Rev Up Your Vav

Engineered Systems, Jan, 2000 by Steven Angle

Commissioning doesn't qualify for thinking "outside the box" anymore, but this example proves that the process does good things for what happens around and inside the (VAV) box.

On your next construction project, it is a new building or a major renovation, you have an important question to answer. Will you, will you not, make commissioning part of the formal construction process?

If you decide not to perform a formal commissioning process, you have made the decision to risk beginning ownership of a building fraught with problems, most of which could have been avoided. Commissioning is the only process that will give you the best chance to not only discover the problems early, but also to get them corrected before the contractor has left and the operations staff takes over.

Although all of the hvac systems should be tested, the following discussion is about the results obtained during the commissioning of variable air volume (VAV) boxes and their components. These are of particular concern because of their quantity and their complexity, and the fact that problems with the VAV boxes can result in:

* Increased energy use;

* Poor occupant comfort;

* Excessive noise;

* Improper ventilation; and

* Excessive maintenance costs.

On the campus of Rensselaer Polytechnic Institute (Troy, NY), we have commissioned the hvac systems for six major building construction projects over the past five years. These have included three lab buildings totaling 187,000 sq ft and two office/classroom buildings totaling 76,000 sq ft. We have commissioned hundreds of pieces of equipment and have discovered many deficiencies with the new systems. The following discussion focuses on the more than 500 VAV boxes that were tested.

HOW THEY WORK

Figure 1 shows a typical fan-powered VAV box, listing the many components that are part of the system. There are many variations that we have tested: VAV with reheat and without fan, VAV without reheat and without fan, constant volume with pneumatic reheat control, and constant volume with DDC reheat control. (For this discussion, the term VAV box includes constant-volume operation.)

The purpose of the VAV box is to provide final control of the air supply to the occupied space. Control is accomplished by reading the space temperature in the occupied space and then using the VAV damper to adjust the primary airflow from the air-handling system (usually at 55[degrees]F to 60[degrees]F) as required. The reheat valve is opened as needed to allow hot water to flow through the reheat coil and temper the supply air. In almost all of the VAV boxes tested, DDC controls have been used.

HOW THEY WERE TESTED

The following is a good overview of the detail required to do an adequate job of testing VAV boxes.

Use the thermostat to control the VAV box. The thermostat should be used to control the VAV box during testing rather than manipulating the software. This ensures that the link between the thermostat and the VAV box is operational. A more complete test of the thermostat and its operation of the VAV box can be accomplished by supplying a hot or cool source to the thermostat to simulate a change in space condition.

Measure the discharge air temperature. During testing of the VAV box, it is important to measure the discharge air temperature from the VAV box to prove that the hot water flow and the DDC control algorithm are working. Many times we have found that while the controls show the reheat valve opening, problems with hand valves or the balancing valve are discovered which restrict water flow to the coil.

Observe the box airflow change from maximum to minimum. During the change of the box from full cooling to full heating, verify that the airflow reaches both the maximum and minimum setpoints as the design requires. Generally, we have used the airflow reading from the DDC display, but airflow hoods or duct traverses can also be used. During this test, sticky dampers, incorrect system static pressure, or incorrect setpoints may be discovered.

INSTALLATION VERIFICATION

During this phase, verify that each of the following components should be verified to be sure they are installed or present according to the drawings and specifications. These items need to be visually verified for each VAV box, making it necessary in some cases to lift ceiling tiles or remove small sections of insulation.

* Reheat valve

* Control wiring

* Isolation unions

* Y-strainer

* Isolation ball valves

* Drain valve, hose tap

* Straight duct

* Access to all components

* Coilfins

* Access doors

* Adequate support

* Balance valve

* Damper linkage

* Flex connection

* Labeling

* Insulation complete

* Pipe unions

* Pressure-temperature tabs Air vent valve

* Thermostat location

FUNCTIONAL PERFORMANCE VERIFICATION

There are three key points about the functional performance testing of the VAV box that warrant discussion in more detail. These items are the most important part of the commissioning process, since this is the time where actual sequences of operations are verified.

Thermostat calibration. Using a handheld thermometer, compare its reading with that of the thermostat to verify calibration of the room thermostat.