In reality, this "bigger is better" rule of thumb is incorporated into nearly every construction project, and usually the expected benefit of this implementation becomes a recurring problem, becoming evident just as the warranty expires on the project. More often than not, the problem is falsely identified as a control system problem.

Too much of a good thing

A notorious case of "bigger is better" thinking is found in the application of pressure-dependent VAV boxes. In the simplest form, a VAV box regulates the amount or volume of cold air discharged into a space in order to maintain a specific space temperature.

To size the VAV box, the volume of cold air (typically 55 degrees F) required for maintaining a comfortable space temperature at the expected maximum load condition must be calculated. This resultant value obtained by this volume calculation is expressed in cfm.

Having calculated the maximum required cfm for the particular space, the proper VAV box can be selected. Most commonly, the size of the VAV box is specified by the diameter of the box's inlet. Sizes can range from 4 in. up to 18 in. and even larger. Selecting the proper size requires that the must be capable of delivering at least the calculated maximum cfm that the space requires.

This is where things begin to go terribly wrong. Instead of choosing a VAV box that is capable of delivering an air volume slightly higher than the calculated maximum required cfm, the tendency is to add a "safety factor" to the design by selecting a VAV box that can deliver an air volume far above the calculated maximum required cfm.

Proper measurement is a must

Controlling a pressure-dependent VAV box requires that the discharge air volume be measured. The controls open and close the VAV damper to maintain the space temperature while always providing a minimum airflow cfm and never exceeding the maximum cfm value.

The cfm is calculated by the controls by reading the velocity pressure signal provided by the VAV box manufacturer's velocity sensor. The velocity sensor measures velocity pressure by subtracting static pressure from total pressure. This velocity pressure signal is a value in an equation that the controls use to calculate air velocity. The air velocity can then be multiplied by the VAV box inlet area to calculate cfm.

Even though the maximum cfm is achievable by the oversized VAV box, the performance of the VAV box is dependent on obtaining a reliable air velocity pressure signal from the VAV box's velocity sensor.

When a VAV box is oversized, the velocity of the air passing the velocity sensor is significantly reduced. In this case, the low air velocity results in a velocity pressure that is below the sensing range of the VAV box manufacturer's velocity sensor.

Verifying proper sizing

The VAV box manufacturer's product data usually includes a chart depicting the relationship between the velocity pressure signal generated by the velocity sensor and the cfm delivery capability of the VAV box. Verifying that a VAV box is sized properly requires that its required minimum cfm value produce a velocity pressure within the readable range of its velocity sensor.

In other words, the design of the velocity sensor only allows it to provide a reliable velocity pressure signal for a certain range of air velocities. If the air velocity is above or below the manufacturer's specified range, the velocity sensor will produces a signal that is unreliable, causing the cfm calculation to be wrong. In the case of oversized VAV boxes, the air velocity is always too low and usually falls below the manufacturer's specified range for the velocity sensor.

Proper sizing in action

The project drawings call for an 8-in. pressure-dependent VAV box to be installed with a minimum cfm of 200 and a maximum cfm of 1,200.

Step 1: Request the velocity pressure signal vs. the cfm chart for the VAV box model. Notice that the lowest velocity pressure listed on the chart is 0.1-in. wc. Any velocity pressure signal reported by the VAV box's velocity pressure sensor that is lower than 0.1-in. wc will be unreliable.

Step 2: Find the 8-in. inlet size on the right-hand side of the chart. Follow the corresponding diagonal line down to the lower left corner until it intersects the 0.1 vertical velocity pressure line. This intersection shows the minimum cfm that can be reliably read from the manufacturer's velocity pressure sensor. In this case, the 8-in. VAV box velocity pressure sensor can only provide a reliable signal to the VAV box controls for airflow rates above 475 cfm.

In this example, the velocity pressure sensor for the 8 in. VAV box that was specified will not provide a reliable signal to the control system, causing inconsistent space temperature and early failure of the VAV box damper actuator.

Preemptive strikes and fixes on the fly

On projects that are in the construction phase, this problem can be exposed in enough time to have the VAV box inlet collars reduced. For VAV boxes that are installed, the only cost-effective method of fixing this problem is to install reduction collars in the inlet. The reduction collar will cause the air velocity to increase, providing a larger velocity pressure.

Some argue that control manufacturers should provide a differential pressure transmitter that can read very low velocity pressures. The fact is that many control manufacturers' differential pressure transmitters can read very low velocity pressures. The problem is that many of the VAV box manufacturers' velocity pressure sensors are incapable of providing a reliable low-velocity pressure signal for the control system to read.

In the interim, the only 100% guaranteed method of eliminating this problem is to properly size and select the VAV box using the manufacturer's published velocity pressure data.

Noticing and fixing this error in an application will ensure that the VAV system is at maximum performance.ES