifications made to the terminal inlets. Or, a correction factor will have to be developed and input into the building computer to allow the boxes to be adjusted near operable values.

The problem becomes even worse when terminal boxes selected are over-sized in order to assure a very low noise level.

(see our publications #37 and #84)

Often, the only way to increase the operating differential pressure is to increase the velocity pressure, which requires an increase in the air velocity at minimum cfm flows. In an attempt to increase the terminal inlet velocity at the terminal's sensor, some designers try to utilize a circular device known as a doughnut.

This device is installed at the inlet to the terminal box. However, in order to assure that the device does not generate turbulence at the box inlet, it must be placed at least 2 1/2 duct diameters upstream from the inlet. The doughnut must also be fabricated with a smooth, round, beveled hole. The device must be stiff enough so it does not cause bending or vibration under maximum cfm flows.

The following data in table1 shows how extensive the problems can be under minimum airflows:

Table 1