Rev Up Your VAV (January 2000)
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 WorkFigure 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°F to 60°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 TestedThe 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 VerificationDuring 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
- Isolation ball valves
- Drain valve, hose tap
- Straight duct
- Access to all components
- Coil fins
- Access doors
- Adequate support
- Balance valve
- Damper linkage
- Flex connection
- Insulation complete
- Pipe unions
- Pressure-temperature tabs
- Air vent valve
- Thermostat location
Functional Performance VerificationThere 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.
Heating. Next, set the box to full heating by raising the thermostat or providing a cold source near it. Observe whether airflow reduces smoothly to the correct heating setpoint, and if the reheat valve opens to 100%. Note the supply air’s exact temperature increase.
Cooling. Finally, set the box to full cooling by lowering the thermostat or placing a warm source near the thermostat. Observe whether the reheat valve closes. The supply air should increase to the correct cooling setpoint, and the supply air temperature should decrease to match system primary supply air temperature. Other steps to confirm proper performance include verifying that the control valve opens upon the failure of control power, and verifying high- and low-temperature alarms by raising or lowering a room’s temperature as appropriate.
Documentation VerificationVerify that the following are complete if applicable:
- Shop drawings;
- O&M manual;
- Balance report; and
- Record drawings.
The ResultsTable 1 shows the number of deficiencies discovered during the commissioning process. Several of these deficiencies are worth discussing in more detail.
Nine constant-volume boxes using a pneumatic reheat valve and thermostat were found to be installed without the main supply air connected. The reheat valve on these boxes had failed to full heating. Fifty-two VAV boxes had problems with the DDC controller and its programming. In some cases, the tuning constants were set “fast” as an aid to the balancer during balancing, then not returned to the correct settings when balancing was completed. In other cases, the minimum or maximum air- flows were not programmed correctly, sometimes because the parameter was changed by the engineer during construction but the information didn’t get communicated. Several had software problems that caused the program to hang up and not control properly.
Twenty-three VAV boxes had problems with the reheat control valve assembly, including several instances where the manual isolation valves were found closed. Control valve problems included poor installation of the pneumatic lines, causing pressure on the control valve body and preventing proper operation. In a few cases, control valve actuators were not yet connected to the valve; this was not easily noticed because they were located above the finished ceiling.
Twenty-five VAV boxes could not achieve the maximum airflow recorded on the balancer’s report. This was sometimes due to an incorrect static pressure setpoint for the system main air handler, because it had not been communicated to the DDC contractor. Elsewhere, problems were due to dampers that were binding and couldn’t reach the full open position. In one case, this problem was noted on the balance report, but it had not yet been corrected by the mechanical contractor.
Eight thermostats were in poor locations. In almost all cases, they were installed according to the design drawings but ended up too close to coffee machines or copiers that generated heat; or, they were located directly in the throw area of a nearby diffuser. Frequently, the owner had located equipment in locations that were different than planned, and the thermostat had to be relocated.
Why Do These Deficiencies Occur?Shouldn’t each VAV box work right the first time? Yes, in an ideal world, but in reality there is:
Human error. Many contractors are involved with each project, accomplishing thousands of tasks during construction. The complexity of the process dictates that zero mistakes would be very difficult to achieve.
Communication. Communication problems between contractors and their subcontractors, owner and contractor, architect and owner, etc., have led to problems in the field. It is unlikely that all the construction document bulletins and changeorders will be incorporated without error, and the commissioning process serves as the double check to be sure that they are.
Project Schedule. It is typical in a construction project that the construction schedule experiences delays creating time pressures at the end of a project. During the last month of construction, the activity level is very high, and many trades are working in the same space. Since VAV box controls and their checkout are one of the last things to occur on the project, there is often significant pressure to complete the work quickly, which leads to problems because there isn’t the time to double check the system.
In addition, the owner sometimes moves in before construction is complete, allowing no time for proper system startup. This schedule pressure at the end of the construction cycle is a major contributing factor to the problems we have experienced.
VAV box complexity. The proper installation and startup of a VAV box system involves the mechanical contractor, the electrician, the controls contractor, the controls contractor’s electrician, the controls contractor’s pneumatic subcontractor, the balancer, and sometimes the general contractor if the box requires painting or special access doors. There are a lot of individuals who have a role to play in the proper installation of the VAV box. The more individuals involved, the more likelihood that errors will occur.
What is the Impact On Costs?The true cost to your organization for allowing deficiencies to go undiscovered is difficult to quantify but is significant, as can be seen by looking at the deficiencies listed. Costs related to personal productivity caused by a poor work environment are the most difficult to quantify, but they are real for your organization.
The cost for additional energy caused by airflow above design requiring additional supply fan horsepower, chiller energy, and reheat energy is also very real. Maintenance calls can last from several minutes to several hours, costing the organization several hundreds of dollars for each call. Meetings between owners and contractors to resolve issues can take hours or continue for weeks and months if problems persist. Clearly, commissioning saves the organization money.
Commissioning has proven to be beneficial at Rensselaer Polytechnic Institute in both new building and major renovation projects over the past five years. The costs involved in performing commissioning have been more than made up in savings through the discovery of deficiencies that would have gone undiscovered, and would have generated operation costs for years to come. The correction of the 288 problems with VAV boxes listed above have allowed us to save untold thousands of dollars by avoiding each of those potential pitfalls. ES